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Archive   : GCCDOC.ZIP
Filename : GCCDOC.LST

 
Output of file : GCCDOC.LST contained in archive : GCCDOC.ZIP

This is Info file gcc.info, produced by Makeinfo-1.44 from the input
file gcc.texi.

This file documents the use and the internals of the GNU compiler.

Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public
License" and this permission notice may be included in translations
approved by the Free Software Foundation instead of in the original
English.

Indirect:
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gcc.info-2: 49440
gcc.info-3: 98138
gcc.info-4: 146799
gcc.info-5: 196465
gcc.info-6: 236935
gcc.info-7: 273049
gcc.info-8: 321228
gcc.info-9: 364288
gcc.info-10: 413942
gcc.info-11: 463799
gcc.info-12: 513545
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gcc.info-15: 647691
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Node: Preprocessor Options75875
Node: Link Options80653
Node: Directory Options84077
Node: Target Options86982
Node: Submodel Options90718
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File: gcc.info, Node: Top, Next: Copying, Up: (DIR)

Introduction
************

This manual documents how to run, install and port the GNU C
compiler, as well as its new features and incompatibilities, and how
to report bugs. It corresponds to GNU CC version 2.0.

* Menu:

* Copying:: GNU General Public License says
how you can copy and share GNU CC.
* Contributors:: People who have contributed to GNU CC.
* Boycott:: Protect your freedom--fight "look and feel".
* Invoking GCC:: Command options supported by 'gcc'.
* Installation:: How to configure, compile and install GNU CC.
* Trouble:: If you have trouble installing GNU CC.
* Service:: How to find suppliers of support for GNU CC.
* Incompatibilities:: Incompatibilities of GNU CC.
* Extensions:: GNU extensions to the C language.
* Bugs:: How to report bugs (if you want to get them fixed).
* VMS:: Using GNU CC on VMS.
* Portability:: Goals of GNU CC's portability features.
* Interface:: Function-call interface of GNU CC output.
* Passes:: Order of passes, what they do, and what each file is for.
* RTL:: The intermediate representation that most passes work on.
* Machine Desc:: How to write machine description instruction patterns.
* Target Macros:: How to write the machine description C macros.
* Config:: Writing the 'xm-MACHINE.h' file.
* Index:: Index of concepts and symbol names.


File: gcc.info, Node: Copying, Next: Contributors, Prev: Top, Up: Top

GNU GENERAL PUBLIC LICENSE
**************************

Version 2, June 1991

Copyright (C) 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

Preamble
========

The licenses for most software are designed to take away your
freedom to share and change it. By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software--to make sure the software is free for all its users. This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it. (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.) You can apply it to
your programs, too.

When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have. You must make sure that they, too, receive or can get the
source code. And you must show them these terms so they know their
rights.

We protect your rights with two steps: (1) copyright the software,
and (2) offer you this license which gives you legal permission to
copy, distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software. If the software is modified by someone else and passed on,
we want its recipients to know that what they have is not the
original, so that any problems introduced by others will not reflect
on the original authors' reputations.

Finally, any free program is threatened constantly by software
patents. We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary. To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and
modification follow.

TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

1. This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be
distributed under the terms of this General Public License. The
"Program", below, refers to any such program or work, and a "work
based on the Program" means either the Program or any derivative
work under copyright law: that is to say, a work containing the
Program or a portion of it, either verbatim or with modifications
and/or translated into another language. (Hereinafter,
translation is included without limitation in the term
"modification".) Each licensee is addressed as "you".

Activities other than copying, distribution and modification
are not covered by this License; they are outside its scope. The
act of running the Program is not restricted, and the output from
the Program is covered only if its contents constitute a work
based on the Program (independent of having been made by running
the Program). Whether that is true depends on what the Program
does.

2. You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an
appropriate copyright notice and disclaimer of warranty; keep
intact all the notices that refer to this License and to the
absence of any warranty; and give any other recipients of the
Program a copy of this License along with the Program.

You may charge a fee for the physical act of transferring a
copy, and you may at your option offer warranty protection in
exchange for a fee.

3. You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

a. You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any
change.

b. You must cause any work that you distribute or publish, that
in whole or in part contains or is derived from the Program
or any part thereof, to be licensed as a whole at no charge
to all third parties under the terms of this License.

c. If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or
display an announcement including an appropriate copyright
notice and a notice that there is no warranty (or else,
saying that you provide a warranty) and that users may
redistribute the program under these conditions, and telling
the user how to view a copy of this License. (Exception: if
the Program itself is interactive but does not normally
print such an announcement, your work based on the Program
is not required to print an announcement.)

These requirements apply to the modified work as a whole. If
identifiable sections of that work are not derived from the
Program, and can be reasonably considered independent and
separate works in themselves, then this License, and its terms,
do not apply to those sections when you distribute them as
separate works. But when you distribute the same sections as
part of a whole which is a work based on the Program, the
distribution of the whole must be on the terms of this License,
whose permissions for other licensees extend to the entire whole,
and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or
contest your rights to work written entirely by you; rather, the
intent is to exercise the right to control the distribution of
derivative or collective works based on the Program.

In addition, mere aggregation of another work not based on the
Program with the Program (or with a work based on the Program) on
a volume of a storage or distribution medium does not bring the
other work under the scope of this License.

4. You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the
terms of Sections 1 and 2 above provided that you also do one of
the following:

a. Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of
Sections 1 and 2 above on a medium customarily used for
software interchange; or,

b. Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than
your cost of physically performing source distribution, a
complete machine-readable copy of the corresponding source
code, to be distributed under the terms of Sections 1 and 2
above on a medium customarily used for software interchange;
or,

c. Accompany it with the information you received as to the
offer to distribute corresponding source code. (This
alternative is allowed only for noncommercial distribution
and only if you received the program in object code or
executable form with such an offer, in accord with
Subsection b above.)

The source code for a work means the preferred form of the
work for making modifications to it. For an executable work,
complete source code means all the source code for all modules it
contains, plus any associated interface definition files, plus
the scripts used to control compilation and installation of the
executable. However, as a special exception, the source code
distributed need not include anything that is normally
distributed (in either source or binary form) with the major
components (compiler, kernel, and so on) of the operating system
on which the executable runs, unless that component itself
accompanies the executable.

If distribution of executable or object code is made by
offering access to copy from a designated place, then offering
equivalent access to copy the source code from the same place
counts as distribution of the source code, even though third
parties are not compelled to copy the source along with the
object code.

5. You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this
License. However, parties who have received copies, or rights,
from you under this License will not have their licenses
terminated so long as such parties remain in full compliance.

6. You are not required to accept this License, since you have not
signed it. However, nothing else grants you permission to modify
or distribute the Program or its derivative works. These actions
are prohibited by law if you do not accept this License.
Therefore, by modifying or distributing the Program (or any work
based on the Program), you indicate your acceptance of this
License to do so, and all its terms and conditions for copying,
distributing or modifying the Program or works based on it.

7. Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program
subject to these terms and conditions. You may not impose any
further restrictions on the recipients' exercise of the rights
granted herein. You are not responsible for enforcing compliance
by third parties to this License.

8. If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent
issues), conditions are imposed on you (whether by court order,
agreement or otherwise) that contradict the conditions of this
License, they do not excuse you from the conditions of this
License. If you cannot distribute so as to satisfy
simultaneously your obligations under this License and any other
pertinent obligations, then as a consequence you may not
distribute the Program at all. For example, if a patent license
would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you,
then the only way you could satisfy both it and this License
would be to refrain entirely from distribution of the Program.

If any portion of this section is held invalid or
unenforceable under any particular circumstance, the balance of
the section is intended to apply and the section as a whole is
intended to apply in other circumstances.

It is not the purpose of this section to induce you to
infringe any patents or other property right claims or to contest
validity of any such claims; this section has the sole purpose of
protecting the integrity of the free software distribution
system, which is implemented by public license practices. Many
people have made generous contributions to the wide range of
software distributed through that system in reliance on
consistent application of that system; it is up to the
author/donor to decide if he or she is willing to distribute
software through any other system and a licensee cannot impose
that choice.

This section is intended to make thoroughly clear what is
believed to be a consequence of the rest of this License.

9. If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces,
the original copyright holder who places the Program under this
License may add an explicit geographical distribution limitation
excluding those countries, so that distribution is permitted only
in or among countries not thus excluded. In such case, this
License incorporates the limitation as if written in the body of
this License.

10. The Free Software Foundation may publish revised and/or new
versions of the General Public License from time to time. Such
new versions will be similar in spirit to the present version,
but may differ in detail to address new problems or concerns.

Each version is given a distinguishing version number. If the
Program specifies a version number of this License which applies
to it and "any later version", you have the option of following
the terms and conditions either of that version or of any later
version published by the Free Software Foundation. If the
Program does not specify a version number of this License, you
may choose any version ever published by the Free Software
Foundation.

11. If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to
the author to ask for permission. For software which is
copyrighted by the Free Software Foundation, write to the Free
Software Foundation; we sometimes make exceptions for this. Our
decision will be guided by the two goals of preserving the free
status of all derivatives of our free software and of promoting
the sharing and reuse of software generally.

NO WARRANTY

12. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO
WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE
LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT
WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT
NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE
QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY
SERVICING, REPAIR OR CORRECTION.

13. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY
MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL,
INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR
INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS
OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH
ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

END OF TERMS AND CONDITIONS

Appendix: How to Apply These Terms to Your New Programs
=======================================================

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

To do so, attach the following notices to the program. It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.

ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
Copyright (C) 19YY NAME OF AUTHOR

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

Also add information on how to contact you by electronic and paper
mail.

If the program is interactive, make it output a short notice like
this when it starts in an interactive mode:

Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type 'show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type 'show c' for details.

The hypothetical commands 'show w' and 'show c' should show the
appropriate parts of the General Public License. Of course, the
commands you use may be called something other than 'show w' and 'show
c'; they could even be mouse-clicks or menu items--whatever suits your
program.

You should also get your employer (if you work as a programmer) or
your school, if any, to sign a "copyright disclaimer" for the program,
if necessary. Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright interest in the program
'Gnomovision' (which makes passes at compilers) written by James Hacker.

SIGNATURE OF TY COON, 1 April 1989
Ty Coon, President of Vice

This General Public License does not permit incorporating your
program into proprietary programs. If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library. If this is what you want to do, use
the GNU Library General Public License instead of this License.


File: gcc.info, Node: Contributors, Next: Boycott, Prev: Copying, Up: Top

Contributors to GNU CC
**********************

In addition to Richard Stallman, several people have written parts
of GNU CC.

* The idea of using RTL and some of the optimization ideas came
from the program PO written at the University of Arizona by Jack
Davidson and Christopher Fraser. See "Register Allocation and
Exhaustive Peephole Optimization", Software Practice and
Experience 14 (9), Sept. 1984, 857-866.

* Paul Rubin wrote most of the preprocessor.

* Leonard Tower wrote parts of the parser, RTL generator, and RTL
definitions, and of the Vax machine description.

* Ted Lemon wrote parts of the RTL reader and printer.

* Jim Wilson implemented loop strength reduction and some other
loop optimizations.

* Nobuyuki Hikichi of Software Research Associates, Tokyo,
contributed the support for the Sony NEWS machine.

* Charles LaBrec contributed the support for the Integrated
Solutions 68020 system.

* Michael Tiemann of Cygnus Support wrote the front end for C++, as
well as the support for inline functions and instruction
scheduling. Also the descriptions of the National Semiconductor
32000 series cpu, the SPARC cpu and part of the Motorola 88000
cpu.

* Jan Stein of the Chalmers Computer Society provided support for
Genix, as well as part of the 32000 machine description.

* Randy Smith finished the Sun FPA support.

* Robert Brown implemented the support for Encore 32000 systems.

* David Kashtan of SRI adapted GNU CC to the Vomit-Making System
(VMS).

* Alex Crain provided changes for the 3b1.

* Greg Satz and Chris Hanson assisted in making GNU CC work on
HP-UX for the 9000 series 300.

* William Schelter did most of the work on the Intel 80386 support.

* Christopher Smith did the port for Convex machines.

* Paul Petersen wrote the machine description for the Alliant FX/8.

* Alain Lichnewsky ported GNU CC to the Mips cpu.

* Devon Bowen, Dale Wiles and Kevin Zachmann ported GNU CC to the
Tahoe.

* Jonathan Stone wrote the machine description for the Pyramid
computer.

* Richard Kenner of New York University wrote the machine
descriptions for the AMD 29000, the IBM RT PC, and the IBM
RS/6000 as well as the support for instruction attributes. He
also made changes to better support RISC processors including
changes to common subexpression elimination, strength reduction,
function calling sequence handling, and condition code support,
in addition to generalizing the code for frame pointer
elimination.

* Richard Kenner and Michael Tiemann jointly developed reorg.c, the
delay slot scheduler.

* Mike Meissner and Tom Wood of Data General finished the port to
the Motorola 88000.

* Masanobu Yuhara of Fujitsu Laboratories implemented the machine
description for the Tron architecture (specifically, the Gmicro).

* NeXT, Inc. donated the front end that supports the Objective C
language.

* James van Artsdalen wrote the code that makes efficient use of
the Intel 80387 register stack.

* Mike Meissner at the Open Software Foundation finished the port
to the MIPS cpu, including adding ECOFF debug support.

* Ron Guilmette implemented the protoize and unprotoize tools, the
support for Dwarf symbolic debugging information, and much of the
support for System V Release 4. He has also worked heavily on
the Intel 386 and 860 support.

* Torbjorn Granlund of the Swedish Institute of Computer Science
implemented multiply-by-constant optimization and better long long
support, and improved leaf function register allocation.


File: gcc.info, Node: Boycott, Next: Invoking GCC, Prev: Contributors, Up: Top

Protect Your Freedom--Fight "Look And Feel"
*******************************************

This section is a political message from the League for
Programming Freedom to the users of GNU CC. It is included here
as an expression of support for the League on the part of the
Free Software Foundation.

Apple and Lotus are trying to create a new form of legal monopoly: a
copyright on a class of user interfaces. These monopolies would cause
serious problems for users and developers of computer software and
systems. Xerox, too, has tried to make a monopoly for itself on window
systems; their suit against Apple was thrown out on a technicality, but
Xerox has not said anything to indicate it wouldn't try again.

Until a few years ago, the law seemed clear: no one could restrict
others from using a user interface; programmers were free to implement
any interface they chose. Imitating interfaces, sometimes with
changes, was standard practice in the computer field. The interfaces
we know evolved gradually in this way; for example, the Macintosh user
interface drew ideas from the Xerox interface, which in turn drew on
work done at Stanford and SRI. 1-2-3 imitated VisiCalc, and dBase
imitated a database program from JPL.

Most computer companies, and nearly all computer users, were happy
with this state of affairs. The companies that are suing say it does
not offer "enough incentive" to develop their products, but they must
have considered it "enough" when they made their decision to do so. It
seems they are not satisfied with the opportunity to continue to
compete in the marketplace--not even with a head start.

If companies like Xerox, Lotus, and Apple are permitted to make law
through the courts, the precedent will hobble the software industry:

* Gratuitous incompatibilities will burden users. Imagine if each
car manufacturer had to arrange the pedals in a different order.

* Software will become and remain more expensive. Users will be
"locked in" to proprietary interfaces, for which there is no real
competition.

* Large companies have an unfair advantage wherever lawsuits become
commonplace. Since they can easily afford to sue, they can
intimidate small companies with threats even when they don't
really have a case.

* User interface improvements will come slower, since incremental
evolution through creative imitation will no longer be permitted.

* Even Apple, etc., will find it harder to make improvements if
they can no longer adapt the good ideas that others introduce, for
fear of weakening their own legal positions. Some users suggest
that this stagnation may already have started.

* If you use GNU software, you might find it of some concern that
user interface copyright will make it hard for the Free Software
Foundation to develop programs compatible with the interfaces
that you already know.

To protect our freedom from lawsuits like these, a group of
programmers and users have formed a new grass-roots political
organization, the League for Programming Freedom.

The purpose of the League is to oppose new monopolistic practices
such as user-interface copyright and software patents; it calls for a
return to the legal policies of the recent past, in which these
practices were not allowed. The League is not concerned with free
software as an issue, and not affiliated with the Free Software
Foundation.

The League's membership rolls include John McCarthy, inventor of
Lisp, Marvin Minsky, founder of the Artificial Intelligence lab, Guy L.
Steele, Jr., author of well-known books on Lisp and C, as well as
Richard Stallman, the developer of GNU CC. Please join and add your
name to the list. Membership dues in the League are $42 per year for
programmers, managers and professionals; $10.50 for students; $21 for
others.

The League needs both activist members and members who only pay
their dues.

To join, or for more information, phone (617) 492-0023 or write to:

League for Programming Freedom
1 Kendall Square #143
P.O. Box 9171
Cambridge, MA 02139

You can also send electronic mail to '[email protected]'.

Here are some suggestions from the League for things you can do to
protect your freedom to write programs:

* Don't buy from Xerox, Lotus or Apple. Buy from their competitors
or from the defendants they are suing.

* Don't develop software to work with the systems made by these
companies.

* Port your existing software to competing systems, so that you
encourage users to switch.

* Write letters to company presidents to let them know their conduct
is unacceptable.

* Tell your friends and colleagues about this issue and how it
threatens to ruin the computer industry.

* Above all, don't work for the look-and-feel plaintiffs, and don't
accept contracts from them.

* Write to Congress to explain the importance of this issue.

House Subcommittee on Intellectual Property
2137 Rayburn Bldg
Washington, DC 20515

Senate Subcommittee on Patents, Trademarks and Copyrights
United States Senate
Washington, DC 20510

(These committees have received lots of mail already; let's give
them even more.)

Express your opinion! You can make a difference.


File: gcc.info, Node: Invoking GCC, Next: Installation, Prev: Boycott, Up: Top

GNU CC Command Options
**********************

When you invoke GNU CC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the '-c' option says
not to run the linker. Then the output consists of object files
output by the assembler.

Other options are passed on to one stage of processing. Some
options control the preprocessor and others the compiler itself. Yet
other options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.

The GNU C compiler uses a command syntax much like the Unix C
compiler. The 'gcc' program accepts options and file names as
operands. Multiple single-letter options may *not* be grouped: '-dr'
is very different from '-d -r'.

You can mix options and other arguments. For the most part, the
order you use doesn't matter; 'gcc' reorders the command-line options
so that the choices specified by option flags are applied to all input
files. Order does matter when you use several options of the same
kind; for example, if you specify '-L' more than once, the directories
are searched in the order specified.

Many options have long names starting with '-f' or with '-W'--for
example, '-fforce-mem', '-fstrength-reduce', '-Wformat' and so on.
Most of these have both positive and negative forms; the negative form
of '-ffoo' would be '-fno-foo'. This manual documents only one of
these two forms, whichever one is not the default.

Here is a summary of all the options, grouped by type.
Explanations are in the following sections.

*Overall Options*
*Note Options Controlling the Kind of Output: Overall Options.
-c -S -E -o FILE -pipe -v -x LANGUAGE

*Language Options*
*Note Options Controlling Dialect: Dialect Options.
-ansi -fbuiltin -fcond-mismatch -fno-asm
-fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char -fwritable-strings
-traditional -traditional-cpp -trigraphs

*Warning Options*
*Note Options to Request or Suppress Warnings: Warning Options.
-fsyntax-only -pedantic -pedantic-errors
-w -W -Wall -Waggregate-return
-Wcast-align -Wcast-qual -Wcomment -Wconversion -Werror
-Wformat -Wid-clash-LEN -Wimplicit -Wimport
-Winline -Wmissing-prototypes
-Wparentheses -Wpointer-arith -Wreturn-type -Wshadow
-Wstrict-prototypes -Wswitch -Wtraditional -Wtrigraphs
-Wuninitialized -Wunused -Wwrite-strings -Wchar-subscripts

*Debugging Options*
*Note Options for Debugging Your Program or GCC: Debugging
Options.
-a -dLETTERS -fpretend-float
-g -gLEVEL -ggdb -gdwarf
-gstabs -gstabs+ -gcoff -gxcoff
-p -pg -save-temps

*Optimization Options*
*Note Options that Control Optimization: Optimize Options.
-fcaller-saves -fcse-follow-jumps -fdelayed-branch
-fexpensive-optimizations -ffloat-store -fforce-addr -fforce-mem
-finline -finline-functions -fkeep-inline-functions
-fno-defer-pop -fno-function-cse -fomit-frame-pointer
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fstrength-reduce -fthread-jumps
-funroll-all-loops -funroll-loops
-O -O2

*Preprocessor Options*
*Note Options Controlling the Preprocessor: Preprocessor Options.
-C -dD -dM -dN
-DMACRO[=DEFN] -E -H
-include FILE -imacros FILE
-M -MD -MM -MMD -nostdinc -P -trigraphs -UMACRO

*Linker Options*
*Note Options for Linking: Link Options.
OBJECT-FILE-NAME
-lLIBRARY -nostdlib -static

*Directory Options*
*Note Options for Directory Search: Directory Options.
-BPREFIX -IDIR -I- -LDIR

*Target Options*
*Note Target Machine and Compiler Version: Target Options.
-b MACHINE -V VERSION

*Machine Dependent Options*
*Note Hardware Models and Configurations: Submodel Options.
*M680x0 Options*
-m68000 -m68020 -m68881 -mbitfield -mc68000 -mc68020 -mfpa
-mnobitfield -mrtd -mshort -msoft-float

*VAX Options*
-mg -mgnu -munix

*SPARC Options*
-mforce-align -mfpu -mno-epilogue

*Convex Options*
-margcount -mc1 -mc2 -mnoargcount

*AMD29K Options*
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge
-mnbw -mnodw -msmall -mstack-check -muser-registers

*M88K Options*
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division
-mhandle-large-shift -midentify-revision
-mno-check-zero-division -mno-ocs-debug-info
-mno-ocs-frame-position -mno-optimize-arg-area -mno-underscores
-mocs-debug-info -mocs-frame-position -moptimize-arg-area
-mshort-data-NUM -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs

*RS/6000 Options*
-mfp-in-toc -mno-fop-in-toc

*RT Options*
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return

*MIPS Options*
-mcpu=CPU TYPE -mips2 -mips3 -mint64 -mlong64 -mlonglong128
-mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats
-mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile
-msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic
-mno-half-pic -G NUM -nocpp

*i386 Options*
-m486 -msoft-float

*Code Generation Options*
*Note Options for Code Generation Conventions: Code Gen Options.
-fcall-saved-REG -fcall-used-REG -ffixed-REG
-fno-common -fpcc-struct-return -fpic -fPIC -fshared-data
-fshort-enums -fshort-double -fvolatile

* Menu:

* Overall Options:: Controlling the kind of output:
an executable, object files, assembler files,
or preprocessed source.
* Dialect Options:: Controlling the variant of C language compiled.
* Warning Options:: How picky should the compiler be?
* Debugging Options:: Symbol tables, measurements, and debugging dumps.
* Optimize Options:: How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
Also, getting dependency information for Make.
* Link Options:: Specifying libraries and so on.
* Directory Options:: Where to find header files and libraries.
Where to find the compiler executable files.
* Target Options:: Running a cross-compiler, or an old version of GNU CC.
* Submodel Options:: Specifying minor hardware or convention variations,
such as 68010 vs 68020.
* Code Gen Options:: Specifying conventions for function calls, data layout
and register usage.
* Environment Variables:: Env vars that affect GNU CC.


File: gcc.info, Node: Overall Options, Next: Dialect Options, Prev: Invoking GCC, Up: Invoking GCC

Options Controlling the Kind of Output
======================================

Compilation can involve up to four stages: preprocessing,
compilation proper, assembly and linking, always in that order. The
first three stages apply to an individual source file, and end by
producing an object file; linking combines all the object files (those
newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind
of compilation is done:

'FILE.c'
C source code which must be preprocessed.

'FILE.i'
C source code which should not be preprocessed.

'FILE.m'
Objective-C source code

'FILE.h'
C header file (not to be compiled or linked).

'FILE.cc'
'FILE.cxx'
'FILE.C'
C++ source code which must be preprocessed.

'FILE.s'
Assembler code.

'FILE.S'
Assembler code which must be preprocessed.

'OTHER'
An object file to be fed straight into linking. Any file name
with no recognized suffix is treated this way.

You can specify the input language explicitly with the '-x' option:

'-x LANGUAGE'
Specify explicitly the LANGUAGE for the following input files
(rather than choosing a default based on the file name suffix).
This option applies to all following input files until the next
'-x' option. Possible values of LANGUAGE are 'c', 'objective-c',
'c-header', 'c++', 'cpp-output', 'assembler', and
'assembler-with-cpp'.

'-x none'
Turn off any specification of a language, so that subsequent
files are handled according to their file name suffixes (as they
are if '-x' has not been used at all).

If you only want some of the stages of compilation, you can use
'-x' (or filename suffixes) to tell 'gcc' where to start, and one of
the options '-c', '-S', or '-E' to say where 'gcc' is to stop. Note
that some combinations (for example, '-x cpp-output -E' instruct 'gcc'
to do nothing at all.

'-c'
Compile or assemble the source files, but do not link. The
linking stage simply is not done. The ultimate output is in the
form of an object file for each source file.

By default, the object file name for a source file is made by
replacing the suffix '.c', '.i', '.s', etc., with '.o'.

Unrecognized input files, not requiring compilation or assembly,
are ignored.

'-S'
Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each
non-assembler input file specified.

By default, the assembler file name for a source file is made by
replacing the suffix '.c', '.i', etc., with '.s'.

Input files that don't require compilation are ignored.

'-E'
Stop after the preprocessing stage; do not run the compiler
proper. The output is in the form of preprocessed source code,
which is sent to the standard output.

Input files which don't require preprocessing are ignored.

'-o FILE'
Place output in file FILE. This applies regardless to whatever
sort of output is being produced, whether it be an executable
file, an object file, an assembler file or preprocessed C code.

Since only one output file can be specified, it does not make
sense to use '-o' when compiling more than one input file, unless
you are producing an executable file as output.

If '-o' is not specified, the default is to put an executable file
in 'a.out', the object file for 'SOURCE.SUFFIX' in 'SOURCE.o',
its assembler file in 'SOURCE.s', and all preprocessed C source
on standard output.

'-v'
Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.

'-pipe'
Use pipes rather than temporary files for communication between
the various stages of compilation. This fails to work on some
systems where the assembler is unable to read from a pipe; but
the GNU assembler has no trouble.


File: gcc.info, Node: Dialect Options, Next: Warning Options, Prev: Overall Options, Up: Invoking GCC

Options Controlling Dialect
===========================

The following options control the dialect of C that the compiler
accepts:

'-ansi'
Support all ANSI standard C programs.

This turns off certain features of GNU C that are incompatible
with ANSI C, such as the 'asm', 'inline' and 'typeof' keywords,
and predefined macros such as 'unix' and 'vax' that identify the
type of system you are using. It also enables the undesirable and
rarely used ANSI trigraph feature, and disallows '$' as part of
identifiers.

The alternate keywords '__asm__', '__extension__', '__inline__'
and '__typeof__' continue to work despite '-ansi'. You would not
want to use them in an ANSI C program, of course, but it useful
to put them in header files that might be included in
compilations done with '-ansi'. Alternate predefined macros such
as '__unix__' and '__vax__' are also available, with or without
'-ansi'.

The '-ansi' option does not cause non-ANSI programs to be
rejected gratuitously. For that, '-pedantic' is required in
addition to '-ansi'. *Note Warning Options::.

The macro '__STRICT_ANSI__' is predefined when the '-ansi' option
is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that
the ANSI standard doesn't call for; this is to avoid interfering
with any programs that might use these names for other things.

'-fno-asm'
Do not recognize 'asm', 'inline' or 'typeof' as a keyword. These
words may then be used as identifiers. You can use '__asm__',
'__inline__' and '__typeof__' instead. '-ansi' implies
'-fno-asm'.

'-fno-builtin'
Don't recognize non-ANSI built-in functions. '-ansi' also has
this effect. Currently, the only function affected is 'alloca'.

'-trigraphs'
Support ANSI C trigraphs. You don't want to know about this
brain-damage. The '-ansi' option implies '-trigraphs'.

'-traditional'
Attempt to support some aspects of traditional C compilers.
Specifically:

* All 'extern' declarations take effect globally even if they
are written inside of a function definition. This includes
implicit declarations of functions.

* The keywords 'typeof', 'inline', 'signed', 'const' and
'volatile' are not recognized. (You can still use the
alternative keywords such as '__typeof__', '__inline__', and
so on.)

* Comparisons between pointers and integers are always allowed.

* Integer types 'unsigned short' and 'unsigned char' promote
to 'unsigned int'.

* Out-of-range floating point literals are not an error.

* String "constants" are not necessarily constant; they are
stored in writable space, and identical looking constants
are allocated separately. (This is the same as the effect of
'-fwritable-strings'.)

* All automatic variables not declared 'register' are
preserved by 'longjmp'. Ordinarily, GNU C follows ANSI C:
automatic variables not declared 'volatile' may be clobbered.

* In the preprocessor, comments convert to nothing at all,
rather than to a space. This allows traditional token
concatenation.

* In the preprocessor, macro arguments are recognized within
string constants in a macro definition (and their values are
stringified, though without additional quote marks, when
they appear in such a context). The preprocessor always
considers a string constant to end at a newline.

* The predefined macro '__STDC__' is not defined when you use
'-traditional', but '__GNUC__' is (since the GNU extensions
which '__GNUC__' indicates are not affected by
'-traditional'). If you need to write header files that work
differently depending on whether '-traditional' is in use, by
testing both of these predefined macros you can distinguish
four situations: GNU C, traditional GNU C, other ANSI C
compilers, and other old C compilers.

'-traditional-cpp'
Attempt to support some aspects of traditional C preprocessors.
This includes the last three items in the table immediately above,
but none of the other effects of '-traditional'.

'-fcond-mismatch'
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.

'-funsigned-char'
Let the type 'char' be unsigned, like 'unsigned char'.

Each kind of machine has a default for what 'char' should be. It
is either like 'unsigned char' by default or like 'signed char'
by default.

Ideally, a portable program should always use 'signed char' or
'unsigned char' when it depends on the signedness of an object.
But many programs have been written to use plain 'char' and
expect it to be signed, or expect it to be unsigned, depending on
the machines they were written for. This option, and its
inverse, let you make such a program work with the opposite
default.

The type 'char' is always a distinct type from each of 'signed
char' or 'unsigned char', even though its behavior is always just
like one of those two.

'-fsigned-char'
Let the type 'char' be signed, like 'signed char'.

Note that this is equivalent to '-fno-unsigned-char', which is
the negative form of '-funsigned-char'. Likewise,
'-fno-signed-char' is equivalent to '-funsigned-char'.

'-fsigned-bitfields'
'-funsigned-bitfields'
'-fno-signed-bitfields'
'-fno-unsigned-bitfields'
These options control whether a bitfield is signed or unsigned,
when the declaration does not use either 'signed' or 'unsigned'.
By default, such a bitfield is signed, because this is
consistent: the basic integer types such as 'int' are signed
types.

However, when '-traditional' is used, bitfields are all unsigned
no matter what.

'-fwritable-strings'
Store string constants in the writable data segment and don't
uniquize them. This is for compatibility with old programs which
assume they can write into string constants. '-traditional' also
has this effect.

Writing into string constants is a very bad idea; "constants"
should be constant.

File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: Dialect Options, Up: Invoking GCC

Options to Request or Suppress Warnings
=======================================

Warnings are diagnostic messages that report constructions which
are not inherently erroneous but which are risky or suggest there may
have been an error.

You can request many specific warnings with options beginning '-W',
for example '-Wimplicit' to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning '-Wno-' to turn off warnings; for example, '-Wno-implicit'.
This manual lists only one of the two forms, whichever is not the
default.

These options control the amount and kinds of warnings produced by
GNU CC:

'-fsyntax-only'
Check the code for syntax errors, but don't emit any output.

'-w'
Inhibit all warning messages.

'-Wno_import'
Inhibit warning messages about the use of '#import'.

'-pedantic'
Issue all the warnings demanded by strict ANSI standard C; reject
all programs that use forbidden extensions.

Valid ANSI standard C programs should compile properly with or
without this option (though a rare few will require '-ansi').
However, without this option, certain GNU extensions and
traditional C features are supported as well. With this option,
they are rejected.

'-pedantic' does not cause warning messages for use of the
alternate keywords whose names begin and end with '__'. Pedantic
warnings are also disabled in the expression that follows
'__extension__'. However, only system header files should use
these escape routes; application programs should avoid them.
*Note Alternate Keywords::.

This option is not intended to be useful; it exists only to
satisfy pedants who would otherwise claim that GNU CC fails to
support the ANSI standard.

Some users try to use '-pedantic' to check programs for strict
ANSI C conformance. They soon find that it does not do quite
what they want: it finds some non-ANSI practices, but not
all--only those for which ANSI C *requires* a diagnostic.

A feature to report any failure to conform to ANSI C might be
useful in some instances, but would require considerable
additional work and would be quite different from '-pedantic'.
We recommend, rather, that users take advantage of the extensions
of GNU C and disregard the limitations of other compilers. Aside
from certain supercomputers and obsolete small machines, there is
less and less reason ever to use any other C compiler other than
for bootstrapping GNU CC.

'-pedantic-errors'
Like '-pedantic', except that errors are produced rather than
warnings.

'-W'
Print extra warning messages for these events:

* A nonvolatile automatic variable might be changed by a call
to 'longjmp'. These warnings as well are possible only in
optimizing compilation.

The compiler sees only the calls to 'setjmp'. It cannot know
where 'longjmp' will be called; in fact, a signal handler
could call it at any point in the code. As a result, you
may get a warning even when there is in fact no problem
because 'longjmp' cannot in fact be called at the place
which would cause a problem.

* A function can return either with or without a value.
(Falling off the end of the function body is considered
returning without a value.) For example, this function
would evoke such a warning:

foo (a)
{
if (a > 0)
return a;
}

* An expression-statement contains no side effects.

* An unsigned value is compared against zero with '>' or '<='.

'-Wimplicit'
Warn whenever a function or parameter is implicitly declared.

'-Wreturn-type'
Warn whenever a function is defined with a return-type that
defaults to 'int'. Also warn about any 'return' statement with no
return-value in a function whose return-type is not 'void'.

'-Wunused'
Warn whenever a local variable is unused aside from its
declaration, whenever a function is declared static but never
defined, and whenever a statement computes a result that is
explicitly not used.

'-Wswitch'
Warn whenever a 'switch' statement has an index of enumeral type
and lacks a 'case' for one or more of the named codes of that
enumeration. (The presence of a 'default' label prevents this
warning.) 'case' labels outside the enumeration range also
provoke warnings when this option is used.

'-Wcomment'
Warn whenever a comment-start sequence '/*' appears in a comment.

'-Wtrigraphs'
Warn if any trigraphs are encountered (assuming they are enabled).

'-Wformat'
Check calls to 'printf' and 'scanf', etc., to make sure that the
arguments supplied have types appropriate to the format string
specified.

'-Wchar-subscripts'
Warn if an array subscript has type 'char'. This is a common
cause of error, as programmers often forget that this type is

signed on some machines.

'-Wuninitialized'
An automatic variable is used without first being initialized.

These warnings are possible only in optimizing compilation,
because they require data flow information that is computed only
when optimizing. If you don't specify '-O', you simply won't get
these warnings.

These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable
that is declared 'volatile', or whose address is taken, or whose
size is other than 1, 2, 4 or 8 bytes. Also, they do not occur
for structures, unions or arrays, even when they are in registers.

Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.

These warnings are made optional because GNU CC is not smart
enough to see all the reasons why the code might be correct
despite appearing to have an error. Here is one example of how
this can happen:

{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}

If the value of 'y' is always 1, 2 or 3, then 'x' is always
initialized, but GNU CC doesn't know this. Here is another
common case:

{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}

This has no bug because 'save_y' is used only if it is set.

Some spurious warnings can be avoided if you declare as
'volatile' all the functions you use that never return. *Note
Function Attributes::.

'-Wparentheses'
Warn if parentheses are omitted in certain contexts.

'-Wall'
All of the above '-W' options combined. These are all the
options which pertain to usage that we recommend avoiding and
that we believe is easy to avoid, even in conjunction with macros.

The remaining '-W...' options are not implied by '-Wall' because
they warn about constructions that we consider reasonable to use, on
occasion, in clean programs.

'-Wtraditional'
Warn about certain constructs that behave differently in
traditional and ANSI C.

* Macro arguments occurring within string constants in the
macro body. These would substitute the argument in
traditional C, but are part of the constant in ANSI C.

* A function declared external in one block and then used
after the end of the block.

* A 'switch' statement has an operand of type 'long'.

'-Wshadow'
Warn whenever a local variable shadows another local variable.

'-Wid-clash-LEN'
Warn whenever two distinct identifiers match in the first LEN
characters. This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.

'-Wpointer-arith'

Warn about anything that depends on the "size of" a function type
or of 'void'. GNU C assigns these types a size of 1, for
convenience in calculations with 'void *' pointers and pointers
to functions.

'-Wcast-qual'
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a 'const char *' is
cast to an ordinary 'char *'.

'-Wcast-align'
Warn whenever a pointer is cast such that the required alignment
of the target is increased. For example, warn if a 'char *' is
cast to an 'int *' on machines where integers can only be
accessed at two- or four-byte boundaries.

'-Wwrite-strings'
Give string constants the type 'const char[LENGTH]' so that
copying the address of one into a non-'const' 'char *' pointer
will get a warning. These warnings will help you find at compile
time code that can try to write into a string constant, but only
if you have been very careful about using 'const' in declarations
and prototypes. Otherwise, it will just be a nuisance; this is
why we did not make '-Wall' request these warnings.

'-Wconversion'
Warn if a prototype causes a type conversion that is different
from what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness
of a fixed point argument except when the same as the default
promotion.

'-Waggregate-return'
Warn if any functions that return structures or unions are
defined or called. (In languages where you can return an array,
this also elicits a warning.)

'-Wstrict-prototypes'
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies
the argument types.)

'-Wmissing-prototypes'
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.

'-Wredundant-decls'
Warn if anything is declared more than once in the same scope,
even in cases where multiple declaration is valid and changes
nothing.

'-Wnested-externs'
Warn if an 'extern' declaration is encountered within an function.

'-Winline'
Warn if a function can not be inlined, and either it was declared
as inline, or else the '-finline-functions' option was given.

'-Werror'
Make all warnings into errors.


File: gcc.info, Node: Debugging Options, Next: Optimize Options, Prev: Warning Options, Up: Invoking GCC

Options for Debugging Your Program or GNU CC
============================================

GNU CC has various special options that are used for debugging
either your program or GCC:

'-g'
Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.

On most systems that use stabs format, '-g' enables use of extra
debugging information that only GDB can use; this extra
information makes debugging work better in GDB but will probably
make DBX crash or refuse to read the program. If you want to
control for certain whether to generate the extra information,
use '-gstabs+' or '-gstabs' (see below).

Unlike most other C compilers, GNU CC allows you to use '-g' with
'-O'. The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not
exist at all; flow of control may briefly move where you did not
expect it; some statements may not be executed because they
compute constant results or their values were already at hand;
some statements may execute in different places because they were
moved out of loops.

Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.

The following options are useful when GNU CC is generated with the
capability for more than one debugging format.

'-ggdb'
Produce debugging information in the native format (if that is
supported), including GDB extensions if at all possible.

'-gstabs'
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by
DBX on most BSD systems.

'-gstabs+'
Produce debugging information in stabs format (if that is
supported), using GDB extensions. The use of these extensions is
likely to make DBX crash or refuse to read the program.

'-gcoff'
Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on COFF systems.

'-gxcoff'
Produce debugging information in XCOFF format (if that is
supported). This is the format used on IBM RS/6000 systems.

'-gdwarf'
Produce debugging information in DWARF format (if that is
supported). This is the format used by SDB on systems that use
DWARF.

'-gLEVEL'
'-ggdbLEVEL'
'-gstabsLEVEL'
'-gcoffLEVEL'
'-gxcoffLEVEL'
'-gdwarfLEVEL'
Request debugging information and also use LEVEL to specify how
much information. The default level is 2.

Level 1 produces minimal information, enough for making
backtraces in parts of the program that you don't plan to debug.
This includes descriptions of functions and external variables,
but no information about local variables and no line numbers.

Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use '-g3'.

'-p'
Generate extra code to write profile information suitable for the
analysis program 'prof'.

'-pg'
Generate extra code to write profile information suitable for the
analysis program 'gprof'.

'-a'
Generate extra code to write profile information for basic blocks,
which will record the number of times each basic block is
executed. This data could be analyzed by a program like 'tcov'.
Note, however, that the format of the data is not what 'tcov'
expects. Eventually GNU 'gprof' should be extended to process
this data.

'-dLETTERS'
Says to make debugging dumps during compilation at times
specified by LETTERS. This is used for debugging the compiler.
The file names for most of the dumps are made by appending a word
to the source file name (e.g. 'foo.c.rtl' or 'foo.c.jump').
Here are the possible letters for use in LETTERS, and their
meanings:

'M'
Dump all macro definitions, at the end of preprocessing, and
write no output.

'N'
Dump all macro names, at the end of preprocessing.

'D'
Dump all macro definitions, at the end of preprocessing, in
addition to normal output.

'y'
Dump debugging information during parsing, to standard error.

'r'
Dump after RTL generation, to 'FILE.rtl'.

'x'
Just generate RTL for a function instead of compiling it.
Usually used with 'r'.

'j'
Dump after first jump optimization, to 'FILE.jump'.

's'
Dump after CSE (including the jump optimization that
sometimes follows CSE), to 'FILE.cse'.

'L'
Dump after loop optimization, to 'FILE.loop'.

't'
Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to 'FILE.cse2'.

'f'
Dump after flow analysis, to 'FILE.flow'.

'c'
Dump after instruction combination, to 'FILE.combine'.

'S'
Dump after the first instruction scheduling pass, to
'FILE.sched'.

'l'
Dump after local register allocation, to 'FILE.lreg'.

'g'
Dump after global register allocation, to 'FILE.greg'.

'R'
Dump after the second instruction scheduling pass, to
'FILE.sched2'.

'J'
Dump after last jump optimization, to 'FILE.jump2'.

'd'
Dump after delayed branch scheduling, to 'FILE.dbr'.

'k'
Dump after conversion from registers to stack, to
'FILE.stack'.

'a'
Produce all the dumps listed above.

'm'
Print statistics on memory usage, at the end of the run, to
standard error.

'p'
Annotate the assembler output with a comment indicating which
pattern and alternative was used.

'-fpretend-float'
When running a cross-compiler, pretend that the target machine
uses the same floating point format as the host machine. This
causes incorrect output of the actual floating constants, but the
actual instruction sequence will probably be the same as GNU CC
would make when running on the target machine.

'-save-temps'
Store the usual "temporary" intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling 'foo.c' with '-c -save-temps' would
produce files 'foo.i' and 'foo.s', as well as 'foo.o'.


File: gcc.info, Node: Optimize Options, Next: Preprocessor Options, Prev: Debugging Options, Up: Invoking GCC

Options That Control Optimization
=================================

These options control various sorts of optimizations:

'-O'
Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.

Without '-O', the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to
any variable or change the program counter to any other statement
in the function and get exactly the results you would expect from
the source code.

Without '-O', only variables declared 'register' are allocated in
registers. The resulting compiled code is a little worse than
produced by PCC without '-O'.

With '-O', the compiler tries to reduce code size and execution
time.

When '-O' is specified, '-fthread-jumps' and '-fdelayed-branch'
are turned on. On some machines other flags may also be turned
on.

'-O2'
Optimize even more. Nearly all supported optimizations that do
not involve a space-speed tradeoff are performed. As compared to
'-O', this option increases both compilation time and the
performance of the generated code.

'-O2' turns on all '-fFLAG' options that enable more
optimization, except for '-funroll-loops', '-funroll-all-loops'
and '-fomit-frame-pointer'.

Options of the form '-fFLAG' specify machine-independent flags.
Most flags have both positive and negative forms; the negative form of
'-ffoo' would be '-fno-foo'. In the table below, only one of the
forms is listed--the one which is not the default. You can figure out
the other form by either removing 'no-' or adding it.

'-ffloat-store'
Do not store floating point variables in registers. This
prevents undesirable excess precision on machines such as the
68000 where the floating registers (of the 68881) keep more
precision than a 'double' is supposed to have.

For most programs, the excess precision does only good, but a few
programs rely on the precise definition of IEEE floating point.
Use '-ffloat-store' for such programs.

'-fno-defer-pop'
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.

'-fforce-mem'
Force memory operands to be copied into registers before doing
arithmetic on them. This may produce better code by making all
memory references potential common subexpressions. When they are
not common subexpressions, instruction combination should
eliminate the separate register-load. I am interested in hearing
about the difference this makes.

'-fforce-addr'
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
'-fforce-mem' may. I am interested in hearing about the
difference this makes.

'-fomit-frame-pointer'
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. *It also makes debugging impossible on some
machines.*

On some machines, such as the Vax, this flag has no effect,
because the standard calling sequence automatically handles the
frame pointer and nothing is saved by pretending it doesn't
exist. The machine-description macro 'FRAME_POINTER_REQUIRED'
controls whether a target machine supports this flag. *Note
Registers::.

'-finline'
Pay attention to the 'inline' keyword. Normally the negation of
this option '-fno-inline' is used to keep the compiler from
expanding any functions inline. However, the opposite effect may
be desirable when compiling without optimization, since inline
expansion is turned off in that case.

'-finline-functions'
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be
worth integrating in this way.

If all calls to a given function are integrated, and the function
is declared 'static', then the function is normally not output as
assembler code in its own right.

'-fcaller-saves'
Enable values to be allocated in registers that will be clobbered
by function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is done
only when it seems to result in better code than would otherwise
be produced.

This option is enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.

'-fkeep-inline-functions'
Even if all calls to a given function are integrated, and the
function is declared 'static', nevertheless output a separate
run-time callable version of the function.

'-fno-function-cse'
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.

This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the
optimizations performed when this option is not used.

The following options control specific optimizations. The '-O2'
option turns on all of these optimizations except '-funroll-loops' and
'-funroll-all-loops'. The '-O' option usually turns on the
'-fthread-jumps' and '-fdelayed-branch' options, but specific machines
may change the default optimizations.

You can use the following flags in the rare cases when "fine-tuning"
of optimizations to be performed is desired.

'-fstrength-reduce'
Perform the optimizations of loop strength reduction and
elimination of iteration variables.

'-fthread-jumps'
Perform optimizations where we check to see if a jump branches to
a location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the
destination of the second branch or a point immediately following
it, depending on whether the condition is known to be true or
false.

'-fcse-follow-jumps'
In common subexpression elimination, scan through jump
instructions in certain cases. This is not as powerful as
completely global CSE, but not as slow either.

'-frerun-cse-after-loop'
Re-run common subexpression elimination after loop optimizations
has been performed.

'-fexpensive-optimizations'
Perform a number of minor optimizations that are relatively
expensive.

'-fdelayed-branch'
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.

'-fschedule-insns'
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions
to be issued until the result of the load or floating point
instruction is required.

'-fschedule-insns2'
Similar to '-fschedule-insns', but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.

'-funroll-loops'
Perform the optimization of loop unrolling. This is only done
for loops whose number of iterations can be determined at compile
time or run time. '-funroll-loop' implies '-fstrength-reduce' and
'-frerun-cse-after-loop'.

'-funroll-all-loops'
Perform the optimization of loop unrolling. This is done for all
loops and usually makes programs run more slowly.
'-funroll-all-loops' implies '-fstrength-reduce' and
'-frerun-cse-after-loop'.

'-fno-peephole'
Disable any machine-specific peephole optimizations.


File: gcc.info, Node: Preprocessor Options, Next: Link Options, Prev: Optimize Options, Up: Invoking GCC

Options Controlling the Preprocessor
====================================

These options control the C preprocessor, which is run on each C
source file before actual compilation.

If you use the '-E' option, nothing is done except preprocessing.
Some of these options make sense only together with '-E' because they
cause the preprocessor output to be unsuitable for actual compilation.

'-include FILE'
Process FILE as input before processing the regular input file.
In effect, the contents of FILE are compiled first. Any '-D' and
'-U' options on the command line are always processed before
'-include FILE', regardless of the order in which they are
written. All the '-include' and '-imacros' options are processed
in the order in which they are written.

'-imacros FILE'
Process FILE as input, discarding the resulting output, before
processing the regular input file. Because the output generated
from FILE is discarded, the only effect of '-imacros FILE' is to
make the macros defined in FILE available for use in the main
input.

Any '-D' and '-U' options on the command line are always
processed before '-imacros FILE', regardless of the order in
which they are written. All the '-include' and '-imacros'
options are processed in the order in which they are written.

'-nostdinc'
Do not search the standard system directories for header files.
Only the directories you have specified with '-I' options (and the
current directory, if appropriate) are searched. *Note Directory
Options::, for information on '-I'.

By using both '-nostdinc' and '-I-', you can limit the
include-file search path to only those directories you specify
explicitly.

'-undef'
Do not predefine any nonstandard macros. (Including architecture
flags).

'-E'
Run only the C preprocessor. Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.

'-C'
Tell the preprocessor not to discard comments. Used with the
'-E' option.

'-P'
Tell the preprocessor not to generate '#line' commands. Used
with the '-E' option.

'-M'
Tell the preprocessor to output a rule suitable for 'make'
describing the dependencies of each object file. For each source
file, the preprocessor outputs one 'make'-rule whose target is
the object file name for that source file and whose dependencies
are all the files '#include'd in it. This rule may be a single
line or may be continued with '\'-newline if it is long. The
list of rules is printed on standard output instead of the
preprocessed C program.

'-M' implies '-E'.

Another way to specify output of a 'make' rule is by setting the
environment variable 'DEPENDENCIES_OUTPUT' (*note Environment
Variables::.).

'-MM'
Like '-M' but the output mentions only the user header files
included with '#include "FILE"'. System header files included
with '#include ' are omitted.

'-MD'
Like '-M' but the dependency information is written to files with
names made by replacing '.c' with '.d' at the end of the input
file names. This is in addition to compiling the file as
specified--'-MD' does not inhibit ordinary compilation the way
'-M' does.

The Mach utility 'md' can be used to merge the '.d' files into a
single dependency file suitable for using with the 'make' command.

'-MMD'
Like '-MD' except mention only user header files, not system
header files.

'-H'
Print the name of each header file used, in addition to other
normal activities.

'-DMACRO'
Define macro MACRO with the string '1' as its definition.

'-DMACRO=DEFN'
Define macro MACRO as DEFN. All instances of '-D' on the command
line are processed before any '-U' options.

'-UMACRO'
Undefine macro MACRO. '-U' options are evaluated after all '-D'
options, but before any '-include' and '-imacros' options.

'-dM'
Tell the preprocessor to output only a list of the macro
definitions that are in effect at the end of preprocessing. Used
with the '-E' option.

'-dD'
Tell the preprocessing to pass all macro definitions into the
output, in their proper sequence in the rest of the output.

'-dN'
Like '-dD' except that the macro arguments and contents are
omitted. Only '#define NAME' is included in the output.

'-trigraphs'
Support ANSI C trigraphs. You don't want to know about this
brain-damage. The '-ansi' option also has this effect.


File: gcc.info, Node: Link Options, Next: Directory Options, Prev: Preprocessor Options, Up: Invoking GCC

Options for Linking
===================

These options come into play when the compiler links object files
into an executable output file. They are meaningless if the compiler
is not doing a link step.

'OBJECT-FILE-NAME'
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as
input to the linker.

'-c'
'-S'
'-E'
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments. *Note Overall
Options::.

'-lLIBRARY'
Search the library named LIBRARY when linking.

It makes a difference where in the command you write this option;
the linker searches processes libraries and object files in the
order they are specified. Thus, 'foo.o -lz bar.o' searches
library 'z' after file 'foo.o' but before 'bar.o'. If 'bar.o'
refers to functions in 'z', those functions may not be loaded.

The linker searches a standard list of directories for the
library, which is actually a file named 'libLIBRARY.a'. The
linker then uses this file as if it had been specified precisely

by name.

The directories searched include several standard system
directories plus any that you specify with '-L'.

Normally the files found this way are library files--archive files
whose members are object files. The linker handles an archive
file by scanning through it for members which define symbols that
have so far been referenced but not defined. But if the file
that is found is an ordinary object file, it is linked in the
usual fashion. The only difference between using an '-l' option
and specifying a file name is that '-l' surrounds LIBRARY with
'lib' and '.a' and searches several directories.

'-nostdlib'
Don't use the standard system libraries and startup files when
linking. Only the files you specify will be passed to the linker.

'-static'
On systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has no
effect.

'-shared'
Produce a shared object which can then be linked with other
objects to form an executable. Only a few systems support this
option.

'-symbolic'
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the
link editor option '-Xlinker -z -Xlinker defs'). Only a few
systems support this option.

'-Xlinker OPTION'
Pass OPTION as an option to the linker. You can use this to
supply system-specific linker options which GNU CC does not know
how to recognize.

If you want to pass an option that takes an argument, you must use
'-Xlinker' twice, once for the option and once for the argument.
For example, to pass '-assert definitions', you must write
'-Xlinker -assert -Xlinker definitions'. It does not work to
write '-Xlinker "-assert definitions"', because this passes the
entire string as a single argument, which is not what the linker
expects.



File: gcc.info, Node: Directory Options, Next: Target Options, Prev: Link Options, Up: Invoking GCC

Options for Directory Search
============================

These options specify directories to search for header files, for
libraries and for parts of the compiler:

'-IDIR'
Append directory DIR to the list of directories searched for
include files.

'-I-'
Any directories you specify with '-I' options before the '-I-'
option are searched only for the case of '#include "FILE"'; they
are not searched for '#include '.

If additional directories are specified with '-I' options after
the '-I-', these directories are searched for all '#include'
directives. (Ordinarily *all* '-I' directories are used this
way.)

In addition, the '-I-' option inhibits the use of the current
directory (where the current input file came from) as the first
search directory for '#include "FILE"'. There is no way to
override this effect of '-I-'. With '-I.' you can specify
searching the directory which was current when the compiler was
invoked. That is not exactly the same as what the preprocessor
does by default, but it is often satisfactory.

'-I-' does not inhibit the use of the standard system directories
for header files. Thus, '-I-' and '-nostdinc' are independent.

'-LDIR'
Add directory DIR to the list of directories to be searched for
'-l'.

'-BPREFIX'
This option specifies where to find the executables, libraries and
data files of the compiler itself.

The compiler driver program runs one or more of the subprograms
'cpp', 'cc1', 'as' and 'ld'. It tries PREFIX as a prefix for
each program it tries to run, both with and without
'MACHINE/VERSION/' (*note Target Options::.).

For each subprogram to be run, the compiler driver first tries the
'-B' prefix, if any. If that name is not found, or if '-B' was
not specified, the driver tries two standard prefixes, which are
'/usr/lib/gcc/' and '/usr/local/lib/gcc-lib/'. If neither of
those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your
'PATH' environment variable.

'-B' prefixes that effectively specify directory names also apply
to libraries in the linker, because the compiler translates these
options into '-L' options for the linker.

The run-time support file 'libgcc.a' can also be searched for
using the '-B' prefix, if needed. If it is not found there, the
two standard prefixes above are tried, and that is all. The file
is left out of the link if it is not found by those means.

Another way to specify a prefix much like the '-B' prefix is to
use the environment variable 'GCC_EXEC_PREFIX'. *Note
Environment Variables::.


File: gcc.info, Node: Target Options, Next: Submodel Options, Prev: Directory Options, Up: Invoking GCC

Specifying Target Machine and Compiler Version
==============================================

By default, GNU CC compiles code for the same type of machine that
you are using. However, it can also be installed as a cross-compiler,
to compile for some other type of machine. In fact, several different
configurations of GNU CC, for different target machines, can be
installed side by side. Then you specify which one to use with the
'-b' option.

In addition, older and newer versions of GNU CC can be installed
side by side. One of them (probably the newest) will be the default,
but you may sometimes wish to use another.

'-b MACHINE'
The argument MACHINE specifies the target machine for compilation.
This is useful when you have installed GNU CC as a cross-compiler.

The value to use for MACHINE is the same as was specified as the
machine type when configuring GNU CC as a cross-compiler. For
example, if a cross-compiler was configured with 'configure

i386v', meaning to compile for an 80386 running System V, then you
would specify '-b i386v' to run that cross compiler.

When you do not specify '-b', it normally means to compile for
the same type of machine that you are using.

'-V VERSION'
The argument VERSION specifies which version of GNU CC to run.
This is useful when multiple versions are installed. For example,
VERSION might be '2.0', meaning to run GNU CC version 2.0.

The default version, when you do not specify '-V', is controlled
by the way GNU CC is installed. Normally, it will be a version
that is recommended for general use.

The '-b' and '-V' options actually work by controlling part of the
file name used for the executable files and libraries used for
compilation. A given version of GNU CC, for a given target machine, is
normally kept in the directory
'/usr/local/lib/gcc-lib/MACHINE/VERSION'.

It follows that sites can customize the effect of '-b' or '-V'
either by changing the names of these directories or adding alternate
names (or symbolic links). Thus, if '/usr/local/lib/gcc-lib/80386' is
a link to '/usr/local/lib/gcc-lib/i386v', then '-b 80386' will be an
alias for '-b i386v'.

In one respect, the '-b' or '-V' do not completely change to a
different compiler: the top-level driver program 'gcc' that you
originally invoked continues to run and invoke the other executables
(preprocessor, compiler per se, assembler and linker) that do the real
work. However, since no real work is done in the driver program, it
usually does not matter that the driver program in use is not the one
for the specified target and version.

The only way that the driver program depends on the target machine
is in the parsing and handling of special machine-specific options.
However, this is controlled by a file which is found, along with the
other executables, in the directory for the specified version and
target machine. As a result, a single installed driver program adapts
to any specified target machine and compiler version.

The driver program executable does control one significant thing,
however: the default version and target machine. Therefore, you can
install different instances of the driver program, compiled for
different targets or versions, under different names.

For example, if the driver for version 2.0 is installed as 'ogcc'
and that for version 2.1 is installed as 'gcc', then the command 'gcc'
will use version 2.1 by default, while 'ogcc' will use 2.0 by default.
However, you can choose either version with either command with the
'-V' option.


File: gcc.info, Node: Submodel Options, Next: Code Gen Options, Prev: Target Options, Up: Invoking GCC

Specifying Hardware Models and Configurations
=============================================

Earlier we discussed the standard option '-b' which chooses among
different installed compilers for completely different target
machines, such as Vax vs. 68000 vs. 80386.

In addition, each of these target machine types can have its own
special options, starting with '-m', to choose among various hardware
models or configurations--for example, 68010 vs 68020, floating
coprocessor or none. A single installed version of the compiler can
compile for any model or configuration, according to the options
specified.

These options are defined by the macro 'TARGET_SWITCHES' in the
machine description. The default for the options is also defined by
that macro, which enables you to change the defaults.

* Menu:

* M680x0 Options::
* VAX Options::
* SPARC Options::
* Convex Options::
* AMD29K Options::
* M88K Options::
* RS/6000 Options::
* RT Options::
* MIPS Options::
* i386 Options::


File: gcc.info, Node: M680x0 Options, Next: Vax Options, Prev: Submodel Options, Up: Submodel Options

M680x0 Options
--------------

These are the '-m' options defined for the 68000 series. The
default values for these options depends on which style of 68000 was

selected when the compiler was configured; the defaults for the most
common choices are given below.

'-m68020'
'-mc68020'
Generate output for a 68020 (rather than a 68000). This is the
default when the compiler is configured for 68020-based systems.

'-m68000'
'-mc68000'
Generate output for a 68000 (rather than a 68020). This is the
default when the compiler is configured for a 68000-based systems.

'-m68881'
Generate output containing 68881 instructions for floating point.
This is the default for most 68020 systems unless '-nfp' was
specified when the compiler was configured.

'-mfpa'
Generate output containing Sun FPA instructions for floating
point.

'-msoft-float'
Generate output containing library calls for floating point.
*Warning:* the requisite libraries are not part of GNU CC.
Normally the facilities of the machine's usual C compiler are
used, but this can't be done directly in cross-compilation. You
must make your own arrangements to provide suitable library
functions for cross-compilation.

'-mshort'
Consider type 'int' to be 16 bits wide, like 'short int'.

'-mnobitfield'
Do not use the bit-field instructions. '-m68000' implies
'-mnobitfield'.

'-mbitfield'
Do use the bit-field instructions. '-m68020' implies
'-mbitfield'. This is the default if you use the unmodified
sources configured for a 68020.

'-mrtd'
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the 'rtd'
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.

This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including 'printf');
otherwise incorrect code will be generated for calls to those
functions.

In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)

The 'rtd' instruction is supported by the 68010 and 68020
processors, but not by the 68000.


File: gcc.info, Node: VAX Options, Next: Sparc Options, Prev: M680x0 Options, Up: Submodel Options

VAX Options
-----------

These '-m' options are defined for the Vax:

'-munix'
Do not output certain jump instructions ('aobleq' and so on) that
the Unix assembler for the Vax cannot handle across long ranges.

'-mgnu'
Do output those jump instructions, on the assumption that you
will assemble with the GNU assembler.

'-mg'
Output code for g-format floating point numbers instead of
d-format.


File: gcc.info, Node: Sparc Options, Next: Convex Options, Prev: Vax Options, Up: Submodel Options

SPARC Options
-------------

These '-m' switches are supported on the Sparc:

'-mforce-align'
Make sure all objects of type 'double' are 8-byte aligned in
memory and use double-word instructions to reference them.

'-mno-epilogue'
Generate separate return instructions for 'return' statements.
This has both advantages and disadvantages; I don't recall what
they are.


File: gcc.info, Node: Convex Options, Next: AMD29K Options, Prev: SPARC Options, Up: Submodel Options

Convex Options
--------------

These '-m' options are defined for the Convex:

'-mc1'
Generate output for a C1. This is the default when the compiler
is configured for a C1.

'-mc2'
Generate output for a C2. This is the default when the compiler
is configured for a C2.

'-margcount'
Generate code which puts an argument count in the word preceding
each argument list. Some nonportable Convex and Vax programs
need this word. (Debuggers don't, except for functions with
variable-length argument lists; this info is in the symbol table.)

'-mnoargcount'
Omit the argument count word. This is the default if you use the
unmodified sources.


File: gcc.info, Node: AMD29K Options, Next: M88K Options, Prev: Convex Options, Up: Submodel Options

AMD29K Options
--------------

These '-m' options are defined for the AMD Am29000:

'-mdw'
Generate code that assumes the 'DW' bit is set, i.e., that byte
and halfword operations are directly supported by the hardware.
This is the default.

'-mnodw'
Generate code that assumes the 'DW' bit is not set.

'-mbw'
Generate code that assumes the system supports byte and halfword
write operations. This is the default.

'-mnbw'
Generate code that assumes the systems does not support byte and
halfword write operations. '-mnbw' implies '-mnodw'.

'-msmall'
Use a small memory model that assumes that all function addresses
are either within a single 256 KB segment or at an absolute
address of less than 256K. This allows the 'call' instruction to
be used instead of a 'const', 'consth', 'calli' sequence.

'-mlarge'
Do not assume that the 'call' instruction can be used; this is the
default.

'-m29050'
Generate code for the Am29050.

'-m29000'
Generate code for the Am29000. This is the default.

'-mkernel-registers'
Generate references to registers 'gr64-gr95' instead of
'gr96-gr127'. This option can be used when compiling kernel code
that wants a set of global registers disjoint from that used by
user-mode code.

Note that when this option is used, register names in '-f' flags
must use the normal, user-mode, names.

'-muser-registers'
Use the normal set of global registers, 'gr96-gr127'. This is the
default.

'-mstack-check'
Insert a call to '__msp_check' after each stack adjustment. This
is often used for kernel code.


File: gcc.info, Node: M88K Options, Next: RS/6000 Options, Prev: AMD29K Options, Up: Submodel Options

M88K Options
------------

These '-m' options are defined for Motorola 88K architectures:

'-m88000'
Generate code that works well on both the m88100 and the m88110.

'-m88100'
Generate code tha Generate code that works best for the m88100,
but that also runs on the m88110.

'-m88110'
Generate code that works best for the m88110, and may not run on
the m88100.

'-midentify-revision'
Include an 'ident' directive in the assembler output recording the
source file name, compiler name and version, timestamp, and
compilation flags used.

'-mno-underscores'
In assembler output, emit symbol names without adding an
underscore character at the beginning of each name. The default
is to use an underscore as prefix on each name.

'-mocs-debug-info'
'-mno-ocs-debug-info'
Include (or omit) additional debugging information (about
registers used in each stack frame) as specified in the 88open
Object Compatibility Standard, "OCS". This extra information
allows debugging of code that has had the frame pointer
eliminated. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is
to include this information; other 88k configurations omit this
information by default.

'-mocs-frame-position'
When emitting COFF debugging information for automatic variables
and parameters stored on the stack, use the offset from the
canonical frame address, which is the stack pointer (register 31)
on entry to the function. The DG/UX, SVr4, Delta88 SVr3.2, and
BCS configurations use '-mocs-frame-position'; other 88k
configurations have the default '-mno-ocs-frame-position'.

'-mno-ocs-frame-position'
When emitting COFF debugging information for automatic variables
and parameters stored on the stack, use the offset from the frame
pointer register (register 30). When this option is in effect,
the frame pointer is not eliminated when debugging information is
selected by the -g switch.

'-moptimize-arg-area'
'-mno-optimize-arg-area'
Control how to store function arguments in stack frames.
'-moptimize-arg-area' saves space, but was ruled illegal by
88open. '-mno-optimize-arg-area' conforms to the 88open
standards. By default GNU CC does not optimize the argument area.

'-mshort-data-NUM'
Generate smaller data references by making them relative to 'r0',
which allows loading a value using a single instruction (rather
than the usual two). You control which data references are
affected by specifying NUM with this option. For example, if you
specify '-mshort-data-512', then the data references affected are
those involving displacements of less than 512 bytes.
'-mshort-data-NUM' is not effective for NUM greater than 64K.

'-msvr4'
'-msvr3'
Turn on ('-msvr4') or off ('-msvr3') compiler extensions related
to System V release 4 (SVr4). This controls the following:

1. Which variant of the assembler syntax to emit (which you can
select independently using '-mversion-03.00').

2. '-msvr4' makes the C preprocessor recognize '#pragma weak'
that is used on System V release 4.

3. '-msvr4' makes GNU CC issue additional declaration
directives used in SVr4.

'-msvr3' is the default for all m88K configurations except the
SVr4 configuration.

'-mversion-03.00'
In the DG/UX configuration, there are two flavors of SVr4. This
option modifies '-msvr4' to select whether the hybrid-COFF or
real-ELF flavor is used. All other configurations ignore this
option.

'-mno-check-zero-division'
'-mcheck-zero-division'
Early models of the 88K architecture had problems with division
by zero; in particular, many of them didn't trap. Use these
options to avoid including (or to include explicitly) additional
code to detect division by zero and signal an exception. All GNU
CC configurations for the 88K use '-mcheck-zero-division' by
default.

'-muse-div-instruction'
Do not emit code to check both the divisor and dividend when doing
signed integer division to see if either is negative, and adjust
the signs so the divide is done using non-negative numbers.
Instead, rely on the operating system to calculate the correct
value when the 'div' instruction traps. This results in
different behavior when the most negative number is divided by
-1, but is useful when most or all signed integer divisions are
done with positive numbers.

'-mtrap-large-shift'
'-mhandle-large-shift'
Include code to detect bit-shifts of more than 31 bits;
respectively, trap such shifts or emit code to handle them
properly. By default GNU CC makes no special provision for large
bit shifts.

'-mwarn-passed-structs'
Warn when a function passes a struct as an argument or result.
Structure-passing conventions have changed during the evolution
of the C language, and are often the source of portability
problems. By default, GNU CC issues no such warning.


File: gcc.info, Node: RS/6000 Options, Next: RT Options, Prev: M88K Options, Up: Submodel Options

IBM RS/6000 Options
-------------------

Only one pair of '-m' options is defined for the IBM RS/6000:

'-mfp-in-toc'
'-mno-fp-in-toc'
Control whether or not floating-point constants go in the Table of
Contents (TOC), a table of all global variable and function
addresses. By default GNU CC puts floating-point constants
there; if the TOC overflows, '-mno-fp-in-toc' will reduce the
size of the TOC, which may avoid the overflow.


File: gcc.info, Node: RT Options, Next: MIPS Options, Prev: RS/6000 Options, Up: Submodel Options

IBM RT Options
--------------

These '-m' options are defined for the IBM RT PC:

'-min-line-mul'
Use an in-line code sequence for integer multiplies. This is the
default.

'-mcall-lib-mul'
Call 'lmul$$' for integer multiples.

'-mfull-fp-blocks'
Generate full-size floating point data blocks, including the
minimum amount of scratch space recommended by IBM. This is the
default.

'-mminimum-fp-blocks'
Do not include extra scratch space in floating point data blocks.
This results in smaller code, but slower execution, since
scratch space must be allocated dynamically.

'-mfp-arg-in-fpregs'
Use a calling sequence incompatible with the IBM calling
convention in which floating point arguments are passed in
floating point registers. Note that 'varargs.h' and 'stdargs.h'
will not work with floating point operands if this option is
specified.

'-mfp-arg-in-gregs'
Use the normal calling convention for floating point arguments.
This is the default.

'-mhc-struct-return'
Return structures of more than one word in memory, rather than in
a register. This provides compatibility with the MetaWare HighC
(hc) compiler. Use '-fpcc-struct-return' for compatibility with
the Portable C Compiler (pcc).

'-mnohc-struct-return'
Return some structures of more than one word in registers, when
convenient. This is the default. For compatibility with the
IBM-supplied compilers, use either '-fpcc-struct-return' or
'-mhc-struct-return'.


File: gcc.info, Node: MIPS Options, Next: i386 Options, Prev: RT Options, Up: Submodel Options

MIPS Options
------------

These '-m' options are defined for the MIPS family of computers:

'-mcpu=CPU TYPE'
Assume the defaults for the machine type CPU TYPE when scheduling
instructions. The default CPU TYPE is 'default', which picks the
longest cycles times for any of the machines, in order that the
code run at reasonable rates on all MIPS cpu's. Other choices
for CPU TYPE are 'r2000', 'r3000', 'r4000', and 'r6000'. While
picking a specific CPU TYPE will schedule things appropriately
for that particular chip, the compiler will not generate any code
that does not meet level 1 of the MIPS ISA (instruction set
architecture) without the '-mips2' or '-mips3' switches being
used.

'-mips2'
Issue instructions from level 2 of the MIPS ISA (branch likely,
square root instructions). The '-mcpu=r4000' or '-mcpu=r6000'
switch must be used in conjuction with '-mips2'.

'-mips3'
Issue instructions from level 3 of the MIPS ISA (64 bit
instructions). You must use the '-mcpu=r4000' switch along with
'-mips3'.

'-mint64'
'-mlong64'
'-mlonglong128'
These options don't work at present.

'-mmips-as'
Generate code for the MIPS assembler, and invoke 'mips-tfile' to
add normal debug information. This is the default for all
platforms except for the OSF/1 reference platform, using the
OSF/rose object format. If the either of the '-gstabs' or
'-gstabs+' switches are used, the 'mips-tfile' program will
encapsulate the stabs within MIPS ECOFF.

'-mgas'
Generate code for the GNU assembler. This is the default on the
OSF/1 reference platform, using the OSF/rose object format.

'-mrnames'
'-mno-rnames'
The '-mrnames' switch says to output code using the MIPS software
names for the registers, instead of the hardware names (ie, A0
instead of $4). The GNU assembler does not support the
'-mrnames' switch, and the MIPS assembler will be instructed to
run the MIPS C preprocessor over the source file. The
'-mno-rnames' switch is default.

'-mgpopt'
'-mno-gpopt'
The '-mgpopt' switch says to write all of the data declarations
before the instructions in the text section, this allows the MIPS
assembler to generate one word memory references instead of using
two words for short global or static data items. This is on by
default if optimization is selected.

'-mstats'
'-mno-stats'
For each non-inline function processed, the '-mstats' switch
causes the compiler to emit one line to the standard error file to
print statistics about the program (number of registers saved,
stack size, etc.).

'-mmemcpy'
'-mno-memcpy'
The '-mmemcpy' switch makes all block moves call the appropriate
string function ('memcpy' or 'bcopy') instead of possibly
generating inline code.

'-mmips-tfile'
'-mno-mips-tfile'
The '-mno-mips-tfile' switch causes the compiler not postprocess
the object file with the 'mips-tfile' program, after the MIPS
assembler has generated it to add debug support. If 'mips-tfile'
is not run, then no local variables will be available to the
debugger. In addition, 'stage2' and 'stage3' objects will have
the temporary file names passed to the assembler embedded in the
object file, which means the objects will not compare the same.
The '-mno-mips-tfile' switch should only be used when there are
bugs in the 'mips-tfile' program that prevents compilation.

'-msoft-float'
Generate output containing library calls for floating point.
*Warning:* the requisite libraries are not part of GNU CC.
Normally the facilities of the machine's usual C compiler are
used, but this can't be done directly in cross-compilation. You
must make your own arrangements to provide suitable library
functions for cross-compilation.

'-mhard-float'
Generate output containing floating point instructions. This is
the default if you use the unmodified sources.

'-mfp64'
Assume that the FR bit in the status word is on, and that there
are 32 64-bit floating point registers, instead of 32 32-bit
floating point registers. You must also specify the
'-mcpu=r4000' and '-mips3' switches.

'-mfp32'
Assume that there are 32 32-bit floating point registers. This
is the default.

'-mabicalls'
'-mno-abicalls'
Emit the '.abicalls', '.cpload', and '.cprestore' pseudo
operations that some System V.4 ports use for position
independent code.

'-mhalf-pic'
'-mno-half-pic'
Put pointers to extern references into the data section and load
them up, rather than put the references in the text section.
These options do not work at present.

'-G NUM'
Put global and static items less than or equal to NUM bytes into
the small data or bss sections instead of the normal data or bss
section. This allows the assembler to emit one word memory
reference instructions based on the global pointer (GP or $28),
instead of the normal two words used. By default, NUM is 8 when
the MIPS assembler is used, and 0 when the GNU assembler is used.
The '-G NUM' switch is also passed to the assembler and linker.
All modules should be compiled with the same '-G NUM' value.

'-nocpp'
Tell the MIPS assembler to not run it's preprocessor over user
assembler files (with a '.s' suffix) when assembling them.

These options are defined by the macro 'TARGET_SWITCHES' in the
machine description. The default for the options is also defined by
that macro, which enables you to change the defaults.


File: gcc.info, Node: i386 Options, Prev: MIPS Options, Up: Submodel Options

Intel 386 Options
-----------------

These '-m' options are defined for the i386 family of computers:

'-m486'
'-mno486'
Control whether or not code is optimized for a 486 instead of an
386. Code generated for an 486 will run on a 386 and vice versa.

'-msoft-float'
Generate output containing library calls for floating point.
*Warning:* the requisite libraries are not part of GNU CC.
Normally the facilities of the machine's usual C compiler are
used, but this can't be done directly in cross-compilation. You
must make your own arrangements to provide suitable library
functions for cross-compilation.

On machines where a function returnings float point results in
the 80387 register stack, some floating point opcodes may be
emitted even if '-msoft-float' is used.


File: gcc.info, Node: Code Gen Options, Next: Environment Variables, Prev: Submodel Options, Up: Invoking GCC

Options for Code Generation Conventions
=======================================

These machine-independent options control the interface conventions
used in code generation.

Most of them have both positive and negative forms; the negative
form of '-ffoo' would be '-fno-foo'. In the table below, only one of
the forms is listed--the one which is not the default. You can figure
out the other form by either removing 'no-' or adding it.

'-fpcc-struct-return'
Use the same convention for returning 'struct' and 'union' values
that is used by the usual C compiler on your system. This
convention is less efficient for small structures, and on many
machines it fails to be reentrant; but it has the advantage of
allowing intercallability between GNU CC-compiled code and
PCC-compiled code.

'-fshort-enums'
Allocate to an 'enum' type only as many bytes as it needs for the
declared range of possible values. Specifically, the 'enum' type
will be equivalent to the smallest integer type which has enough
room.

'-fshort-double'
Use the same size for 'double' as for 'float'.

'-fshared-data'
Requests that the data and non-'const' variables of this
compilation be shared data rather than private data. The
distinction makes sense only on certain operating systems, where
shared data is shared between processes running the same program,
while private data exists in one copy per process.

'-fno-common'
Allocate even uninitialized global variables in the bss section
of the object file, rather than generating them as common blocks.
This has the effect that if the same variable is declared
(without 'extern') in two different compilations, you will get an
error when you link them. The only reason this might be useful
is if you wish to verify that the program will work on other
systems which always work this way.

'-fno-ident'
Ignore the '#ident' directive.

'-fno-gnu-linker'
Don't output global initializations such as C++ constructors and
destructors in the form used by the GNU linker (on systems where
the GNU linker is the standard method of handling them). Use
this option when you want to use a "collect" program and a
non-GNU linker.

'-finhibit-size-directive'
Don't output a '.size' assembler directive, or anything else that
would cause trouble if the function is split in the middle, and
the two halves are placed at locations far apart in memory. This
option is used when compiling 'crtstuff.c'; you should not need
to use it for anything else.

'-fvolatile'

Consider all memory references through pointers to be volatile.

'-fpic'
If supported for the target machines, generate
position-independent code, suitable for use in a shared library.
All addresses will be accessed through a global offset table
(GOT). If the GOT size for the linked executable exceeds a
machine-specific maximum size, you will get an error message from
the linker indicating that '-fpic' does not work; recompile with
'-fPIC' instead. (These maximums are 16k on the m88k, 8k on the
Sparc, and 32k on the m68k and RS/6000. The 386 has no such
limit.)

Position-independent code requires special support, and therefore
works only on certain machines. Code generated for the IBM
RS/6000 is always position-independent.

The GNU assembler does not fully support PIC. Currently, you
must use some other assembler in order for PIC to work. We would
welcome volunteers to upgrade GAS to handle this; the first part
of the job is to figure out what the assembler must do
differently.

'-fPIC'
If supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on the
size of the global offset table. This option makes a difference
on the m68k, m88k and the Sparc.

Position-independent code requires special support, and therefore
works only on certain machines.

'-ffixed-REG'
Treat the register named REG as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).

REG must be the name of a register. The register names accepted
are machine-specific and are defined in the 'REGISTER_NAMES'
macro in the machine description macro file.

This flag does not have a negative form, because it specifies a
three-way choice.

'-fcall-used-REG'
Treat the register named REG as an allocatable register that is
clobbered by function calls. It may be allocated for temporaries
or variables that do not live across a call. Functions compiled
this way will not save and restore the register REG.

Use of this flag for a register that has a fixed pervasive role
in the machine's execution model, such as the stack pointer or
frame pointer, will produce disastrous results.

This flag does not have a negative form, because it specifies a
three-way choice.

'-fcall-saved-REG'
Treat the register named REG as an allocatable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way will save
and restore the register REG if they use it.

Use of this flag for a register that has a fixed pervasive role
in the machine's execution model, such as the stack pointer or
frame pointer, will produce disastrous results.

A different sort of disaster will result from the use of this
flag for a register in which function values may be returned.

This flag does not have a negative form, because it specifies a
three-way choice.


File: gcc.info, Node: Environment Variables, Prev: Code Gen Options, Up: Invoking GCC

Environment Variables Affecting GNU CC
======================================

This section describes several environment variables that affect
how GNU CC operates. They work by specifying directories or prefixes
to use when searching for various kinds of files.

Note that you can also specify places to search using options such
as '-B', '-I' and '-L' (*note Directory Options::.). These take
precedence over places specified using environment variables, which in
turn take precedence over those specified by the configuration of GNU
CC. *Note Driver::.

'TMPDIR'
If 'TMPDIR' is set, it specifies the directory to use for
temporary files. GNU CC uses temporary files to hold the output
of one stage of compilation which is to be used as input to the
next stage: for example, the output of the preprocessor, which is
the input to the compiler proper.

'GCC_EXEC_PREFIX'
If 'GCC_EXEC_PREFIX' is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.

If GNU CC cannot find the subprogram using the specified prefix,
it tries looking in the usual places for the subprogram.

Other prefixes specified with '-B' take precedence over this
prefix.

This prefix is also used for finding files such as 'crt0.o' that
are used for linking.

In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with
'/usr/local/lib/gcc-lib' (more precisely, with the value of
'GCC_INCLUDE_DIR'), GNU CC tries replacing that beginning with
the specified prefix to produce an alternate directory name.
Thus, with '-Bfoo/', GNU CC will search 'foo/bar' where it would
normally search '/usr/local/lib/bar'. These alternate
directories are searched first; the standard directories come
next.

'COMPILER_PATH'
The value of 'COMPILER_PATH' is a colon-separated list of
directories, much like 'PATH'. GNU CC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using 'GCC_EXEC_PREFIX'.

'LIBRARY_PATH'
The value of 'LIBRARY_PATH' is a colon-separated list of
directories, much like 'PATH'. GNU CC tries the directories thus
specified when searching for special linker files, if it can't
find them using 'GCC_EXEC_PREFIX'. Linking using GNU CC also
uses these directories when searching for ordinary libraries for
the '-l' option (but directories specified with '-L' come first).

'C_INCLUDE_PATH'
'CPLUS_INCLUDE_PATH'
'OBJC_INCLUDE_PATH'
These environment variables pertain to particular languages. Each
variable's value is a colon-separated list of directories, much
like 'PATH'. When GNU CC searches for header files, it tries the
directories listed in the variable for the language you are
using, after the directories specified with '-I' but before the
standard header file directories.

'DEPENDENCIES_OUTPUT'
If this variable is set, its value specifies how to output
dependencies for Make based on the header files processed by the
compiler. This output looks much like the output from the '-M'
option (*note Preprocessor Options::.), but it goes to a separate
file, and is in addition to the usual results of compilation.

The value of 'DEPENDENCIES_OUTPUT' can be just a file name, in
which case the Make rules are written to that file, guessing the
target name from the source file name. Or the value can have the
form 'FILE TARGET', in which case the rules are written to file
FILE using TARGET as the target name.


File: gcc.info, Node: Installation, Next: Trouble, Prev: Invoking GCC, Up: Top

Installing GNU CC
*****************

Here is the procedure for installing GNU CC on a Unix system.

* Menu:

* Other Dir:: Compiling in a separate directory (not where the source is).
* Sun Install:: See below for installation on the Sun.
* 3B1 Install:: See below for installation on the 3B1.
* VMS Install:: See below for installation on VMS.
* Unos Install:: See below for installation on Unos (from CRDS).

1. If you have built GNU CC previously in the same directory for a
different target machine, do 'make distclean' to delete all files
that might be invalid.

2. On a Sequent system, go to the Berkeley universe.

3. On a System V release 4 system, make sure '/usr/bin' precedes
'/usr/ucb' in 'PATH'. The 'cc' command in '/usr/ucb' uses
libraries which have bugs.

4. Specify the host and target machine configurations. You do this
by running the file 'configure' with appropriate arguments.

If you are building a compiler to produce code for the machine
it runs on, specify just one machine type. To build a
cross-compiler, specify two configurations, one for the "host
machine" (which the compiler runs on), and one for the "target
machine" (which the compiler produces code for). The command
looks like this:

configure --host=sun3-sunos3 --target=sparc-sun-sunos4.1

A configuration name may be canonical or it may be more or less
abbreviated.

A canonical configuration name has three parts, separated by
dashes. It looks like this: 'CPU-COMPANY-SYSTEM'. (The three
parts may themselves contain dashes; 'configure' can figure out
which dashes serve which purpose.) For example,
'm68k-sun-sunos4.1' specifies a Sun 3.

You can also replace parts of the configuration by nicknames
or aliases. For example, 'sun3' stands for 'm68k-sun', so
'sun3-sunos4.1' is another way to specify a Sun 3. You can also
use simply 'sun3-sunos', since the version of SunOS is assumed by
default to be version 4. 'sun3-bsd' also works, since
'configure' knows that the only BSD variant on a Sun 3 is SunOS.

You can specify a version number after any of the system
types, and some of the CPU types. In most cases, the version is
irrelevant, and will be ignored. So you might as well specify
the version if you know it.

Here are the possible CPU types:

a29k, arm, cN, hppa, i386, i860, m68000, m68k, m88k, mips,
ns32k, romp, rs6000, sparc, vax.

Here are the recognized company names. As you can see,
customary abbreviations are used rather than the longer official
names.

alliant, altos, apollo, att, convergent, convex, crds, dec,
dg, encore, harris, hp, ibm, mips, motorola, ncr, next, ns,
omron, sequent, sgi, sony, sun, tti, unicom.

The company name is meaningful only to disambiguate when the
rest of the information supplied is insufficient. You can omit
it, writing just 'CPU-SYSTEM', if it is not needed. For example,
'vax-ultrix4.2' is equivalent to 'vax-dec-ultrix4.2'.

Here is a list of system types:

bsd, sysv, mach, minix, genix, ultrix, vms, sco, isc, aix,
sunos, hpux, unos, luna, dgux, newsos, osfrose, osf, dynix,
aos, ctix.

You can omit the system type; then 'configure' guesses the
operating system from the CPU and company.

You can add a version number to the system type; this may or
may not make a difference. For example, you can write 'bsd4.3' or
'bsd4.4' to distinguish versions of BSD. In practice, the version
number is most needed for 'sysv3' and 'sysv4', which are often
treated differently.

If you specify an impossible combination such as 'i860-dg-vms',
then you may get an error message from 'configure', or it may
ignore part of the information and do the best it can with the
rest. 'configure' always prints the canonical name for the
alternative that it used.

Often a particular model of machine has a name. Many machine
names are recognized as aliases for CPU/company combinations.
Thus, the machine name 'sun3', mentioned above, is an alias for
'm68k-sun'. Sometimes we accept a company name as a machine
name, when the name is popularly used for a particular machine.
Here is a table of the known machine names:

3300, 3b1, 7300, altos3068, altos, apollo68, att-7300,
balance, convex-cN, crds, decstation-3100, decstation-dec,
decstation, delta, encore, gmicro, hp7NN, hp8NN, hp9k2NN,
hp9k3NN, hp9k7NN, hp9k8NN, iris4d, iris, isi68, m3230,
magnum, merlin, miniframe, mmax, news-3600, news800, news,
next, pbd, pc532, pmax, ps2, risc-news, rtpc, sun2, sun386i,
sun386, sun3, sun4, symmetry, tower-32, tower.

Remember that a machine name specifies both the cpu type and the
company name.

On certain systems, you must specify whether you want GNU CC
to work with the usual compilation tools or with the GNU
compilation tools (including GAS). Use the '--gas' argument when
you run 'configure', if you want to use the GNU tools. The
systems were this makes a difference are 'i386-ANYTHING-sysv',
'i860-ANYTHING-bsd', 'm68k-hp-hpux', 'm68k-sony-bsd',
'm68k-altos-sysv', 'm68000-hp-hpux', and 'm68000-att-sysv'. On
any other system, '--gas' has no effect.

On certain systems, you must specify whether the machine has a
floating point unit. These systems are 'm68k-sun-sunosN' and
'm68k-isi-bsd'. On any other system, '--nfp' currently has no
effect, though perhaps there are other systems where it could
usefully make a difference.

If you want to install your own homemade configuration files,
you can use 'local' as the company name to access them. If you
use configuration 'CPU-local', the entire configuration name is
used to form the configuration file names.

Thus, if you specify 'm68k-local', then the files used are
'm68k-local.md', 'm68k-local.h', 'm68k-local.c',
'xm-m68k-local.h', 't-m68k-local', and 'x-m68k-local'.

Here is a list of configurations that have special treatment
or special things you must know:

'hppa-hp-hpux'
HP precision architecture, running HP-UX. '-g' does not work
on this configuration, since the system uses a peculiar
debugging format which GNU CC does not know about.

'i386-*-sco'
Compilation with RCC is recommended, but it produces lots of
spurious warnings. They do not necessarily indicate that
anything is wrong.

'm68000-att'
AT&T 3b1, a.k.a. 7300 PC. Special procedures are needed to
compile GNU CC with this machine's standard C compiler, due
to bugs in that compiler. *Note 3b1 Install::. You can
bootstrap it more easily with previous versions of GNU CC if
you have them.

'm68000-hp-bsd'
HP 9000 series 200 running BSD. Note that the C compiler
that comes with this system cannot compile GNU CC; contact
'[email protected]' to get binaries of GNU CC for bootstrapping.

'm68k-altos'
Altos 3068. You must use the GNU assembler, linker and
debugger, with COFF-encapsulation. Also, you must fix a
kernel bug. Details in the file 'ALTOS-README'.

'm68k-hp-hpux'
HP 9000 series 300 or 400 running HP-UX. HP-UX version 8.0
has a bug in the assembler that prevents compilation of GNU
CC. To fix it, get patch PHCO_0800 from HP.

In addition, '--gas' does not currently work with this
configuration. Changes in HP-UX have broken the library
conversion tool and the linker.

'm68k-sun'
Sun 3. We do not provide a configuration file to use the
Sun FPA by default, because programs that establish signal
handlers for floating point traps inherently cannot work
with the FPA.

'm88k-dgux'
Motorola m88k running DG/UX. To build native or cross
compilers on DG/UX, you must first change to the 88open BCS
software development environment. This is done by issuing
this command:

eval 'sde-target m88kbcs'

'mips-mips-bsd'
MIPS machines running the MIPS operating system in BSD mode.
It's possible that some old versions of the system lack the
functions 'memcpy', 'memcmp', and 'memset'. If your system
lacks these, you must remove or undo the definition of
'TARGET_MEM_FUNCTIONS' in 'mips-bsd.h'.

'ns32k-encore'
Encore ns32000 system. Encore systems are supported only
under BSD.

'ns32k-*-genix'
National Semiconductor ns32000 system. Genix has bugs in
'alloca' and 'malloc'; you must get the compiled versions of
these from GNU Emacs.

'ns32k-utek'
UTEK ns32000 system ("merlin"). The C compiler that comes
with this system cannot compile GNU CC; contact
'tektronix!reed!mason' to get binaries of GNU CC for
bootstrapping.

'pyramid'
The Pyramid C compler is reported to be unable to compile
GNU CC. You must use an older version of GNU CC for
bootstrapping.

'vax-dec-ultrix'
Don't try compiling with Vax C ('vcc'). It produces
incorrect code in some cases (for example, when 'alloca' is
used).

Meanwhile, compiling 'cp-parse.c' with pcc does not work
because of an internal table size limitation in that
compiler. To avoid this problem, compile just the GNU C
compiler first, and use it to recompile building all the
languages that you want to run.

Here we spell out what files will be set up by 'configure'.
Normally you need not be concerned with these files.

* A symbolic link named 'config.h' is made to the top-level
config file for the machine you will run the compiler on
(*note Config::.). This file is responsible for defining
information about the host machine. It includes 'tm.h'.

The top-level config file is located in the subdirectory
'config'. Its name is always 'xm-SOMETHING.h'; usually
'xm-MACHINE.h', but there are some exceptions.

If your system does not support symbolic links, you might
want to set up 'config.h' to contain a '#include' command
which refers to the appropriate file.

* A symbolic link named 'tconfig.h' is made to the top-level
config file for your target machine. This is used for
compiling certain programs to run on that machine.

* A symbolic link named 'tm.h' is made to the
machine-description macro file for your target machine. It
should be in the subdirectory 'config' and its name is often
'MACHINE.h'.

* A symbolic link named 'md' will be made to the machine
description pattern file. It should be in the 'config'
subdirectory and its name should be 'MACHINE.md'; but
MACHINE is often not the same as the name used in the 'tm.h'
file because the 'md' files are more general.

* A symbolic link named 'aux-output.c' will be made to the
output subroutine file for your machine. It should be in
the 'config' subdirectory and its name should be 'MACHINE.c'.

* The command file 'configure' also constructs 'Makefile' by
adding some text to the template file 'Makefile.in'. The
additional text comes from files in the 'config' directory,
named 't-TARGET' and 'h-HOST'. If these files do not exist,
it means nothing needs to be added for a given target or
host.

5. Make sure the Bison parser generator is installed. (This is
unnecessary if the Bison output files 'c-parse.c' and 'cexp.c'
are more recent than 'c-parse.y' and 'cexp.y' and you do not plan
to change the '.y' files.)

Bison versions older than Sept 8, 1988 will produce incorrect
output for 'c-parse.c'.

6. Build the compiler. Just type 'make LANGUAGES=c' in the compiler
directory.

'LANGUAGES=c' specifies that only the C compiler should be
compiled. The makefile normally builds compilers for all the
supported languages; currently, C, C++ and Objective C. However,
C is the only language that is sure to work when you build with
other non-GNU C compilers. In addition, building anything but C
at this stage is a waste of time.

In general, you can specify the languages to build by typing
the argument 'LANGUAGES="LIST"', where LIST is one or more words
from the list 'c', 'c++', and 'objective-c'.

Ignore any warnings you may see about "statement not reached"
in 'insn-emit.c'; they are normal. Any other compilation errors
may represent bugs in the port to your machine or operating
system, and should be investigated and reported (*note Bugs::.).

Some commercial compilers fail to compile GNU CC because they
have bugs or limitations. For example, the Microsoft compiler is
said to run out of macro space. Some Ultrix compilers run out of
expression space; then you need to break up the statement where
the problem happens.

7. If you are using COFF-encapsulation, you must convert 'libgcc.a'
to a GNU-format library at this point. See the file
'README-ENCAP' in the directory containing the GNU binary file
utilities, for directions.

8. Move the first-stage object files and executables into a
subdirectory with this command:

make stage1

The files are moved into a subdirectory named 'stage1'. Once
installation is complete, you may wish to delete these files with
'rm -r stage1'.

9. Recompile the compiler with itself, with this command:

make CC=stage1/gcc CFLAGS="-g -O -Bstage1/"

This is called making the stage 2 compiler.

The command shown above builds compilers for all the supported
languages. If you don't want them all, you can specify the
languages to build by typing the argument 'LANGUAGES="LIST"'.
LIST should contain one or more words from the list 'c', 'c++',
and 'objective-c', separated by spaces.

On a 68000 or 68020 system lacking floating point hardware,
unless you have selected a 'tm.h' file that expects by default
that there is no such hardware, do this instead:

make CC=stage1/gcc CFLAGS="-g -O -Bstage1/ -msoft-float"

10. If you wish to test the compiler by compiling it with itself one
more time, do this:

make stage2
make CC=stage2/gcc CFLAGS="-g -O -Bstage2/"

This is called making the stage 3 compiler. Aside from the '-B'
option, the options should be the same as when you made the stage
2 compiler.

Then compare the latest object files with the stage 2 object
files--they ought to be identical, unless they contain time
stamps. On systems where object files do not contain time
stamps, you can do this (in Bourne shell):

for file in *.o; do
cmp $file stage2/$file
done

This will mention any object files that differ between stage 2
and stage 3. Any difference, no matter how innocuous, indicates
that the stage 2 compiler has compiled GNU CC incorrectly, and is
therefore a potentially serious bug which you should investigate
and report (*note Bugs::.).

On systems that use COFF object files, or an object file
format that is a superset of COFF (such as ECOFF or XCOFF), bytes
5 to 8 will always be different, since it is a timestamp. On
these systems, you can do the comparison as follows (in Bourne
shell):

for file in *.o; do
tail +10c $file > foo1
tail +10c stage2/$file > foo2
cmp foo1 foo2 || echo $file
done

If you have built the compiler with the '-mno-mips-tfile'
option on MIPS machines, you will not be able to compare the
files.

11. Install the compiler driver, the compiler's passes and run-time
support. You can use the following command:

make CC=stage2/gcc install

(Use the same value for 'CC' that you used when compiling the
files that are being installed.)

This copies the files 'cc1', 'cpp' and 'libgcc.a' to files
'cc1', 'cpp' and 'libgcc.a' in directory
'/usr/local/lib/gcc-lib/TARGET/VERSION', which is where the
compiler driver program looks for them. Here TARGET is the target
machine type specified when you ran 'configure', and VERSION is
the version number of GNU CC. This naming scheme permits various
versions and/or cross-compilers to coexist.

It also copies the driver program 'gcc' into the directory
'/usr/local/bin', so that it appears in typical execution search
paths.

*Warning: there is a bug in 'alloca' in the Sun library. To
avoid this bug, be sure to install the executables of GNU CC that
were compiled by GNU CC. (That is, the executables from stage 2
or 3, not stage 1.) They use 'alloca' as a built-in function and
never the one in the library.*

(It is usually better to install GNU CC executables from stage
2 or 3, since they usually run faster than the ones compiled with
some other compiler.)

12. If you will be using C++ or Objective C, and your operating
system does not handle constructors, then you must build and
install the program 'collect2'. Do this with the following
command:

make CC="stage2/gcc -O" install-collect2

The systems that *do* handle constructors on their own include
system V release 4, and system V release 3 on the Intel 386.

Berkeley systems that use the "a.out" object file format handle
constructors without 'collect2' if you use the GNU linker. But if
you don't use the GNU linker, then you need 'collect2' on these
systems.

13. Build and install 'protoize' if you want it. Type

make CC="stage2/gcc -O" install-proto

There is as yet no documentation for 'protoize'. Sorry.

14. Correct errors in the header files on your machine.

Various system header files often contain constructs which are
incompatible with ANSI C, and they will not work when you compile
programs with GNU CC. This behavior consists of substituting for
macro argument names when they appear inside of character
constants. The most common offender is 'ioctl.h'.

You can overcome this problem when you compile by specifying
the '-traditional' option.

Alternatively, on Sun systems and 4.3BSD at least, you can
correct the include files by running the shell script
'fixincludes'. This installs modified, corrected copies of the
files 'ioctl.h', 'ttychars.h' and many others, in a special
directory where only GNU CC will normally look for them. This
script will work on various systems because it chooses the files
by searching all the system headers for the problem cases that we
know about.

Use the following command to do this:

make install-fixincludes

If you selected a different directory for GNU CC installation
when you installed it, by specifying the Make variable 'prefix' or
'libdir', specify it the same way in this command.

Note that some systems are starting to come with ANSI C system
header files. On these systems, don't run 'fixincludes'; it may
not work, and is certainly not necessary.

If you cannot install the compiler's passes and run-time support in
'/usr/local/lib', you can alternatively use the '-B' option to specify
a prefix by which they may be found. The compiler concatenates the
prefix with the names 'cpp', 'cc1' and 'libgcc.a'. Thus, you can put
the files in a directory '/usr/foo/gcc' and specify '-B/usr/foo/gcc/'
when you run GNU CC.

Also, you can specify an alternative default directory for these
files by setting the Make variable 'libdir' when you make GNU CC.


File: gcc.info, Node: Other Dir, Next: Sun Install, Prev: Installation, Up: Installation

Compilation in a Separate Directory
===================================

If you wish to build the object files and executables in a directory
other than the one containing the source files, here is what you must
do differently:

1. Make sure you have a version of Make that supports the 'VPATH'
feature. (GNU Make supports it, as do Make versions on most BSD
systems.)

2. If you have ever run 'configure' in the source directory, you
must undo the configuration. Do this by running:

make cleanconfig

3. Go to the directory in which you want to build the compiler before
running 'configure':

mkdir gcc-sun3
cd gcc-sun3

On systems that do not support symbolic links, this directory
must be on the same file system as the source code directory.

4. Specify where to find 'configure' when you run it:

../gcc-2.00/configure ...

This also tells 'configure' where to find the compiler sources;
'configure' takes the directory from the file name that was used
to invoke it. But if you want to be sure, you can specify the
source directory with the '--srcdir' option, like this:

../gcc-2.00/configure --srcdir=../gcc-2.00 sun3

The directory you specify with '--srcdir' need not be the same
as the one that 'configure' is found in.

Now, you can run 'make' in that directory. You need not repeat the
configuration steps shown above, when ordinary source files change.
You must, however, run 'configure' again when the configuration files
change, if your system does not support symbolic links.


File: gcc.info, Node: Sun Install, Next: 3b1 Install, Prev: Other Dir, Up: Installation

Installing GNU CC on the Sun
============================

Make sure the environment variable 'FLOAT_OPTION' is not set when
you compile 'libgcc.a'. If this option were set to 'f68881' when
'libgcc.a' is compiled, the resulting code would demand to be linked
with a special startup file and would not link properly without
special pains.

There is a bug in 'alloca' in certain versions of the Sun library.
To avoid this bug, install the binaries of GNU CC that were compiled by
GNU CC. They use 'alloca' as a built-in function and never the one in
the library.

Some versions of the Sun compiler crash when compiling GNU CC. The
problem is a segmentation fault in cpp. This problem seems to be due
to the bulk of data in the environment variables. You may be able to
avoid it by using the following command to compile GNU CC with Sun CC:

make CC="TERMCAP=x OBJS=x LIBFUNCS=x STAGESTUFF=x cc"


File: gcc.info, Node: 3b1 Install, Next: Unos Install, Prev: Sun Install, Up: Installation

Installing GNU CC on the 3b1
============================

Installing GNU CC on the 3b1 is difficult if you do not already have
GNU CC running, due to bugs in the installed C compiler. However, the
following procedure might work. We are unable to test it.

1. Comment out the '#include "config.h"' line on line 37 of 'cccp.c'
and do 'make cpp'. This makes a preliminary version of GNU cpp.

2. Save the old '/lib/cpp' and copy the preliminary GNU cpp to that
file name.

3. Undo your change in 'cccp.c', or reinstall the original version,
and do 'make cpp' again.

4. Copy this final version of GNU cpp into '/lib/cpp'.

5. Replace every occurrence of 'obstack_free' in the file 'tree.c'
with '_obstack_free'.

6. Run 'make' to get the first-stage GNU CC.

7. Reinstall the original version of '/lib/cpp'.

8. Now you can compile GNU CC with itself and install it in the
normal fashion.


File: gcc.info, Node: Unos Install, Next: VMS Install, Prev: 3b1 Install, Up: Installation

Installing GNU CC on Unos
=========================

Use 'configure unos' for building on Unos.

The Unos assembler is named 'casm' instead of 'as'. For some
strange reason linking '/bin/as' to '/bin/casm' changes the behavior,
and does not work. So, when installing GNU CC, you should install the
following script as 'as' in the subdirectory where the passes of GCC
are installed:

#!/bin/sh
casm $*

The default Unos library is named 'libunos.a' instead of 'libc.a'.
To allow GNU CC to function, either change all references to '-lc' in
'gcc.c' to '-lunos' or link '/lib/libc.a' to '/lib/libunos.a'.

When compiling GNU CC with the standard compiler, to overcome bugs
in the support of 'alloca', do not use '-O' when making stage 2. Then
use the stage 2 compiler with '-O' to make the stage 3 compiler. This
compiler will have the same characteristics as the usual stage 2
compiler on other systems. Use it to make a stage 4 compiler and
compare that with stage 3 to verify proper compilation.

Unos uses memory segmentation instead of demand paging, so you will
need a lot of memory. 5 Mb is barely enough if no other tasks are
running. If linking 'cc1' fails, try putting the object files into a
library and linking from that library.


File: gcc.info, Node: VMS Install, Prev: Unos Install, Up: Installation

Installing GNU CC on VMS
========================

The VMS version of GNU CC is distributed in a backup saveset
containing both source code and precompiled binaries.

To install the 'gcc' command so you can use the compiler easily, in
the same manner as you use the VMS C compiler, you must install the
VMS CLD file for GNU CC as follows:

1. Define the VMS logical names 'GNU_CC' and 'GNU_CC_INCLUDE' to
point to the directories where the GNU CC executables ('gcc-cpp',
'gcc-cc1', etc.) and the C include files are kept. This should
be done with the commands:

$ assign /system /translation=concealed -
disk:[gcc.] gnu_cc
$ assign /system /translation=concealed -
disk:[gcc.include.] gnu_cc_include

with the appropriate disk and directory names. These commands
can be placed in your system startup file so they will be
executed whenever the machine is rebooted. You may, if you
choose, do this via the 'GCC_INSTALL.COM' script in the '[GCC]'
directory.

2. Install the 'GCC' command with the command line:

$ set command /table=sys$common:[syslib]dcltables -
/output=sys$common:[syslib]dcltables gnu_cc:[000000]gcc
$ install replace sys$common:[syslib]dcltables

3. To install the help file, do the following:

$ lib/help sys$library:helplib.hlb gcc.hlp

Now you can invoke the compiler with a command like 'gcc /verbose
file.c', which is equivalent to the command 'gcc -v -c file.c' in
Unix.

If you wish to use GNU C++ you must first install GNU CC, and then
perform the following steps:

1. Define the VMS logical name 'GNU_GXX_INCLUDE' to point to the
directory where the preprocessor will search for the C++ header
files. This can be done with the command:

$ assign /system /translation=concealed -
disk:[gcc.gxx_include.] gnu_gxx_include

with the appropriate disk and directory name. If you are going
to be using libg++, this is where the libg++ install procedure
will install the libg++ header files.

2. Obtain the file 'gcc-cc1plus.exe', and place this in the same
directory that 'gcc-cc1.exe' is kept.

The GNU C++ compiler can be invoked with a command like 'gcc
/plus /verbose file.cc', which is equivalent to the command 'g++
-v -c file.cc' in Unix.

We try to put corresponding binaries and sources on the VMS
distribution tape. But sometimes the binaries will be from an older
version that the sources, because we don't always have time to update
them. (Use the '/version' option to determine the version number of
the binaries and compare it with the source file 'version.c' to tell
whether this is so.) In this case, you should use the binaries you
get to recompile the sources. If you must recompile, here is how:

1. Copy the file 'vms.h' to 'tm.h', 'xm-vms.h' to 'config.h',
'vax.md' to 'md.' and 'vax.c' to 'aux-output.c'. The files to be
copied are found in the subdirectory named 'config'; they should
be copied to the main directory of GNU CC. If you wish, you may
use the command file 'config-gcc.com' to perform these steps for
you.

2. Setup the logical names and command tables as defined above. In
addition, define the VMS logical name 'GNU_BISON' to point at the
to the directories where the Bison executable is kept. This
should be done with the command:

$ assign /system /translation=concealed -
disk:[bison.] gnu_bison

You may, if you choose, use the 'INSTALL_BISON.COM' script in
the '[BISON]' directory.

3. Install the 'BISON' command with the command line:

$ set command /table=sys$common:[syslib]dcltables -
/output=sys$common:[syslib]dcltables -
gnu_bison:[000000]bison
$ install replace sys$common:[syslib]dcltables

4. Type '@make-gcc' to recompile everything (alternatively, you may
submit the file 'make-gcc.com' to a batch queue). If you wish to
build the GNU C++ compiler as well as the GNU CC compiler, you
must first edit 'make-gcc.com' and follow the instructions that
appear in the comments.

5. In order to use GCC, you need a library of functions which GCC
compiled code will call to perform certain tasks, and these
functions are defined in the file 'libgcc2.c'. To compile this
you should use the command procedure 'make-l2.com', which will
generate the library 'libgcc2.olb'. 'libgcc2.olb' should be
built using the compiler built from the same distribution that
'libgcc2.c' came from, and 'make-gcc.com' will automatically do
all of this for you.

To install the library, use the following commands:

$ lib gnu_cc:[000000]gcclib/delete=(new,eprintf)
$ lib libgcc2/extract=*/output=libgcc2.obj
$ lib gnu_cc:[000000]gcclib libgcc2.obj

The first command simply removes old modules that will be
replaced with modules from libgcc2. If the VMS librarian
complains about those modules not being present, simply ignore
the message and continue on with the next command.

Whenever you update the compiler on your system, you should
also update the library with the above procedure.

*If you are building GNU CC with a previous version of GNU CC,
you also should check to see that you have the newest version of
the assembler*. In particular, GNU CC version 2 treats global
constant variables slightly differently from GNU CC version 1,
and GAS version 1.38.1 does not have the patches required to work
with GCC version 2. If you use GAS 1.38.1, then 'extern const'
variables will not have the read-only bit set, and the linker
will generate warning messages about mismatched psect attributes
for these variables. These warning messages are merely a
nuisance, and can safely be ignored.

If you are compiling with a version of GNU CC older than 1.33,
specify '/DEFINE=("inline=")' as an option in all the
compilations. This requires editing all the 'gcc' commands in
'make-cc1.com'. (The older versions had problems supporting
'inline'.) Once you have a working 1.33 or newer GNU CC, you can
change this file back.

Under previous versions of GNU CC, the generated code would
occasionally give strange results when linked to the sharable
'VAXCRTL' library. Now this should work.

Even with this version, however, GNU CC itself should not be linked
to the sharable 'VAXCRTL'. The 'qsort' routine supplied with
'VAXCRTL' has a bug which can cause a compiler crash.

Similarly, the preprocessor should not be linked to the sharable
'VAXCRTL'. The 'strncat' routine supplied with 'VAXCRTL' has a bug
which can cause the preprocessor to go into an infinite loop.

If you attempt to link to the sharable 'VAXCRTL', the VMS linker
will strongly resist any effort to force it to use the 'qsort' and
'strncat' routines from 'gcclib'. Until the bugs in 'VAXCRTL' have
been fixed, linking any of the compiler components to the sharable
VAXCRTL is not recommended. (These routines can be bypassed by
placing duplicate copies of 'qsort' and 'strncat' in 'gcclib' under
different names, and patching the compiler sources to use these
routines). Both of the bugs in 'VAXCRTL' are still present in VMS
version 5.4-1, which is the most recent version as of this writing.

The executables that are generated by 'make-cc1.com' and
'make-cccp.com' use the nonshared version of 'VAXCRTL' (and thus use
the 'qsort' and 'strncat' routines from 'gcclib.olb').


File: gcc.info, Node: Trouble, Next: Service, Prev: Installation, Up: Top

Known Causes of Trouble with GNU CC
***********************************

Here are some of the things that have caused trouble for people
installing or using GNU CC.

* On certain systems, defining certain environment variables such as
'CC' can interfere with the functioning of 'make'.

* If you encounter seemingly strange errors when trying to build the
compiler in a directory other than the source directory, make
sure you have done all the necessary preparations. *Note Other
Dir::.

* In previous versions of GNU CC, the 'gcc' driver program looked
for 'as' and 'ld' in various places such as files beginning with
'/usr/local/lib/gcc-'. GNU CC version 2 looks for them in the
directory '/usr/local/lib/gcc/TARGET/VERSION'.

Thus, to use a version of 'as' or 'ld' that is not the system
default, for example 'gas' or GNU 'ld', you must put them in that
directory (or make links to them from that directory).

* Some commands executed when making the compiler may fail (return a
non-zero status) and be ignored by 'make'. These failes, which
are often due to files that were not found, are expected, and can
safely be ignored.

* Cross compilation can run into trouble for certain machines
because some target machines' assemblers require floating point
numbers to be written as *integer* constants in certain contexts.

The compiler writes these integer constants by examining the
floating point value as an integer and printing that integer,
because this is simple to write and independent of the details of
the floating point representation. But this does not work if the
compiler is running on a different machine with an incompatible
floating point format, or even a different byte-ordering.

In addition, correct constant folding of floating point values
requires representing them in the target machine's format. (The
C standard does not quite require this, but in practice it is the
only way to win.)

It is now possible to overcome these problems by defining macros
such as 'REAL_VALUE_TYPE'. But doing so is a substantial amount
of work for each target machine. *Note Cross-compilation::.

* Users often think it is a bug when GNU CC reports an error for
code like this:

int foo (short);

int foo (x)
short x;
{...}

The error message is correct: this code really is erroneous,
because the old-style non-prototype definition passes subword
integers in their promoted types. In other words, the argument
is really an 'int', not a 'short'. The correct prototype is this:

int foo (int);

* Users often think it is a bug when GNU CC reports an error for
code like this:

int foo (struct mumble *);

struct mumble { ... };

int foo (struct mumble *x)
{ ... }

This code really is erroneous, because the scope of 'struct
mumble' the prototype is limited to the argument list containing
it. It does not refer to the 'struct mumble' defined with file
scope immediately below--they are two unrelated types with
similar names in different scopes.

But in the definition of 'foo', the file-scope type is used
because that is available to be inherited. Thus, the definition
and the prototype do not match, and you get an error.

This behavior may seem silly, but it's what the ANSI standard
specifies. It is easy enough for you to make your code work by
moving the definition of 'struct mumble' above the prototype.
It's not worth being incompatible with ANSI C just to avoid an
error for the example shown above.

* Programs that use preprocessor directives in the middle of macro
arguments do not work with GNU CC. For example, a program like
this will not work:

foobar (
#define luser
hack)

ANSI C does not permit such a construct, and it does not seem
worth the trouble of implementing, given that it is not required.

* Certain local variables aren't recognized by debuggers when you
compile with optimization.

This occurs because sometimes GNU CC optimizes the variable out of
existence. There is no way to tell the debugger how to compute
the value such a variable "would have had", and it is not clear
that would be desirable anyway. So GNU CC simply does not
mention the eliminated variable when it writes debugging
information.

You have to expect a certain amount of disagreement between the
executable and your source code, when you use optimization.

* '-2147483648' is positive.

This is because 2147483648 cannot fit in the type 'int', so
(following the ANSI C rules) its data type is 'unsigned long int'.
Negating this value yields 2147483648 again.

* The GNU assembler (GAS) does not support PIC. To generate PIC
code, you must use some other assembler, such as '/bin/as'.

* Sometimes on a Sun 4 you may observe a crash in the program
'genflags' or 'genoutput' while building GCC. This is said to be
due to a bug in 'sh'. You can probably get around it by running
'genflags' or 'genoutput' manually and then retrying the 'make'.

* On a Sun, linking using GNU CC fails to find a shared library and
reports that the library doesn't exist at all.

This happens if you are using the GNU linker, because it does only
static linking and looks only for unshared libraries. If you
have a shared library with no unshared counterpart, the GNU
linker won't find anything.

We hope to make a linker which supports Sun shared libraries, but
please don't ask when it will be finished--we don't know.

* On some versions of Ultrix, the system supplied compiler cannot
compile 'cp-parse.c' because it cannot handle so many cases in a
'switch' statement. You can work around this problem by
compiling with GNU CC.

* On some BSD systems including some versions of Ultrix, use of
profiling causes static variable destructors (currently used only
in C++) not to be run.

* On HP 9000 series 300 or 400 running HP-UX release 8.0, there is
a bug in the assembler that must be fixed before GNU CC can be
built. This bug manifests itself during the first stage of
compilation, while building 'libgcc2.a':

_floatdisf
cc1: warning: '-g' option not supported on this version of GCC
cc1: warning: '-g1' option not supported on this version of GCC
./gcc: Internal compiler error: program as got fatal signal 11

A patched version of the assembler is available by anonymous ftp
from 'altdorf.ai.mit.edu' as the file
'archive/cph/hpux-8.0-assembler'. If you have HP software
support, the patch can also be obtained directly from HP, as
described in the following note:

This is the patched assembler, to patch SR#1653-010439,
where the assembler aborts on floating point constants.

The bug is not really in the assembler, but in the shared
library version of the function "cvtnum(3c)". The bug on
"cvtnum(3c)" is SR#4701-078451. Anyway, the attached
assembler uses the archive library version of "cvtnum(3c)"
and thus does not exhibit the bug.

This patch is also known as PHCO_0800.

* The current version of the assembler ('/bin/as') for the RS/6000
has certain problems that prevent the '-g' option in GCC from
working.

IBM has produced a fixed version of the assembler. The
replacement assembler is not a standard component of either AIX
3.1.5 or AIX 3.2, but is expected to become standard in a future
distribution. This assembler is available from IBM as APAR
IX22829. See the file 'README.RS6000' for more details on how to
obtain this assembler.

* Sun forgot to include a static version of 'libdl.a' with some
versions of SunOS (mainly 4.1). This results in undefined
symbols when linking static binaries (that is, if you use
'-static'). If you see undefined symbols '_dlclose', '_dlsym' or
'_dlopen' when linking, compile and link against the file
'mit/util/misc/dlsym.c' from the MIT version of X windows.

* On the IBM RS/6000, compiling code of the form

extern int foo;

... foo ...

static int foo;

will cause the linker to report an undefined symbol 'foo'.
Although this behavior differs from most other systems, it is not
a bug because redefining an 'extern' variable as 'static' is
undefined in ANSI C.

* On VMS, GAS versions 1.38.1 and earlier may cause spurious warning
messages from the linker. These warning messages complain of
mismatched psect attributes. You can ignore them. *Note VMS
Install::.

For additional common problems, see *Note Incompatibilities::.


File: gcc.info, Node: Service, Next: Incompatibilities, Prev: Trouble, Up: Top

How To Get Help with GNU CC
***************************

If you need help installing, using or changing GNU CC, there are two
ways to find it:

* Send a message to a suitable network mailing list. First try
'[email protected]', and if that brings no response, try
'[email protected]'.

* Look in the service directory for someone who might help you for
a fee. The service directory is found in the file named
'SERVICE' in the GNU CC distribution.


File: gcc.info, Node: Incompatibilities, Next: Extensions, Prev: Service, Up: Top

Incompatibilities of GNU CC
***************************

There are several noteworthy incompatibilities between GNU C and
most existing (non-ANSI) versions of C. The '-traditional' option
eliminates most of these incompatibilities, *but not all*, by telling
GNU C to behave like the other C compilers.

* GNU CC normally makes string constants read-only. If several
identical-looking string constants are used, GNU CC stores only
one copy of the string.

One consequence is that you cannot call 'mktemp' with a string
constant argument. The function 'mktemp' always alters the
string its argument points to.

Another consequence is that 'sscanf' does not work on some systems
when passed a string constant as its format control string or
input. This is because 'sscanf' incorrectly tries to write into
the string constant. Likewise 'fscanf' and 'scanf'.

The best solution to these problems is to change the program to
use 'char'-array variables with initialization strings for these
purposes instead of string constants. But if this is not
possible, you can use the '-fwritable-strings' flag, which
directs GNU CC to handle string constants the same way most C
compilers do. '-traditional' also has this effect, among others.

* GNU CC does not substitute macro arguments when they appear
inside of string constants. For example, the following macro in
GNU CC

#define foo(a) "a"

will produce output '"a"' regardless of what the argument A is.

The '-traditional' option directs GNU CC to handle such cases
(among others) in the old-fashioned (non-ANSI) fashion.

* When you use 'setjmp' and 'longjmp', the only automatic variables
guaranteed to remain valid are those declared 'volatile'. This
is a consequence of automatic register allocation. Consider this
function:

jmp_buf j;

foo ()
{
int a, b;

a = fun1 ();
if (setjmp (j))
return a;

a = fun2 ();
/* 'longjmp (j)' may occur in 'fun3'. */
return a + fun3 ();
}

Here 'a' may or may not be restored to its first value when the
'longjmp' occurs. If 'a' is allocated in a register, then its
first value is restored; otherwise, it keeps the last value stored
in it.

If you use the '-W' option with the '-O' option, you will get a
warning when GNU CC thinks such a problem might be possible.

The '-traditional' option directs GNU C to put variables in the
stack by default, rather than in registers, in functions that
call 'setjmp'. This results in the behavior found in traditional
C compilers.

* Declarations of external variables and functions within a block
apply only to the block containing the declaration. In other
words, they have the same scope as any other declaration in the
same place.

In some other C compilers, a 'extern' declaration affects all the
rest of the file even if it happens within a block.

The '-traditional' option directs GNU C to treat all 'extern'
declarations as global, like traditional compilers.

* In traditional C, you can combine 'long', etc., with a typedef
name, as shown here:

typedef int foo;
typedef long foo bar;

In ANSI C, this is not allowed: 'long' and other type modifiers
require an explicit 'int'. Because this criterion is expressed
by Bison grammar rules rather than C code, the '-traditional'
flag cannot alter it.

* PCC allows typedef names to be used as function parameters. The
difficulty described immediately above applies here too.

* PCC allows whitespace in the middle of compound assignment
operators such as '+='. GNU CC, following the ANSI standard,
does not allow this. The difficulty described immediately above
applies here too.

* GNU CC will flag unterminated character constants inside of
preprocessor conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if
these comments contain apostrophes, GNU CC will probably report
an error. For example, this code would produce an error:

#if 0
You can't expect this to work.
#endif

The best solution to such a problem is to put the text into an
actual C comment delimited by '/*...*/'. However, '-traditional'
suppresses these error messages.

* When compiling functions that return 'float', PCC converts it to
a double. GNU CC actually returns a 'float'. If you are
concerned with PCC compatibility, you should declare your
functions to return 'double'; you might as well say what you mean.

* When compiling functions that return structures or unions, GNU CC
output code normally uses a method different from that used on
most versions of Unix. As a result, code compiled with GNU CC
cannot call a structure-returning function compiled with PCC, and
vice versa.

The method used by GNU CC is as follows: a structure or union
which is 1, 2, 4 or 8 bytes long is returned like a scalar. A
structure or union with any other size is stored into an address
supplied by the caller (usually in a special, fixed register, but
on some machines it is passed on the stack). The
machine-description macros 'STRUCT_VALUE' and
'STRUCT_INCOMING_VALUE' tell GNU CC where to pass this address.

By contrast, PCC on most target machines returns structures and
unions of any size by copying the data into an area of static
storage, and then returning the address of that storage as if it
were a pointer value. The caller must copy the data from that
memory area to the place where the value is wanted. GNU CC does
not use this method because it is slower and nonreentrant.

On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GNU CC on most of these machines
uses a compatible convention when returning structures and unions
in memory, but still returns small structures and unions in
registers.

You can tell GNU CC to use a compatible convention for all
structure and union returning with the option
'-fpcc-struct-return'.

There are also system-specific incompatibilities.

* On the Alliant, the system's own convention for returning
structures and unions is unusual, and is not compatible with GNU
CC no matter what options are used.

* On the IBM RT PC, the MetaWare HighC compiler (hc) uses yet
another convention for structure and union returning. Use
'-mhc-struct-return' to tell GNU CC to use a convention compatible
with it.

* On Ultrix, the Fortran compiler expects registers 2 through 5 to
be saved by function calls. However, the C compiler uses
conventions compatible with BSD Unix: registers 2 through 5 may
be clobbered by function calls.

GNU CC uses the same convention as the Ultrix C compiler. You
can use these options to produce code compatible with the Fortran
compiler:

-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5

* DBX rejects some files produced by GNU CC, though it accepts
similar constructs in output from PCC. Until someone can supply
a coherent description of what is valid DBX input and what is
not, there is nothing I can do about these problems. You are on
your own.


File: gcc.info, Node: Extensions, Next: Bugs, Prev: Incompatibilities, Up: Top

GNU Extensions to the C Language
********************************

GNU C provides several language features not found in ANSI standard
C. (The '-pedantic' option directs GNU CC to print a warning message
if any of these features is used.) To test for the availability of
these features in conditional compilation, check for a predefined macro
'__GNUC__', which is always defined under GNU CC.

* Menu:

* Statement Exprs:: Putting statements and declarations inside expressions.
* Local Labels:: Labels local to a statement-expression.
* Labels as Values:: Getting pointers to labels, and computed gotos.
* Nested Functions:: As in Algol and Pascal, lexical scoping of functions.
* Naming Types:: Giving a name to the type of some expression.
* Typeof:: 'typeof': referring to the type of an expression.
* Lvalues:: Using '?:', ',' and casts in lvalues.
* Conditionals:: Omitting the middle operand of a '?:' expression.
* Long Long:: Double-word integers--'long long int'.
* Zero Length:: Zero-length arrays.
* Variable Length:: Arrays whose length is computed at run time.
* Subscripting:: Any array can be subscripted, even if not an lvalue.
* Pointer Arith:: Arithmetic on 'void'-pointers and function pointers.
* Initializers:: Non-constant initializers.
* Constructors:: Constructor expressions give structures, unions
or arrays as values.
* Labeled Elements:: Labeling elements of initializers.
* Cast to Union:: Casting to union type from any member of the union.
* Case Ranges:: 'case 1 ... 9' and such.
* Function Attributes:: Declaring that functions have no side effects,
or that they can never return.
* Dollar Signs:: Dollar sign is allowed in identifiers.
* Character Escapes:: '\e' stands for the character ESC.
* Variable Attributes:: Specifying attributes of variables.
* Alignment:: Inquiring about the alignment of a type or variable.
* Inline:: Defining inline functions (as fast as macros).
* Extended Asm:: Assembler instructions with C expressions as operands.
(With them you can define "built-in" functions.)
* Asm Labels:: Specifying the assembler name to use for a C symbol.
* Explicit Reg Vars:: Defining variables residing in specified registers.
* Alternate Keywords:: '__const__', '__asm__', etc., for header files.
* Incomplete Enums:: 'enum foo;', with details to follow.


File: gcc.info, Node: Statement Exprs, Next: Local Labels, Up: Extensions

Statements and Declarations within Expressions
==============================================

A compound statement in parentheses may appear inside an expression
in GNU C. This allows you to declare variables within an expression.
For example:

({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })

is a valid (though slightly more complex than necessary) expression
for the absolute value of 'foo ()'.

This feature is especially useful in making macro definitions
"safe" (so that they evaluate each operand exactly once). For
example, the "maximum" function is commonly defined as a macro in
standard C as follows:

#define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either A or B twice, with bad results if
the operand has side effects. In GNU C, if you know the type of the
operands (here let's assume 'int'), you can define the macro safely as
follows:

#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Embedded statements are not allowed in constant expressions, such as
the value of an enumeration constant, the width of a bit field, or the
initial value of a static variable.

If you don't know the type of the operand, you can still do this,
but you must use 'typeof' (*note Typeof::.) or type naming (*note
Naming Types::.).


File: gcc.info, Node: Local Labels, Next: Labels as Values, Prev: Statement Exprs, Up: Extensions

Locally Declared Labels
=======================

Each statement expression is a scope in which "local labels" can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary 'goto' statement, but only from within the statement
expression it belongs to.

A local label declaration looks like this:

__label__ LABEL;

or

__label__ LABEL1, LABEL2, ...;

Local label declarations must come at the beginning of the statement
expression, right after the '({', before any ordinary declarations.

The label declaration defines the label *name*, but does not define
the label itself. You must do this in the usual way, with 'LABEL:',
within the statements of the statement expression.

The local label feature is useful because statement expressions are
often used in macros. If the macro contains nested loops, a 'goto'
can be useful for breaking out of them. However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function. A local label avoids this problem. For
example:

#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})


File: gcc.info, Node: Labels as Values, Next: Nested Functions, Prev: Local Labels, Up: Extensions

Labels as Values
================

You can get the address of a label defined in the current function
(or a containing function) with the unary operator '&&'. The value
has type 'void *'. This value is a constant and can be used wherever
a constant of that type is valid. For example:

void *ptr;
...
ptr = &&foo;

To use these values, you need to be able to jump to one. This is
done with the computed goto statement(1), 'goto *EXP;'. For example,

goto *ptr;

Any expression of type 'void *' is allowed.

One way of using these constants is in initializing a static array
that will serve as a jump table:

static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

goto *array[i];

Note that this does not check whether the subscript is in bounds--array
indexing in C never does that.

Such an array of label values serves a purpose much like that of the
'switch' statement. The 'switch' statement is cleaner, so use that
rather than an array unless the problem does not fit a 'switch'
statement very well.

Another use of label values is in an interpreter for threaded code.
The labels within the interpreter function can be stored in the
threaded code for super-fast dispatching.

---------- Footnotes ----------

(1) The analogous feature in Fortran is called an assigned goto,
but that name seems inappropriate in C, where one can do more than
simply store label addresses in label variables.


File: gcc.info, Node: Nested Functions, Next: Naming Types, Prev: Labels as Values, Up: Extensions

Nested Functions
================


A "nested function" is a function defined inside another function.
The nested function's name is local to the block where it is defined.
For example, here we define a nested function named 'square', and call
it twice:

foo (double a, double b)
{
double square (double z) { return z * z; }

return square (a) + square (b);
}

The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called "lexical scoping". For example, here we show a nested function
which uses an inherited variable named 'offset':

bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
...
for (i = 0; i < size; i++)
... access (array, i) ...
}

It is possible to call the nested function from outside the scope
of its name by storing its address or passing the address to another
function:

hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }

intermediate (store, size);
}

Here, the function 'intermediate' receives the address of 'store'
as an argument. If 'intermediate' calls 'store', the arguments given
to 'store' are used to store into 'array'. But this technique works
only so long as the containing function ('hack', in this example) does
not exit. If you try to call the nested function through its address
after the containing function has exited, all hell will break loose.

A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (*note Local Labels::.). Such a jump returns instantly to the
containing function, exiting the nested function which did the 'goto'
and any intermediate functions as well. Here is an example:

bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
...
for (i = 0; i < size; i++)
... access (array, i) ...
...
return 0;

/* Control comes here from 'access'
if it detects an error. */
failure:
return -1;
}

A nested function always has internal linkage. Declaring one with
'extern' is erroneous. If you need to declare the nested function
before its definition, use 'auto' (which is otherwise meaningless for
function declarations).

bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
...
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
...
}


File: gcc.info, Node: Naming Types, Next: Typeof, Prev: Nested Functions, Up: Extensions

Naming an Expression's Type
===========================

You can give a name to the type of an expression using a 'typedef'
declaration with an initializer. Here is how to define NAME as a type
name for the type of EXP:

typedef NAME = EXP;

This is useful in conjunction with the statements-within-expressions
feature. Here is how the two together can be used to define a safe
"maximum" macro that operates on any arithmetic type:

#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })

The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within
the expressions that are substituted for 'a' and 'b'. Eventually we
hope to design a new form of declaration syntax that allows you to
declare variables whose scopes start only after their initializers;
this will be a more reliable way to prevent such conflicts.


File: gcc.info, Node: Typeof, Next: Lvalues, Prev: Naming Types, Up: Extensions

Referring to a Type with 'typeof'
=================================

Another way to refer to the type of an expression is with 'typeof'.
The syntax of using of this keyword looks like 'sizeof', but the
construct acts semantically like a type name defined with 'typedef'.

There are two ways of writing the argument to 'typeof': with an
expression or with a type. Here is an example with an expression:

typeof (x[0](1))

This assumes that 'x' is an array of functions; the type described is
that of the values of the functions.

Here is an example with a typename as the argument:

typeof (int *)

Here the type described is that of pointers to 'int'.

If you are writing a header file that must work when included in
ANSI C programs, write '__typeof__' instead of 'typeof'. *Note
Alternate Keywords::.

A 'typeof'-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or
inside of 'sizeof' or 'typeof'.

* This declares 'y' with the type of what 'x' points to.

typeof (*x) y;

* This declares 'y' as an array of such values.

typeof (*x) y[4];

* This declares 'y' as an array of pointers to characters:

typeof (typeof (char *)[4]) y;

It is equivalent to the following traditional C declaration:

char *y[4];

To see the meaning of the declaration using 'typeof', and why it
might be a useful way to write, let's rewrite it with these
macros:

#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])

Now the declaration can be rewritten this way:

array (pointer (char), 4) y;

Thus, 'array (pointer (char), 4)' is the type of arrays of 4
pointers to 'char'.


File: gcc.info, Node: Lvalues, Next: Conditionals, Prev: Typeof, Up: Extensions

Generalized Lvalues
===================

Compound expressions, conditional expressions and casts are allowed
as lvalues provided their operands are lvalues. This means that you
can take their addresses or store values into them.

For example, a compound expression can be assigned, provided the
last expression in the sequence is an lvalue. These two expressions
are equivalent:

(a, b) += 5
a, (b += 5)

Similarly, the address of the compound expression can be taken.
These two expressions are equivalent:

&(a, b)
a, &b

A conditional expression is a valid lvalue if its type is not void
and the true and false branches are both valid lvalues. For example,
these two expressions are equivalent:

(a ? b : c) = 5
(a ? b = 5 : (c = 5))

A cast is a valid lvalue if its operand is an lvalue. A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression. After this is stored, the value is
converted back to the specified type to become the value of the
assignment. Thus, if 'a' has type 'char *', the following two
expressions are equivalent:

(int)a = 5
(int)(a = (char *)(int)5)

An assignment-with-arithmetic operation such as '+=' applied to a
cast performs the arithmetic using the type resulting from the cast,
and then continues as in the previous case. Therefore, these two
expressions are equivalent:

(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))

You cannot take the address of an lvalue cast, because the use of
its address would not work out coherently. Suppose that '&(int)f' were
permitted, where 'f' has type 'float'. Then the following statement
would try to store an integer bit-pattern where a floating point
number belongs:

*&(int)f = 1;

This is quite different from what '(int)f = 1' would do--that would
convert 1 to floating point and store it. Rather than cause this
inconsistency, we think it is better to prohibit use of '&' on a cast.

If you really do want an 'int *' pointer with the address of 'f',
you can simply write '(int *)&f'.


File: gcc.info, Node: Conditionals, Next: Long Long, Prev: Lvalues, Up: Extensions

Conditional Expressions with Omitted Operands
=============================================

The middle operand in a conditional expression may be omitted. Then
if the first operand is nonzero, its value is the value of the
conditional expression.

Therefore, the expression

x ? : y

has the value of 'x' if that is nonzero; otherwise, the value of 'y'.

This example is perfectly equivalent to

x ? x : y

In this simple case, the ability to omit the middle operand is not
especially useful. When it becomes useful is when the first operand
does, or may (if it is a macro argument), contain a side effect. Then
repeating the operand in the middle would perform the side effect
twice. Omitting the middle operand uses the value already computed
without the undesirable effects of recomputing it.


File: gcc.info, Node: Long Long, Next: Zero Length, Prev: Conditionals, Up: Extensions

Double-Word Integers
====================

GNU C supports data types for integers that are twice as long as
'long int'. Simply write 'long long int' for a signed integer, or
'unsigned long long int' for an unsigned integer.

You can use these types in arithmetic like any other integer types.
Addition, subtraction, and bitwise boolean operations on these types
are open-coded on all types of machines. Multiplication is open-coded
if the machine supports fullword-to-doubleword a widening multiply
instruction. Division and shifts are open-coded only on machines that
provide special support. The operations that are not open-coded use
special library routines that come with GNU CC.

There may be pitfalls when you use 'long long' types for function
arguments, unless you declare function prototypes. If a function
expects type 'int' for its argument, and you pass a value of type
'long long int', confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects 'long long int' and you pass 'int'.
The best way to avoid such problems is to use prototypes.


File: gcc.info, Node: Zero Length, Next: Variable Length, Prev: Long Long, Up: Extensions

Arrays of Length Zero
=====================

Zero-length arrays are allowed in GNU C. They are very useful as
the last element of a structure which is really a header for a
variable-length object:

struct line {
int length;
char contents[0];
};

{
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
}

In standard C, you would have to give 'contents' a length of 1,
which means either you waste space or complicate the argument to
'malloc'.


File: gcc.info, Node: Variable Length, Next: Subscripting, Prev: Zero Length, Up: Extensions

Arrays of Variable Length
=========================

Variable-length automatic arrays are allowed in GNU C. These
arrays are declared like any other automatic arrays, but with a length
that is not a constant expression. The storage is allocated at the
point of declaration and deallocated when the brace-level is exited.
For example:

FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}

Jumping or breaking out of the scope of the array name deallocates
the storage. Jumping into the scope is not allowed; you get an error
message for it.

You can use the function 'alloca' to get an effect much like
variable-length arrays. The function 'alloca' is available in many
other C implementations (but not in all). On the other hand,
variable-length arrays are more elegant.

There are other differences between these two methods. Space
allocated with 'alloca' exists until the containing *function* returns.
The space for a variable-length array is deallocated as soon as the
array name's scope ends. (If you use both variable-length arrays and
'alloca' in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with 'alloca'.)

You can also use variable-length arrays as arguments to functions:

struct entry
tester (int len, char data[len][len])
{
...
}

The length of an array is computed once when the storage is
allocated and is remembered for the scope of the array in case you
access it with 'sizeof'.

If you want to pass the array first and the length afterward, you
can use a forward declaration in the parameter list--another GNU
extension.

struct entry
tester (int len; char data[len][len], int len)
{
...
}

The 'int len' before the semicolon is a "parameter forward
declaration", and it serves the purpose of making the name 'len' known
when the declaration of 'data' is parsed.

You can write any number of such parameter forward declarations in
the parameter list. They can be separated by commas or semicolons,
but the last one must end with a semicolon, which is followed by the
"real" parameter declarations. Each forward declaration must match a
"real" declaration in parameter name and data type.


File: gcc.info, Node: Subscripting, Next: Pointer Arith, Prev: Variable Length, Up: Extensions

Non-Lvalue Arrays May Have Subscripts
=====================================

Subscripting is allowed on arrays that are not lvalues, even though
the unary '&' operator is not. For example, this is valid in GNU C
though not valid in other C dialects:

struct foo {int a[4];};

struct foo f();

bar (int index)
{
return f().a[index];
}


File: gcc.info, Node: Pointer Arith, Next: Initializers, Prev: Subscripting, Up: Extensions

Arithmetic on 'void'- and Function-Pointers
===========================================

In GNU C, addition and subtraction operations are supported on
pointers to 'void' and on pointers to functions. This is done by
treating the size of a 'void' or of a function as 1.

A consequence of this is that 'sizeof' is also allowed on 'void'
and on function types, and returns 1.

The option '-Wpointer-arith' requests a warning if these extensions
are used.


File: gcc.info, Node: Initializers, Next: Constructors, Prev: Pointer Arith, Up: Extensions

Non-Constant Initializers
=========================

The elements of an aggregate initializer for an automatic variable
are not required to be constant expressions in GNU C. Here is an
example of an initializer with run-time varying elements:

foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
...
}


File: gcc.info, Node: Constructors, Next: Labeled Elements, Prev: Initializers, Up: Extensions

Constructor Expressions
=======================

GNU C supports constructor expressions. A constructor looks like a
cast containing an initializer. Its value is an object of the type
specified in the cast, containing the elements specified in the
initializer.

Usually, the specified type is a structure. Assume that 'struct
foo' and 'structure' are declared as shown:

struct foo {int a; char b[2];} structure;

Here is an example of constructing a 'struct foo' with a constructor:

structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}

You can also construct an array. If all the elements of the
constructor are (made up of) simple constant expressions, suitable for
use in initializers, then the constructor is an lvalue and can be
coerced to a pointer to its first element, as shown here:

char **foo = (char *[]) { "x", "y", "z" };

Array constructors whose elements are not simple constants are not
very useful, because the constructor is not an lvalue. There are only
two valid ways to use it: to subscript it, or initialize an array
variable with it. The former is probably slower than a 'switch'
statement, while the latter does the same thing an ordinary C
initializer would do. Here is an example of subscripting an array
constructor:

output = ((int[]) { 2, x, 28 }) [input];

Constructor expressions for scalar types and union types are is
also allowed, but then the constructor expression is equivalent to a
cast.


File: gcc.info, Node: Labeled Elements, Next: Cast to Union, Prev: Constructors, Up: Extensions

Labeled Elements in Initializers
================================

Standard C requires the elements of an initializer to appear in a
fixed order, the same as the order of the elements in the array or
structure being initialized.

In GNU C you can give the elements in any order, specifying the
array indices or structure field names they apply to.

To specify an array index, write '[INDEX]' before the element
value. For example,

int a[6] = { [4] 29, [2] 15 };

is equivalent to

int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being
initialized is automatic.

In a structure initializer, specify the name of a field to
initialize with 'FIELDNAME:' before the element value. For example,
given the following structure,

struct point { int x, y; };

the following initialization

struct point p = { y: yvalue, x: xvalue };

is equivalent to

struct point p = { xvalue, yvalue };

You can also use an element label when initializing a union, to
specify which element of the union should be used. For example,

union foo { int i; double d; };

union foo f = { d: 4 };

will convert 4 to a 'double' to store it in the union using the second
element. By contrast, casting 4 to type 'union foo' would store it
into the union as the integer 'i', since it is an integer. (*Note
Cast to Union::.)

You can combine this technique of naming elements with ordinary C
initialization of successive elements. Each initializer element that
does not have a label applies to the next consecutive element of the
array or structure. For example,

int a[6] = { [1] v1, v2, [4] v4 };

is equivalent to

int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an 'enum' type. For
example:

int whitespace[256]
= { [' '] 1, ['\t'] 1, ['\h'] 1,
['\f'] 1, ['\n'] 1, ['\r'] 1 };


File: gcc.info, Node: Case Ranges, Next: Function Attributes, Prev: Cast to Union, Up: Extensions

Case Ranges
===========

You can specify a range of consecutive values in a single 'case'
label, like this:

case LOW ... HIGH:

This has the same effect as the proper number of individual 'case'
labels, one for each integer value from LOW to HIGH, inclusive.

This feature is especially useful for ranges of ASCII character
codes:

case 'A' ... 'Z':

*Be careful:* Write spaces around the '...', for otherwise it may
be parsed wrong when you use it with integer values. For example,
write this:

case 1 ... 5:

rather than this:

case 1...5:


File: gcc.info, Node: Cast to Union, Next: Case Ranges, Prev: Labeled Elements, Up: Extensions

Cast to a Union Type
====================

A cast to union type is like any other cast, except that the type
specified is a union type. You can specify the type either with
'union TAG' or with a typedef name.

The types that may be cast to the union type are those of the
members of the union. Thus, given the following union and variables:

union foo { int i; double d; };
int x;
double y;

both 'x' and 'y' can be cast to type 'union' foo.

Using the cast as the right-hand side of an assignment to a
variable of union type is equivalent to storing in a member of the
union:

union foo u;
...
u = (union foo) x == u.i = x
u = (union foo) y == u.d = y

You can also use the union cast as a function argument:

void hack (union foo);
...
hack ((union foo) x);


File: gcc.info, Node: Function Attributes, Next: Dollar Signs, Prev: Case Ranges, Up: Extensions

Declaring Attributes of Functions
=================================

In GNU C, you declare certain things about functions called in your
program which help the compiler optimize function calls.

A few standard library functions, such as 'abort' and 'exit',
cannot return. GNU CC knows this automatically. Some programs define
their own functions that never return. You can declare them
'volatile' to tell the compiler this fact. For example,

extern void volatile fatal ();

void
fatal (...)
{
... /* Print error message. */ ...

exit (1);
}

The 'volatile' keyword tells the compiler to assume that 'fatal'
cannot return. This makes slightly better code, but more importantly
it helps avoid spurious warnings of uninitialized variables.

It does not make sense for a 'volatile' function to have a return
type other than 'void'.

Many functions do not examine any values except their arguments, and
have no effects except the return value. Such a function can be
subject to common subexpression elimination and loop optimization just
as an arithmetic operator would be. These functions should be declared
'const'. For example,

extern int const square ();

says that the hypothetical function 'square' is safe to call fewer
times than the program says.

Note that a function that has pointer arguments and examines the
data pointed to must *not* be declared 'const'. Likewise, a function
that calls a non-'const' function usually must not be 'const'. It
does not make sense for a 'const' function to return 'void'.

We recommend placing the keyword 'const' after the function's
return type. It makes no difference in the example above, but when the
return type is a pointer, it is the only way to make the function
itself const. For example,

const char *mincp (int);

says that 'mincp' returns 'const char *'--a pointer to a const object.
To declare 'mincp' const, you must write this:

char * const mincp (int);

Some people object to this feature, suggesting that ANSI C's
'#pragma' should be used instead. There are two reasons for not doing
this.

1. It is impossible to generate '#pragma' commands from a macro.

2. The '#pragma' command is just as likely as these keywords to mean
something else in another compiler.

These two reasons apply to almost any application that might be
proposed for '#pragma'. It is basically a mistake to use '#pragma' for
*anything*.


File: gcc.info, Node: Dollar Signs, Next: Character Escapes, Prev: Function Attributes, Up: Extensions

Dollar Signs in Identifier Names
================================

In GNU C, you may use dollar signs in identifier names. This is
because many traditional C implementations allow such identifiers.

Dollar signs are allowed on certain machines if you specify
'-traditional'. On a few systems they are allowed by default, even if
'-traditional' is not used. But they are never allowed if you specify
'-ansi'.

There are certain ANSI C programs (obscure, to be sure) that would
compile incorrectly if dollar signs were permitted in identifiers. For
example:

#define foo(a) #a
#define lose(b) foo (b)
#define test$
lose (test)


File: gcc.info, Node: Character Escapes, Next: Variable Attributes, Prev: Dollar Signs, Up: Extensions

The Character ESC in Constants
==============================

You can use the sequence '\e' in a string or character constant to
stand for the ASCII character ESC.


File: gcc.info, Node: Alignment, Next: Inline, Prev: Variable Attributes, Up: Extensions

Inquiring on Alignment of Types or Variables
============================================

The keyword '__alignof__' allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type. Its
syntax is just like 'sizeof'.

For example, if the target machine requires a 'double' value to be
aligned on an 8-byte boundary, then '__alignof__ (double)' is 8. This
is true on many RISC machines. On more traditional machine designs,
'__alignof__ (double)' is 4 or even 2.

Some machines never actually require alignment; they allow
reference to any data type even at an odd addresses. For these
machines, '__alignof__' reports the *recommended* alignment of a type.

When the operand of '__alignof__' is an lvalue rather than a type,
the value is the largest alignment that the lvalue is known to have.
It may have this alignment as a result of its data type, or because it
is part of a structure and inherits alignment from that structure. For
example, after this declaration:

struct foo { int x; char y; } foo1;

the value of '__alignof__ (foo1.y)' is probably 2 or 4, the same as
'__alignof__ (int)', even though the data type of 'foo1.y' does not
itself demand any alignment.


File: gcc.info, Node: Variable Attributes, Next: Alignment, Prev: Character Escapes, Up: Extensions

Specifying Attributes of Variables
==================================

The keyword '__attribute__' allows you to specify special
attributes of variables or structure fields. The only attributes
currently defined are the 'aligned' and 'format' attributes.

The 'aligned' attribute specifies the alignment of the variable or
structure field. For example, the declaration:

int x __attribute__ ((aligned (16))) = 0;

causes the compiler to allocate the global variable 'x' on a 16-byte
boundary. On a 68000, this could be used in conjunction with an 'asm'
expression to access the 'move16' instruction which requires 16-byte
aligned operands.

You can also specify the alignment of structure fields. For
example, to create a double-word aligned 'int' pair, you could write:

struct foo { int x[2] __attribute__ ((aligned (8))); };

This is an alternative to creating a union with a 'double' member that
forces the union to be double-word aligned.

It is not possible to specify the alignment of functions; the
alignment of functions is determined by the machine's requirements and
cannot be changed.

The 'format' attribute specifies that a function takes 'printf' or
'scanf' style arguments which should be type-checked against a format
string. For example, the declaration:

extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));

causes the compiler to check the arguments in calls to 'my_printf' for
consistency with the 'printf' style format string argument 'my_format'.

The first parameter of the 'format' attribute determines how the
format string is interpreted, and should be either 'printf' or
'scanf'. The second parameter specifies the number of the format
string argument (starting from 1). The third parameter specifies the
number of the first argument which should be checked against the
format string. For functions where the arguments are not available to
be checked (such as 'vprintf'), specify the third parameter as zero.
In this case the compiler only checks the format string for
consistency.

In the example above, the format string ('my_format') is the second
argument to 'my_print' and the arguments to check start with the third
argument, so the correct parameters for the format attribute are 2 and
3.

The 'format' attribute allows you to identify your own functions
which take format strings as arguments, so that GNU CC can check the
calls to these functions for errors. The compiler always checks
formats for the ANSI library functions 'printf', 'fprintf', 'sprintf',
'scanf', 'fscanf', 'sscanf', 'vprintf', 'vfprintf' and 'vsprintf'
whenever such warnings are requested (using '-Wformat'), so there is no
need to modify the header file 'stdio.h'.


File: gcc.info, Node: Inline, Next: Extended Asm, Prev: Alignment, Up: Extensions

An Inline Function is As Fast As a Macro
========================================

By declaring a function 'inline', you can direct GNU CC to integrate
that function's code into the code for its callers. This makes
execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their
known values may permit simplifications at compile time so that not
all of the inline function's code needs to be included.

To declare a function inline, use the 'inline' keyword in its
declaration, like this:

inline int
inc (int *a)
{
(*a)++;
}

(If you are writing a header file to be included in ANSI C
programs, write '__inline__' instead of 'inline'. *Note Alternate
Keywords::.)

You can also make all "simple enough" functions inline with the
option '-finline-functions'. Note that certain usages in a function
definition can make it unsuitable for inline substitution.

When a function is both inline and 'static', if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GNU CC does not actually output assembler code for the
function, unless you specify the option '-fkeep-inline-functions'.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition). If there is a
nonintegrated call, then the function is compiled to assembler code as
usual. The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.

When an inline function is not 'static', then the compiler must
assume that there may be calls from other source files; since a global
symbol can be defined only once in any program, the function must not
be defined in the other source files, so the calls therein cannot be
integrated. Therefore, a non-'static' inline function is always
compiled on its own in the usual fashion.

If you specify both 'inline' and 'extern' in the function
definition, then the definition is used only for inlining. In no case
is the function compiled on its own, not even if you refer to its
address explicitly. Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.

This combination of 'inline' and 'extern' has almost the effect of
a macro. The way to use it is to put a function definition in a
header file with these keywords, and put another copy of the
definition (lacking 'inline' and 'extern') in a library file. The
definition in the header file will cause most calls to the function to
be inlined. If any uses of the function remain, they will refer to
the single copy in the library.


File: gcc.info, Node: Extended Asm, Next: Asm Labels, Prev: Inline, Up: Extensions

Assembler Instructions with C Expression Operands
=================================================

In an assembler instruction using 'asm', you can now specify the
operands of the instruction using C expressions. This means no more
guessing which registers or memory locations will contain the data you
want to use.

You must specify an assembler instruction template much like what
appears in a machine description, plus an operand constraint string
for each operand.

For example, here is how to use the 68881's 'fsinx' instruction:

asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here 'angle' is the C expression for the input operand while 'result'
is that of the output operand. Each has '"f"' as its operand
constraint, saying that a floating point register is required. The
'=' in '=f' indicates that the operand is an output; all output
operands' constraints must use '='. The constraints use the same
language used in the machine description (*note Constraints::.).

Each operand is described by an operand-constraint string followed by
the C expression in parentheses. A colon separates the assembler
template from the first output operand, and another separates the last
output operand from the first input, if any. Commas separate output
operands and separate inputs. The total number of operands is limited
to ten or to the maximum number of operands in any instruction pattern
in the machine description, whichever is greater.

If there are no output operands, and there are input operands, then
there must be two consecutive colons surrounding the place where the
output operands would go.

Output operand expressions must be lvalues; the compiler can check
this. The input operands need not be lvalues. The compiler cannot
check whether the operands have data types that are reasonable for the
instruction being executed. It does not parse the assembler
instruction template and does not know what it means, or whether it is
valid assembler input. The extended 'asm' feature is most often used
for machine instructions that the compiler itself does not know exist.

The output operands must be write-only; GNU CC will assume that the
values in these operands before the instruction are dead and need not
be generated. Extended asm does not support input-output or read-write
operands. For this reason, the constraint character '+', which
indicates such an operand, may not be used.

When the assembler instruction has a read-write operand, or an
operand in which only some of the bits are to be changed, you must
logically split its function into two separate operands, one input
operand and one write-only output operand. The connection between
them is expressed by constraints which say they need to be in the same
location when the instruction executes. You can use the same C
expression for both operands, or different expressions. For example,
here we write the (fictitious) 'combine' instruction with 'bar' as its
read-only source operand and 'foo' as its read-write destination:

asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint '"0"' for operand 1 says that it must occupy the same
location as operand 0. A digit in constraint is allowed only in an
input operand, and it must refer to an output operand.

Only a digit in the constraint can guarantee that one operand will
be in the same place as another. The mere fact that 'foo' is the
value of both operands is not enough to guarantee that they will be in
the same place in the generated assembler code. The following would
not work:

asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

Various optimizations or reloading could cause operands 0 and 1 to
be in different registers; GNU CC knows no reason not to do so. For
example, the compiler might find a copy of the value of 'foo' in one
register and use it for operand 1, but generate the output operand 0
in a different register (copying it afterward to 'foo''s own address).
Of course, since the register for operand 1 is not even mentioned in
the assembler code, the result will not work, but GNU CC can't tell
that.

Some instructions clobber specific hard registers. To describe
this, write a third colon after the input operands, followed by the
names of the clobbered hard registers (given as strings). Here is a
realistic example for the Vax:

asm volatile ("movc3 %0,%1,%2"
: /* no outputs */
: "g" (from), "g" (to), "g" (count)
: "r0", "r1", "r2", "r3", "r4", "r5");

If you refer to a particular hardware register from the assembler
code, then you will probably have to list the register after the third
colon to tell the compiler that the register's value is modified. In
many assemblers, the register names begin with '%'; to produce one '%'
in the assembler code, you must write '%%' in the input.

You can put multiple assembler instructions together in a single
'asm' template, separated either with newlines (written as '\n') or
with semicolons if the assembler allows such semicolons. The GNU
assembler allows semicolons and all Unix assemblers seem to do so.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like.
Here is an example of multiple instructions in a template; it assumes
that the subroutine '_foo' accepts arguments in registers 9 and 10:

asm ("movl %0,r9;movl %1,r10;call _foo"
: /* no outputs */
: "g" (from), "g" (to)
: "r9", "r10");

Unless an output operand has the '&' constraint modifier, GNU CC may
allocate it in the same register as an unrelated input operand, on the
assumption that the inputs are consumed before the outputs are
produced. This assumption may be false if the assembler code actually
consists of more than one instruction. In such a case, use '&' for
each output operand that may not overlap an input. *Note Modifiers::.

If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the 'asm'
construct, as follows:

asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
: "g" (result)
: "g" (input));

This assumes your assembler supports local labels, as the GNU assembler
and most Unix assemblers do.

Usually the most convenient way to use these 'asm' instructions is
to encapsulate them in macros that look like functions. For example,

#define sin(x) \
({ double __value, __arg = (x); \
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
__value; })

Here the variable '__arg' is used to make sure that the instruction
operates on a proper 'double' value, and to accept only those
arguments 'x' which can convert automatically to a 'double'.

Another way to make sure the instruction operates on the correct
data type is to use a cast in the 'asm'. This is different from using
a variable '__arg' in that it converts more different types. For
example, if the desired type were 'int', casting the argument to 'int'
would accept a pointer with no complaint, while assigning the argument
to an 'int' variable named '__arg' would warn about using a pointer
unless the caller explicitly casts it.

If an 'asm' has output operands, GNU CC assumes for optimization
purposes that the instruction has no side effects except to change the
output operands. This does not mean that instructions with a side
effect cannot be used, but you must be careful, because the compiler
may eliminate them if the output operands aren't used, or move them
out of loops, or replace two with one if they constitute a common
subexpression. Also, if your instruction does have a side effect on a
variable that otherwise appears not to change, the old value of the
variable may be reused later if it happens to be found in a register.

You can prevent an 'asm' instruction from being deleted, moved
significantly, or combined, by writing the keyword 'volatile' after
the 'asm'. For example:

#define set_priority(x) \
asm volatile ("set_priority %0": /* no outputs */ : "g" (x))

An instruction without output operands will not be deleted or moved
significantly, regardless, unless it is unreachable.

Note that even a volatile 'asm' instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions. You can't expect a sequence of volatile 'asm'
instructions to remain perfectly consecutive. If you want consecutive
output, use a single 'asm'.

It is a natural idea to look for a way to give access to the
condition code left by the assembler instruction. However, when we
attempted to implement this, we found no way to make it work reliably.
The problem is that output operands might need reloading, which would
result in additional following "store" instructions. On most
machines, these instructions would alter the condition code before
there was time to test it. This problem doesn't arise for ordinary
"test" and "compare" instructions because they don't have any output
operands.

If you are writing a header file that should be includable in ANSI C
programs, write '__asm__' instead of 'asm'. *Note Alternate
Keywords::.


File: gcc.info, Node: Asm Labels, Next: Explicit Reg Vars, Prev: Extended Asm, Up: Extensions

Controlling Names Used in Assembler Code
========================================

You can specify the name to be used in the assembler code for a C
function or variable by writing the 'asm' (or '__asm__') keyword after
the declarator as follows:

int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable 'foo' in the
assembler code should be 'myfoo' rather than the usual '_foo'.

On systems where an underscore is normally prepended to the name of
a C function or variable, this feature allows you to define names for
the linker that do not start with an underscore.

You cannot use 'asm' in this way in a function *definition*; but
you can get the same effect by writing a declaration for the function
before its definition and putting 'asm' there, like this:

extern func () asm ("FUNC");

func (x, y)
int x, y;
...

It is up to you to make sure that the assembler names you choose do
not conflict with any other assembler symbols. Also, you must not use
a register name; that would produce completely invalid assembler code.
GNU CC does not as yet have the ability to store static variables in
registers. Perhaps that will be added.


File: gcc.info, Node: Explicit Reg Vars, Next: Alternate Keywords, Prev: Asm Labels, Up: Extensions

Variables in Specified Registers
================================

GNU C allows you to put a few global variables into specified
hardware registers. You can also specify the register in which an
ordinary register variable should be allocated.

* Global register variables reserve registers throughout the
program. This may be useful in programs such as programming
language interpreters which have a couple of global variables
that are accessed very often.

* Local register variables in specific registers do not reserve the
registers. The compiler's data flow analysis is capable of
determining where the specified registers contain live values,
and where they are available for other uses.

These local variables are sometimes convenient for use with the
extended 'asm' feature (*note Extended Asm::.), if you want to
write one output of the assembler instruction directly into a
particular register. (This will work provided the register you
specify fits the constraints specified for that operand in the
'asm'.)

* Menu:

* Global Reg Vars::
* Local Reg Vars::


File: gcc.info, Node: Global Reg Vars, Next: Local Reg Vars, Up: Explicit Reg Vars

Defining Global Register Variables
----------------------------------

You can define a global register variable in GNU C like this:

register int *foo asm ("a5");

Here 'a5' is the name of the register which should be used. Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type. The register 'a5'
would be a good choice on a 68000 for a variable of pointer type. On
machines with register windows, be sure to choose a "global" register
that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals.
For example, some 68000 operating systems call this register '%a5'.

Eventually there may be a way of asking the compiler to choose a
register automatically, but first we need to figure out how it should
choose and how to enable you to guide the choice. No solution is
evident.

Defining a global register variable in a certain register reserves
that register entirely for this use, at least within the current
compilation. The register will not be allocated for any other purpose
in the functions in the current compilation. The register will not be
saved and restored by these functions. Stores into this register are
never deleted even if they would appear to be dead, but references may
be deleted or moved or simplified.

It is not safe to access the global register variables from signal
handlers, or from more than one thread of control, because the system
library routines may temporarily use the register for other things
(unless you recompile them specially for the task at hand).

It is not safe for one function that uses a global register
variable to call another such function 'foo' by way of a third function
'lose' that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared). This is
because 'lose' might save the register and put some other value there.
For example, you can't expect a global register variable to be
available in the comparison-function that you pass to 'qsort', since
'qsort' might have put something else in that register. (If you are
prepared to recompile 'qsort' with the same global register variable,
you can solve this problem.)

If you want to recompile 'qsort' or other source files which do not
actually use your global register variable, so that they will not use
that register for any other purpose, then it suffices to specify the
compiler option '-ffixed-REG'. You need not actually add a global
register declaration to their source code.

A function which can alter the value of a global register variable
cannot safely be called from a function compiled without this
variable, because it could clobber the value the caller expects to
find there on return. Therefore, the function which is the entry
point into the part of the program that uses the global register
variable must explicitly save and restore the value which belongs to
its caller.

On most machines, 'longjmp' will restore to each global register
variable the value it had at the time of the 'setjmp'. On some
machines, however, 'longjmp' will not change the value of global
register variables. To be portable, the function that called 'setjmp'
should make other arrangements to save the values of the global
register variables, and to restore them in a 'longjmp'. This way, the
same thing will happen regardless of what 'longjmp' does.

All global register variable declarations must precede all function
definitions. If such a declaration could appear after function
definitions, the declaration would be too late to prevent the register
from being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an
executable file has no means to supply initial contents for a register.

On the Sparc, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as 'getwd', as well as
the subroutines for division and remainder, modify g3 and g4. g1 and
g2 are local temporaries.

On the 68000, a2 ... a5 should be suitable, as should d2 ... d7.
Of course, it will not do to use more than a few of those.


File: gcc.info, Node: Local Reg Vars, Prev: Global Reg Vars, Up: Explicit Reg Vars

Specifying Registers for Local Variables
----------------------------------------

You can define a local register variable with a specified register
like this:

register int *foo asm ("a5");

Here 'a5' is the name of the register which should be used. Note that
this is the same syntax used for defining global register variables,
but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a
problem, since specific registers are most often useful with explicit
assembler instructions (*note Extended Asm::.). Both of these things
generally require that you conditionalize your program according to
cpu type.

In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals.
For example, some 68000 operating systems call this register '%a5'.

Eventually there may be a way of asking the compiler to choose a
register automatically, but first we need to figure out how it should
choose and how to enable you to guide the choice. No solution is
evident.

Defining such a register variable does not reserve the register; it
remains available for other uses in places where flow control
determines the variable's value is not live. However, these registers
are made unavailable for use in the reload pass. I would not be
surprised if excessive use of this feature leaves the compiler too few
available registers to compile certain functions.


File: gcc.info, Node: Alternate Keywords, Next: Incomplete Enums, Prev: Explicit Reg Vars, Up: Extensions

Alternate Keywords
==================

The option '-traditional' disables certain keywords; '-ansi'
disables certain others. This causes trouble when you want to use GNU
C extensions, or ANSI C features, in a general-purpose header file that
should be usable by all programs, including ANSI C programs and
traditional ones. The keywords 'asm', 'typeof' and 'inline' cannot be
used since they won't work in a program compiled with '-ansi', while
the keywords 'const', 'volatile', 'signed', 'typeof' and 'inline'
won't work in a program compiled with '-traditional'.

The way to solve these problems is to put '__' at the beginning and
end of each problematical keyword. For example, use '__asm__' instead
of 'asm', '__const__' instead of 'const', and '__inline__' instead of
'inline'.

Other C compilers won't accept these alternative keywords; if you
want to compile with another compiler, you can define the alternate
keywords as macros to replace them with the customary keywords. It
looks like this:

#ifndef __GNUC__
#define __asm__ asm
#endif

'-pedantic' causes warnings for many GNU C extensions. You can
prevent such warnings within one expression by writing '__extension__'
before the expression. '__extension__' has no effect aside from this.


File: gcc.info, Node: Incomplete Enums, Prev: Alternate Keywords, Up: Extensions

Incomplete 'enum' Types
=======================

You can define an 'enum' tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
'struct foo' without describing the elements. A later declaration
which does specify the possible values completes the type.

You can't allocate variables or storage using the type while it is
incomplete. However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of
'enum' more consistent with the way 'struct' and 'union' are handled.


File: gcc.info, Node: Bugs, Next: VMS, Prev: Extensions, Up: Top

Reporting Bugs
**************

Your bug reports play an essential role in making GNU CC reliable.

When you encounter a problem, the first thing to do is to see if it
is already known. *Note Trouble::. Also look in *Note
Incompatibilities::. If it isn't known, then you should report the
problem.

Reporting a bug may help you by bringing a solution to your
problem, or it may not. (If it does not, look in the service
directory; see *Note Service::.) In any case, the principal function
of a bug report is to help the entire community by making the next
version of GNU CC work better. Bug reports are your contribution to
the maintenance of GNU CC.

In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.

* Menu:

* Criteria: Bug Criteria. Have you really found a bug?
* Reporting: Bug Reporting. How to report a bug effectively.
* Non-bugs:: Some things we think are not problems.
* Known: Trouble. Known problems.
* Help: Service. Where to ask for help.


File: gcc.info, Node: Bug Criteria, Next: Bug Reporting, Prev: Bugs, Up: Bugs

Have You Found a Bug?
=====================

If you are not sure whether you have found a bug, here are some
guidelines:

* If the compiler gets a fatal signal, for any input whatever, that
is a compiler bug. Reliable compilers never crash.

* If the compiler produces invalid assembly code, for any input
whatever (except an 'asm' statement), that is a compiler bug,
unless the compiler reports errors (not just warnings) which
would ordinarily prevent the assembler from being run.

* If the compiler produces valid assembly code that does not
correctly execute the input source code, that is a compiler bug.

However, you must double-check to make sure, because you may have
run into an incompatibility between GNU C and traditional C
(*note Incompatibilities::.). These incompatibilities might be
considered bugs, but they are inescapable consequences of
valuable features.

Or you may have a program whose behavior is undefined, which
happened by chance to give the desired results with another C
compiler.

For example, in many nonoptimizing compilers, you can write 'x;'
at the end of a function instead of 'return x;', with the same
results. But the value of the function is undefined if 'return'
is omitted; it is not a bug when GNU CC produces different
results.

Problems often result from expressions with two increment
operators, as in 'f (*p++, *p++)'. Your previous compiler might
have interpreted that expression the way you intended; GNU CC
might interpret it another way. Neither compiler is wrong. The
bug is in your code.

After you have localized the error to a single source line, it
should be easy to check for these things. If your program is
correct and well defined, you have found a compiler bug.

* If the compiler produces an error message for valid input, that
is a compiler bug.

Note that the following is not valid input, and the error message
for it is not a bug:

int foo (char);

int
foo (x)
char x;
{ ... }

The prototype says to pass a 'char', while the definition says to
pass an 'int' and treat the value as a 'char'. This is what the
ANSI standard says, and it makes sense.

* If the compiler does not produce an error message for invalid
input, that is a compiler bug. However, you should note that
your idea of "invalid input" might be my idea of "an extension"
or "support for traditional practice".

* If you are an experienced user of C compilers, your suggestions
for improvement of GNU CC are welcome in any case.


File: gcc.info, Node: Bugs Reporting
How to Report Bugs
==================

Send bug reports for GNU C to one of these addresses:

[email protected]
{ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gcc

*Do not send bug reports to 'help-gcc', or to the newsgroup
'gnu.gcc.help'.* Most users of GNU CC do not want to receive bug
reports. Those that do, have asked to be on 'bug-gcc'.

The mailing list 'bug-gcc' has a newsgroup which serves as a
repeater. The mailing list and the newsgroup carry exactly the same
messages. Often people think of posting bug reports to the newsgroup
instead of mailing them. This appears to work, but it has one problem
which can be crucial: a newsgroup posting does not contain a mail path
back to the sender. Thus, if I need to ask for more information, I
may be unable to reach you. For this reason, it is better to send bug
reports to the mailing list.

As a last resort, send bug reports on paper to:

GNU Compiler Bugs
Free Software Foundation
675 Mass Ave
Cambridge, MA 02139

The fundamental principle of reporting bugs usefully is this:
*report all the facts*. If you are not sure whether to state a fact
or leave it out, state it!

Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter. Thus, you
might assume that the name of the variable you use in an example does
not matter. Well, probably it doesn't, but one cannot be sure.
Perhaps the bug is a stray memory reference which happens to fetch
from the location where that name is stored in memory; perhaps, if the
name were different, the contents of that location would fool the
compiler into doing the right thing despite the bug. Play it safe and
give a specific, complete example. That is the easiest thing for you
to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable me to fix
the bug if it is not known. It isn't very important what happens if
the bug is already known. Therefore, always write your bug reports on
the assumption that the bug is not known.

Sometimes people give a few sketchy facts and ask, "Does this ring a
bell?" Those bug reports are useless, and I urge everyone to *refuse
to respond to them* except to chide the sender to report bugs properly.

To enable me to fix the bug, you should include all these things:

* The version of GNU CC. You can get this by running it with the
'-v' option.

Without this, I won't know whether there is any point in looking
for the bug in the current version of GNU CC.

* A complete input file that will reproduce the bug. If the bug is
in the C preprocessor, send me a source file and any header files
that it requires. If the bug is in the compiler proper ('cc1'),
run your source file through the C preprocessor by doing 'gcc -E
SOURCEFILE > OUTFILE', then include the contents of OUTFILE in
the bug report. (Any '-I', '-D' or '-U' options that you used in
actual compilation should also be used when doing this.)

A single statement is not enough of an example. In order to
compile it, it must be embedded in a function definition; and the
bug might depend on the details of how this is done.

Without a real example I can compile, all I can do about your bug
report is wish you luck. It would be futile to try to guess how
to provoke the bug. For example, bugs in register allocation and
reloading frequently depend on every little detail of the function
they happen in.

* The command arguments you gave GNU CC to compile that example and
observe the bug. For example, did you use '-O'? To guarantee
you won't omit something important, list them all.

If I were to try to guess the arguments, I would probably guess
wrong and then I would not encounter the bug.

* The type of machine you are using, and the operating system name
and version number.

* The operands you gave to the 'configure' command when you
installed the compiler.

* A description of what behavior you observe that you believe is
incorrect. For example, "It gets a fatal signal," or, "There is
an incorrect assembler instruction in the output."

Of course, if the bug is that the compiler gets a fatal signal,
then I will certainly notice it. But if the bug is incorrect
output, I might not notice unless it is glaringly wrong. I won't
study all the assembler code from a 50-line C program just on the
off chance that it might be wrong.

Even if the problem you experience is a fatal signal, you should
still say so explicitly. Suppose something strange is going on,
such as, your copy of the compiler is out of synch, or you have
encountered a bug in the C library on your system. (This has
happened!) Your copy might crash and mine would not. If you
told me to expect a crash, then when mine fails to crash, I would
know that the bug was not happening for me. If you had not told
me to expect a crash, then I would not be able to draw any
conclusion from my observations.

Often the observed symptom is incorrect output when your program
is run. Sad to say, this is not enough information for me unless
the program is short and simple. If you send me a large program,
I don't have time to figure out how it would work if compiled
correctly, much less which line of it was compiled wrong. So you
will have to do that. Tell me which source line it is, and what
incorrect result happens when that line is executed. A person
who understands the program can find this as easily as a bug in
the program itself.

* If you send me examples of output from GNU CC, please use '-g'
when you make them. The debugging information includes source
line numbers which are essential for correlating the output with
the input.

* If you wish to suggest changes to the GNU CC source, send me
context diffs. If you even discuss something in the GNU CC
source, refer to it by context, not by line number.

The line numbers in my development sources don't match those in
your sources. Your line numbers would convey no useful
information to me.

* Additional information from a debugger might enable me to find a
problem on a machine which I do not have available myself.
However, you need to think when you collect this information if
you want it to have any chance of being useful.

For example, many people send just a backtrace, but that is never
useful by itself. A simple backtrace with arguments conveys
little about GNU CC because the compiler is largely data-driven;
the same functions are called over and over for different RTL
insns, doing different things depending on the details of the
insn.

Most of the arguments listed in the backtrace are useless because
they are pointers to RTL list structure. The numeric values of
the pointers, which the debugger prints in the backtrace, have no
significance whatever; all that matters is the contents of the
objects they point to (and most of the contents are other such
pointers).

In addition, most compiler passes consist of one or more loops
that scan the RTL insn sequence. The most vital piece of
information about such a loop--which insn it has reached--is
usually in a local variable, not in an argument.

What you need to provide in addition to a backtrace are the
values of the local variables for several stack frames up. When
a local variable or an argument is an RTX, first print its value
and then use the GDB command 'pr' to print the RTL expression
that it points to. (If GDB doesn't run on your machine, use your
debugger to call the function 'debug_rtx' with the RTX as an
argument.) In general, whenever a variable is a pointer, its
value is no use without the data it points to.

In addition, include a debugging dump from just before the pass
in which the crash happens. Most bugs involve a series of insns,
not just one.

Here are some things that are not necessary:

* A description of the envelope of the bug.

Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and
which changes will not affect it.

This is often time consuming and not very useful, because the way
I will find the bug is by running a single example under the
debugger with breakpoints, not by pure deduction from a series of
examples. I recommend that you save your time for something else.

Of course, if you can find a simpler example to report *instead*
of the original one, that is a convenience for me. Errors in the
output will be easier to spot, running under the debugger will
take less time, etc. Most GNU CC bugs involve just one function,
so the most straightforward way to simplify an example is to
delete all the function definitions except the one where the bug
occurs. Those earlier in the file may be replaced by external
declarations if the crucial function depends on them.
(Exception: inline functions may affect compilation of functions
defined later in the file.)

However, simplification is not vital; if you don't want to do
this, report the bug anyway and send me the entire test case you
used.

* A patch for the bug.

A patch for the bug does help me if it is a good one. But don't
omit the necessary information, such as the test case, on the
assumption that a patch is all I need. I might see problems with
your patch and decide to fix the problem another way, or I might
not understand it at all.

Sometimes with a program as complicated as GNU CC it is very hard
to construct an example that will make the program follow a
certain path through the code. If you don't send me the example,
I won't be able to construct one, so I won't be able to verify
that the bug is fixed.

And if I can't understand what bug you are trying to fix, or why
your patch should be an improvement, I won't install it. A test
case will help me to understand.

* A guess about what the bug is or what it depends on.

Such guesses are usually wrong. Even I can't guess right about
such things without first using the debugger to find the facts.


File: gcc.info, Node: Non-bugs, Prev: Bug Reporting, Up: Bugs

Certain Changes We Don't Want to Make
=====================================

This section lists changes that people frequently request, but which
we do not make because we think GNU CC is better without them.

* Checking the number and type of arguments to a function which has
an old-fashioned definition and no prototype.

Such a feature would work only occasionally--only for calls that
appear in the same file as the called function, following the
definition. The only way to check all calls reliably is to add a
prototype for the function. But adding a prototype eliminates
the motivation for this feature. So the feature is not
worthwhile.

* Warning about using an expression whose type is signed as a shift
count.

Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.

* Warning about assigning a signed value to an unsigned variable.

Such assignments must be very common; warning about them would
cause more annoyance than good.

* Warning when a non-void function value is ignored.

Coming as I do from a Lisp background, I balk at the idea that
there is something dangerous about discarding a value. There are
functions that return values which some callers may find useful;
it makes no sense to clutter the program with a cast to 'void'
whenever the value isn't useful.

* Making bitfields unsigned by default on particular machines where
"the ABI standard" says to do so.

The ANSI C standard leaves it up to the implementation whether a
bitfield declared plain 'int' is signed or not. This in effect
creates two alternative dialects of C.

The GNU C compiler supports both dialects; you can specify the
dialect you want with the option '-fsigned-bitfields' or
'-funsigned-bitfields'. However, this leaves open the question
of which dialect to use by default.

Currently, the preferred dialect makes plain bitfields signed,
because this is simplest. Since 'int' is the same as 'signed
int' in every other context, it is cleanest for them to be the
same in bitfields as well.

Some computer manufacturers have published Application Binary
Interface standards which specify that plain bitfields should be
unsigned. It is a mistake, however, to say anything about this
issue in an ABI. This is because the handling of plain bitfields
distinguishes two dialects of C. Both dialects are meaningful on
every type of machine. Whether a particular object file was
compiled using signed bitfields or unsigned is of no concern to
other object files, even if they access the same bitfields in the
same data structures.

A given program is written in one or the other of these two
dialects. The program stands a chance to work on most any
machine if it is compiled with the proper dialect. It is
unlikely to work at all if compiled with the wrong dialect.

Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields
differently on certain machines.

Occasionally users write programs intended only for a particular
machine type. On these occasions, the users would benefit if the
GNU C compiler were to support by default the same dialect as the
other compilers on that machine. But such applications are rare.
And users writing a program to run on more than one type of
machine cannot possibly benefit from this kind of compatibility.

This is why GNU CC does and will treat plain bitfields in the same
fashion on all types of machines (by default).

There are some arguments for making bitfields unsigned by default
on all machines. If, for example, this becomes a universal de
facto standard, it would make sense for GNU CC to go along with
it. This is something to be considered in the future.

(Of course, users strongly concerned about portability should
indicate explicitly in each bitfield whether it is signed or not.
In this way, they write programs which have the same meaning in
both C dialects.)

* Undefining '__STDC__' when '-ansi' is not used.

Currently, GNU CC defines '__STDC__' as long as you don't use
'-traditional'. This provides good results in practice.

Programmers normally use conditionals on '__STDC__' to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation. Since plain 'gcc'
supports all the features of ANSI C, the correct answer to these
questions is "yes".

Some users try to use '__STDC__' to check for the availability of
certain library facilities. This is actually incorrect usage in
an ANSI C program, because the ANSI C standard says that a
conforming freestanding implementation should define '__STDC__'
even though it does not have the library facilities. 'gcc -ansi
-pedantic' is a conforming freestanding implementation, and it is
therefore required to define '__STDC__', even though it does not
come with an ANSI C library.

Sometimes people say that defining '__STDC__' in a compiler that
does not completely conform to the ANSI C standard somehow
violates the standard. This is illogical. The standard is a
standard for compilers that claim to support ANSI C, such as 'gcc
-ansi'--not for other compilers such as plain 'gcc'. Whatever
the ANSI C standard says is relevant to the design of plain 'gcc'
without '-ansi' only for pragmatic reasons, not as a requirement.

* Undefining '__STDC__' in C++.

Programs written to compile with C++-to-C translators get the
value of '__STDC__' that goes with the C compiler that is
subsequently used. These programs must test '__STDC__' to
determine what kind of C preprocessor that compiler uses: whether
they should concatenate tokens in the ANSI C fashion or in the
traditional fashion.

These programs work properly with GNU C++ if '__STDC__' is
defined. They would not work otherwise.

In addition, many header files are written to provide prototypes
in ANSI C but not in traditional C. Many of these header files
can work without change in C++ provided '__STDC__' is defined.
If '__STDC__' is not defined, they will all fail, and will all
need to be changed to test explicitly for C++ as well.


File: gcc.info, Node: VMS, Next: Portability, Prev: Bugs, Up: Top

Using GNU CC on VMS
*******************

* Menu:

* Include Files and VMS:: Where the preprocessor looks for the include files.
* Global Declarations:: How to do globaldef, globalref and globalvalue with
GNU CC.
* VMS Misc:: Misc information.


File: gcc.info, Node: Include Files and VMS, Next: Global Declarations, Prev: VMS, Up: VMS

Include Files and VMS
=====================

Due to the differences between the filesystems of Unix and VMS, GNU
CC attempts to translate file names in '#include' into names that VMS
will understand. The basic strategy is to prepend a prefix to the
specification of the include file, convert the whole filename to a VMS
filename, and then try to open the file. GNU CC tries various prefixes
one by one until one of them succeeds:

1. The first prefix is the 'GNU_CC_INCLUDE:' logical name: this is
where GNU C header files are traditionally stored. If you wish
to store header files in non-standard locations, then you can
assign the logical 'GNU_CC_INCLUDE' to be a search list, where
each element of the list is suitable for use with a rooted
logical.

2. The next prefix tried is 'SYS$SYSROOT:[SYSLIB.]'. This is where
VAX-C header files are traditionally stored.

3. If the include file specification by itself is a valid VMS
filename, the preprocessor then uses this name with no prefix in
an attempt to open the include file.

4. If the file specification is not a valid VMS filename (i.e. does
not contain a device or a directory specifier, and contains a '/'
character), the preprocessor tries to convert it from Unix syntax
to VMS syntax.

Conversion works like this: the first directory name becomes a
device, and the rest of the directories are converted into
VMS-format directory names. For example, 'X11/foobar.h' is
translated to 'X11:[000000]foobar.h' or 'X11:foobar.h', whichever
one can be opened. This strategy allows you to assign a logical
name to point to the actual location of the header files.

5. If none of these strategies succeeds, the '#include' fails.

Include directives of the form:

#include foobar

are a common source of incompatibility between VAX-C and GNU CC. VAX-C
treats this much like a standard '#include ' directive.
That is incompatible with the ANSI C behavior implemented by GNU CC: to
expand the name 'foobar' as a macro. Macro expansion should
eventually yield one of the two standard formats for '#include':

#include "FILE"
#include

If you have this problem, the best solution is to modify the source
to convert the '#include' directives to one of the two standard forms.
That will work with either compiler. If you want a quick and dirty
fix, define the file names as macros with the proper expansion, like
this:

#define stdio

This will work, as long as the name doesn't conflict with anything else
in the program.

Another source of incompatibility is that VAX-C assumes that:

#include "foobar"

is actually asking for the file 'foobar.h'. GNU CC does not make this
assumption, and instead takes what you ask for literally; it tries to
read the file 'foobar'. The best way to avoid this problem is to
always specify the desired file extension in your include directives.

GNU CC for VMS is distributed with a set of include files that is
sufficient to compile most general purpose programs. Even though the
GNU CC distribution does not contain header files to define constants
and structures for some VMS system-specific functions, there is no
reason why you cannot use GNU CC with any of these functions. You
first may have to generate or create header files, either by using the
public domain utility 'UNSDL' (which can be found on a DECUS tape), or
by extracting the relevant modules from one of the system macro
libraries, and using an editor to construct a C header file.


File: gcc.info, Node: Global Declarations, Next: VMS Misc, Prev: Include Files and VMS, Up: VMS

Global Declarations and VMS
===========================

GNU CC does not provide the 'globalref', 'globaldef' and
'globalvalue' keywords of VAX-C. You can get the same effect with an
obscure feature of GAS, the GNU assembler. (This requires GAS version
1.39 or later.) The following macros allow you to use this feature in
a fairly natural way:

#ifdef __GNUC__
#define GLOBALREF(TYPE,NAME) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
#define GLOBALDEF(TYPE,NAME,VALUE) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
= VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \
= {VALUE}
#else
#define GLOBALREF(TYPE,NAME) \
globalref TYPE NAME
#define GLOBALDEF(TYPE,NAME,VALUE) \
globaldef TYPE NAME = VALUE
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
globalvalue TYPE NAME = VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
globalvalue TYPE NAME
#endif

(The '_$$PsectAttributes_GLOBALSYMBOL' prefix at the start of the name
is removed by the assembler, after it has modified the attributes of
the symbol). These macros are provided in the VMS binaries
distribution in a header file 'GNU_HACKS.H'. An example of the usage
is:

GLOBALREF (int, ijk);
GLOBALDEF (int, jkl, 0);

The macros 'GLOBALREF' and 'GLOBALDEF' cannot be used
straightforwardly for arrays, since there is no way to insert the array
dimension into the declaration at the right place. However, you can
declare an array with these macros if you first define a typedef for
the array type, like this:

typedef int intvector[10];
GLOBALREF (intvector, foo);

Array and structure initializers will also break the macros; you can
define the initializer to be a macro of its own, or you can expand the
'GLOBALDEF' macro by hand. You may find a case where you wish to use
the 'GLOBALDEF' macro with a large array, but you are not interested
in explicitly initializing each element of the array. In such cases
you can use an initializer like: '{0,}', which will initialize the
entire array to '0'.

A shortcoming of this implementation is that a variable declared
with 'GLOBALVALUEREF' or 'GLOBALVALUEDEF' is always an array. For
example, the declaration:

GLOBALVALUEREF(int, ijk);

declares the variable 'ijk' as an array of type 'int [1]'. This is
done because a globalvalue is actually a constant; its "value" is what
the linker would normally consider an address. That is not how an
integer value works in C, but it is how an array works. So treating
the symbol as an array name gives consistent results--with the
exception that the value seems to have the wrong type. *Don't try to
access an element of the array.* It doesn't have any elements. The
array "address" may not be the address of actual storage.

The fact that the symbol is an array may lead to warnings where the
variable is used. Insert type casts to avoid the warnings. Here is an
example; it takes advantage of the ANSI C feature allowing macros that
expand to use the same name as the macro itself.

GLOBALVALUEREF (int, ss$_normal);
GLOBALVALUEDEF (int, xyzzy,123);
#ifdef __GNUC__
#define ss$_normal ((int) ss$_normal)
#define xyzzy ((int) xyzzy)
#endif

Don't use 'globaldef' or 'globalref' with a variable whose type is
an enumeration type; this is not implemented. Instead, make the
variable an integer, and use a 'globalvaluedef' for each of the
enumeration values. An example of this would be:

#ifdef __GNUC__
GLOBALDEF (int, color, 0);
GLOBALVALUEDEF (int, RED, 0);
GLOBALVALUEDEF (int, BLUE, 1);
GLOBALVALUEDEF (int, GREEN, 3);
#else
enum globaldef color {RED, BLUE, GREEN = 3};
#endif


File: gcc.info, Node: VMS Misc, Prev: Global Declarations, Up: VMS

Other VMS Issues
================

GNU CC automatically arranges for 'main' to return 1 by default if
you fail to specify an explicit return value. This will be interpreted
by VMS as a status code indicating a normal successful completion.

Version 1 of GNU CC did not provide this default.

GNU CC on VMS works only with the GNU assembler, GAS. You need
version 1.37 or later of GAS in order to produce value debugging
information for the VMS debugger. Use the ordinary VMS linker with
the object files produced by GAS.

Under previous versions of GNU CC, the generated code would
occasionally give strange results when linked to the sharable
'VAXCRTL' library. Now this should work.

A caveat for use of 'const' global variables: the 'const' modifier
must be specified in every external declaration of the variable in all
of the source files that use that variable. Otherwise the linker will
issue warnings about conflicting attributes for the variable. Your
program will still work despite the warnings, but the variable will be
placed in writable storage.

The VMS linker does not distinguish between upper and lower case
letters in function and variable names. However, usual practice in C
is to distinguish case. Normally GNU CC (by means of the assembler
GAS) implements usual C behavior by augmenting each name that is not
all lower-case. A name is augmented by truncating it to at most 23
characters and then adding more characters at the end which encode the
case pattern the rest.

Name augmentation yields bad results for programs that use
precompiled libraries (such as Xlib) which were generated by another
compiler. You can use the compiler option '/NOCASE_HACK' to inhibit
augmentation; it makes external C functions and variables
case-independent as is usual on VMS. Alternatively, you could write
all references to the functions and variables in such libraries using
lower case; this will work on VMS, but is not portable to other
systems.

Function and variable names are handled somewhat differently with
GNU C++. The GNU C++ compiler performs "name mangling" on function
names, which means that it adds information to the function name to
describe the data types of the arguments that the function takes. One
result of this is that the name of a function can become very long.
Since the VMS linker only recognizes the first 31 characters in a name,
special action is taken to ensure that each function and variable has a
unique name that can be represented in 31 characters.

If the name (plus a name augmentation, if required) is less than 32
characters in length, then no special action is performed. If the name
is longer than 31 characters, the assembler (GAS) will generate a hash
string based upon the function name, truncate the function name to 23
characters, and append the hash string to the truncated name. If the
'/VERBOSE' compiler option is used, the assembler will print both the
full and truncated names of each symbol that is truncated.

The '/NOCASE_HACK' compiler option should not be used when you are
compiling programs that use libg++. libg++ has several instances of
objects (i.e. 'Filebuf' and 'filebuf') which become indistinguishable
in a case-insensitive environment. This leads to cases where you need
to inhibit augmentation selectively (if you were using libg++ and Xlib
in the same program, for example). There is no special feature for
doing this, but you can get the result by defining a macro for each
mixed case symbol for which you wish to inhibit augmentation. The
macro should expand into the lower case equivalent of itself. For
example:

#define StuDlyCapS studlycaps

These macro definitions can be placed in a header file to minimize
the number of changes to your source code.


File: gcc.info, Node: Portability, Next: Interface, Prev: VMS, Up: Top

GNU CC and Portability
**********************

The main goal of GNU CC was to make a good, fast compiler for
machines in the class that the GNU system aims to run on: 32-bit
machines that address 8-bit bytes and have several general registers.
Elegance, theoretical power and simplicity are only secondary.

GNU CC gets most of the information about the target machine from a
machine description which gives an algebraic formula for each of the
machine's instructions. This is a very clean way to describe the
target. But when the compiler needs information that is difficult to
express in this fashion, I have not hesitated to define an ad-hoc
parameter to the machine description. The purpose of portability is
to reduce the total work needed on the compiler; it was not of
interest for its own sake.

GNU CC does not contain machine dependent code, but it does contain
code that depends on machine parameters such as endianness (whether
the most significant byte has the highest or lowest address of the
bytes in a word) and the availability of autoincrement addressing. In
the RTL-generation pass, it is often necessary to have multiple
strategies for generating code for a particular kind of syntax tree,
strategies that are usable for different combinations of parameters.
Often I have not tried to address all possible cases, but only the
common ones or only the ones that I have encountered. As a result, a
new target may require additional strategies. You will know if this
happens because the compiler will call 'abort'. Fortunately, the new
strategies can be added in a machine-independent fashion, and will
affect only the target machines that need them.


File: gcc.info, Node: Interface, Next: Passes, Prev: Portability, Up: Top

Interfacing to GNU CC Output
****************************

GNU CC is normally configured to use the same function calling
convention normally in use on the target system. This is done with the
machine-description macros described (*note Target Macros::.).

However, returning of structure and union values is done
differently on some target machines. As a result, functions compiled
with PCC returning such types cannot be called from code compiled with
GNU CC, and vice versa. This does not cause trouble often because few
Unix library routines return structures or unions.

GNU CC code returns structures and unions that are 1, 2, 4 or 8
bytes long in the same registers used for 'int' or 'double' return
values. (GNU CC typically allocates variables of such types in
registers also.) Structures and unions of other sizes are returned by
storing them into an address passed by the caller (usually in a
register). The machine-description macros 'STRUCT_VALUE' and
'STRUCT_INCOMING_VALUE' tell GNU CC where to pass this address.

By contrast, PCC on most target machines returns structures and
unions of any size by copying the data into an area of static storage,
and then returning the address of that storage as if it were a pointer
value. The caller must copy the data from that memory area to the
place where the value is wanted. This is slower than the method used
by GNU CC, and fails to be reentrant.

On some target machines, such as RISC machines and the 80386, the
standard system convention is to pass to the subroutine the address of
where to return the value. On these machines, GNU CC has been
configured to be compatible with the standard compiler, when this
method is used. It may not be compatible for structures of 1, 2, 4 or
8 bytes.

GNU CC uses the system's standard convention for passing arguments.
On some machines, the first few arguments are passed in registers; in
others, all are passed on the stack. It would be possible to use
registers for argument passing on any machine, and this would probably
result in a significant speedup. But the result would be complete
incompatibility with code that follows the standard convention. So
this change is practical only if you are switching to GNU CC as the
sole C compiler for the system. We may implement register argument
passing on certain machines once we have a complete GNU system so that
we can compile the libraries with GNU CC.

On some machines (particularly the Sparc), certain types of
arguments are passed "by invisible reference". This means that the
value is stored in memory, and the address of the memory location is
passed to the subroutine.

If you use 'longjmp', beware of automatic variables. ANSI C says
that automatic variables that are not declared 'volatile' have
undefined values after a 'longjmp'. And this is all GNU CC promises
to do, because it is very difficult to restore register variables
correctly, and one of GNU CC's features is that it can put variables
in registers without your asking it to.

If you want a variable to be unaltered by 'longjmp', and you don't
want to write 'volatile' because old C compilers don't accept it, just
take the address of the variable. If a variable's address is ever
taken, even if just to compute it and ignore it, then the variable
cannot go in a register:

{
int careful;
&careful;
...
}

Code compiled with GNU CC may call certain library routines. Most
of them handle arithmetic for which there are no instructions. This
includes multiply and divide on some machines, and floating point
operations on any machine for which floating point support is disabled
with '-msoft-float'. Some standard parts of the C library, such as
'bcopy' or 'memcpy', are also called automatically. The usual
function call interface is used for calling the library routines.

These library routines should be defined in the library 'libgcc.a',
which GNU CC automatically searches whenever it links a program. On
machines that have multiply and divide instructions, if hardware
floating point is in use, normally 'libgcc.a' is not needed, but it is
searched just in case.

Each arithmetic function is defined in 'libgcc1.c' to use the
corresponding C arithmetic operator. As long as the file is compiled
with another C compiler, which supports all the C arithmetic operators,
this file will work portably. However, 'libgcc1.c' does not work if
compiled with GNU CC, because each arithmetic function would compile
into a call to itself!


File: gcc.info, Node: Passes, Next: RTL, Prev: Interface, Up: Top

Passes and Files of the Compiler
********************************

The overall control structure of the compiler is in 'toplev.c'.
This file is responsible for initialization, decoding arguments,
opening and closing files, and sequencing the passes.

The parsing pass is invoked only once, to parse the entire input.
The RTL intermediate code for a function is generated as the function
is parsed, a statement at a time. Each statement is read in as a
syntax tree and then converted to RTL; then the storage for the tree
for the statement is reclaimed. Storage for types (and the
expressions for their sizes), declarations, and a representation of
the binding contours and how they nest, remain until the function is
finished being compiled; these are all needed to output the debugging
information.

Each time the parsing pass reads a complete function definition or
top-level declaration, it calls the function 'rest_of_compilation' or
'rest_of_decl_compilation' in 'toplev.c', which are responsible for
all further processing necessary, ending with output of the assembler
language. All other compiler passes run, in sequence, within
'rest_of_compilation'. When that function returns from compiling a
function definition, the storage used for that function definition's
compilation is entirely freed, unless it is an inline function (*note
Inline::.).

Here is a list of all the passes of the compiler and their source
files. Also included is a description of where debugging dumps can be
requested with '-d' options.

* Parsing. This pass reads the entire text of a function
definition, constructing partial syntax trees. This and RTL
generation are no longer truly separate passes (formerly they
were), but it is easier to think of them as separate.

The tree representation does not entirely follow C syntax,
because it is intended to support other languages as well.

Language-specific data type analysis is also done in this pass,
and every tree node that represents an expression has a data type
attached. Variables are represented as declaration nodes.

Constant folding and some arithmetic simplifications are also done
during this pass.

The language-independent source files for parsing are
'stor-layout.c', 'fold-const.c', and 'tree.c'. There are also
header files 'tree.h' and 'tree.def' which define the format of
the tree representation.

The source files for parsing C are 'c-parse.y', 'c-decl.c',
'c-typeck.c', 'c-convert.c', 'c-lang.c', and 'c-aux-info.c' along
with header files 'c-lex.h', and 'c-tree.h'.

The source files for parsing C++ are 'cp-parse.y', 'cp-class.c',
'cp-cvt.c',
'cp-decl.c', 'cp-decl.c', 'cp-decl2.c', 'cp-dem.c', 'cp-except.c',
'cp-expr.c', 'cp-init.c', 'cp-lex.c', 'cp-method.c', 'cp-ptree.c',
'cp-search.c', 'cp-tree.c', 'cp-type2.c', and 'cp-typeck.c',
along with header files 'cp-tree.def', 'cp-tree.h', and
'cp-decl.h'.

The special source files for parsing Objective C are
'objc-parse.y', 'objc-actions.c', 'objc-tree.def', and
'objc-actions.h'. Certain C-specific files are used for this as
well.

The file 'c-common.c' is also used for all of the above languages.

* RTL generation. This is the conversion of syntax tree into RTL
code. It is actually done statement-by-statement during parsing,
but for most purposes it can be thought of as a separate pass.

This is where the bulk of target-parameter-dependent code is
found, since often it is necessary for strategies to apply only
when certain standard kinds of instructions are available. The
purpose of named instruction patterns is to provide this
information to the RTL generation pass.

Optimization is done in this pass for 'if'-conditions that are
comparisons, boolean operations or conditional expressions. Tail
recursion is detected at this time also. Decisions are made
about how best to arrange loops and how to output 'switch'
statements.

The source files for RTL generation include 'stmt.c',
'function.c', 'expr.c', 'calls.c', 'explow.c', 'expmed.c',
'optabs.c' and 'emit-rtl.c'. Also, the file 'insn-emit.c',
generated from the machine description by the program 'genemit',
is used in this pass. The header file 'expr.h' is used for
communication within this pass.

The header files 'insn-flags.h' and 'insn-codes.h', generated
from the machine description by the programs 'genflags' and
'gencodes', tell this pass which standard names are available for
use and which patterns correspond to them.

Aside from debugging information output, none of the following
passes refers to the tree structure representation of the
function (only part of which is saved).

The decision of whether the function can and should be expanded
inline in its subsequent callers is made at the end of rtl
generation. The function must meet certain criteria, currently
related to the size of the function and the types and number of
parameters it has. Note that this function may contain loops,
recursive calls to itself (tail-recursive functions can be
inlined!), gotos, in short, all constructs supported by GNU CC.
The file 'integrate.c' contains the code to save a function's rtl
for later inlining and to inline that rtl when the function is
called. The header file 'integrate.h' is also used for this
purpose.

The option '-dr' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.rtl' to
the input file name.

* Jump optimization. This pass simplifies jumps to the following
instruction, jumps across jumps, and jumps to jumps. It deletes
unreferenced labels and unreachable code, except that unreachable
code that contains a loop is not recognized as unreachable in
this pass. (Such loops are deleted later in the basic block
analysis.) It also converts some code originally written with
jumps into sequences of instructions that directly set values
from the results of comparisons, if the machine has such
instructions.

Jump optimization is performed two or three times. The first
time is immediately following RTL generation. The second time is
after CSE, but only if CSE says repeated jump optimization is
needed. The last time is right before the final pass. That
time, cross-jumping and deletion of no-op move instructions are
done together with the optimizations described above.

The source file of this pass is 'jump.c'.

The option '-dj' causes a debugging dump of the RTL code after
this pass is run for the first time. This dump file's name is
made by appending '.jump' to the input file name.

* Register scan. This pass finds the first and last use of each
register, as a guide for common subexpression elimination. Its
source is in 'regclass.c'.

* Jump threading. This pass detects a condition jump that branches
to an identical or inverse test. Such jumps can be 'threaded'
through the second conditional test. The source code for this
pass is in 'jump.c'. This optimization is only performed if
'-fthread-jumps' is enabled.

* Common subexpression elimination. This pass also does constant
propagation. Its source file is 'cse.c'. If constant
propagation causes conditional jumps to become unconditional or to
become no-ops, jump optimization is run again when CSE is
finished.

The option '-ds' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.cse' to
the input file name.

* Loop optimization. This pass moves constant expressions out of
loops, and optionally does strength-reduction and loop unrolling
as well. Its source files are 'loop.c' and 'unroll.c', plus the
header 'loop.h' used for communication between them. Loop
unrolling uses some functions in 'integrate.c' and the header
'integrate.h'.

The option '-dL' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.loop' to
the input file name.

* If '-frerun-cse-after-loop' was enabled, a second common
subexpression elimination pass is performed after the loop
optimization pass. Jump threading is also done again at this
time if it was specified.

The option '-dt' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.cse2' to
the input file name.

* Stupid register allocation is performed at this point in a
nonoptimizing compilation. It does a little data flow analysis as
well. When stupid register allocation is in use, the next pass
executed is the reloading pass; the others in between are skipped.
The source file is 'stupid.c'.

* Data flow analysis ('flow.c'). This pass divides the program
into basic blocks (and in the process deletes unreachable loops);
then it computes which pseudo-registers are live at each point in
the program, and makes the first instruction that uses a value
point at the instruction that computed the value.

This pass also deletes computations whose results are never used,
and combines memory references with add or subtract instructions
to make autoincrement or autodecrement addressing.

The option '-df' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.flow' to
the input file name. If stupid register allocation is in use,
this dump file reflects the full results of such allocation.

* Instruction combination ('combine.c'). This pass attempts to
combine groups of two or three instructions that are related by
data flow into single instructions. It combines the RTL
expressions for the instructions by substitution, simplifies the
result using algebra, and then attempts to match the result
against the machine description.

The option '-dc' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.combine'
to the input file name.

* Instruction scheduling ('sched.c'). This pass looks for
instructions whose output will not be available by the time that
it is used in subsequent instructions. (Memory loads and
floating point instructions often have this behavior on RISC
machines). It re-orders instructions within a basic block to try
to separate the definition and use of items that otherwise would
cause pipeline stalls.

Instruction scheduling is performed twice. The first time is
immediately after instruction combination and the second is
immediately after reload.

The option '-dS' causes a debugging dump of the RTL code after
this pass is run for the first time. The dump file's name is
made by appending '.sched' to the input file name.

* Register class preferencing. The RTL code is scanned to find out
which register class is best for each pseudo register. The source
file is 'regclass.c'.

* Local register allocation ('local-alloc.c'). This pass allocates
hard registers to pseudo registers that are used only within one
basic block. Because the basic block is linear, it can use fast
and powerful techniques to do a very good job.

The option '-dl' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.lreg' to
the input file name.

* Global register allocation ('global-alloc.c'). This pass
allocates hard registers for the remaining pseudo registers (those
whose life spans are not contained in one basic block).

* Reloading. This pass renumbers pseudo registers with the hardware
registers numbers they were allocated. Pseudo registers that did
not get hard registers are replaced with stack slots. Then it
finds instructions that are invalid because a value has failed to
end up in a register, or has ended up in a register of the wrong
kind. It fixes up these instructions by reloading the
problematical values temporarily into registers. Additional
instructions are generated to do the copying.

The reload pass also optionally eliminates the frame pointer and
inserts instructions to save and restore call-clobbered registers
around calls.

Source files are 'reload.c' and 'reload1.c', plus the header
'reload.h' used for communication between them.

The option '-dg' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.greg' to
the input file name.

* Instruction scheduling is repeated here to try to avoid pipeline

stalls due to memory loads generated for spilled pseudo registers.

The option '-dR' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.sched2'
to the input file name.

* Jump optimization is repeated, this time including cross-jumping
and deletion of no-op move instructions.

The option '-dJ' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.jump2'
to the input file name.

* Delayed branch scheduling. This optional pass attempts to find
instructions that can go into the delay slots of other
instructions, usually jumps and calls. The source file name is
'reorg.c'.

The option '-dd' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.dbr' to
the input file name.

* Conversion from usage of some hard registers to usage of a
register stack may be done at this point. Currently, this is
supported only for the floating-point registers of the Intel
80387 coprocessor. The source file name is 'reg-stack.c'.

The options '-dk' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending '.stack'
to the input file name.

* Final. This pass outputs the assembler code for the function.
It is also responsible for identifying spurious test and compare
instructions. Machine-specific peephole optimizations are
performed at the same time. The function entry and exit
sequences are generated directly as assembler code in this pass;
they never exist as RTL.

The source files are 'final.c' plus 'insn-output.c'; the latter
is generated automatically from the machine description by the
tool 'genoutput'. The header file 'conditions.h' is used for
communication between these files.

* Debugging information output. This is run after final because it
must output the stack slot offsets for pseudo registers that did
not get hard registers. Source files are 'dbxout.c' for DBX
symbol table format, 'sdbout.c' for SDB symbol table format, and
'dwarfout.c' for DWARF symbol table format.

Some additional files are used by all or many passes:

* Every pass uses 'machmode.def' and 'machmode.h' which define the
machine modes.

* Several passes use 'real.h', which defines the default
representation of floating point constants and how to operate on
them.

* All the passes that work with RTL use the header files 'rtl.h'
and 'rtl.def', and subroutines in file 'rtl.c'. The tools 'gen*'
also use these files to read and work with the machine
description RTL.

* Several passes refer to the header file 'insn-config.h' which
contains a few parameters (C macro definitions) generated
automatically from the machine description RTL by the tool
'genconfig'.

* Several passes use the instruction recognizer, which consists of
'recog.c' and 'recog.h', plus the files 'insn-recog.c' and
'insn-extract.c' that are generated automatically from the
machine description by the tools 'genrecog' and 'genextract'.

* Several passes use the header files 'regs.h' which defines the
information recorded about pseudo register usage, and
'basic-block.h' which defines the information recorded about
basic blocks.

* 'hard-reg-set.h' defines the type 'HARD_REG_SET', a bit-vector
with a bit for each hard register, and some macros to manipulate
it. This type is just 'int' if the machine has few enough hard
registers; otherwise it is an array of 'int' and some of the
macros expand into loops.

* Several passes use instruction attributes. A definition of the
attributes defined for a particular machine is in file
'insn-attr.h', which is generated from the machine description by
the program 'genattr'. The file 'insn-attrtab.c' contains
subroutines to obtain the attribute values for insns. It is
generated from the machine description by the program
'genattrtab'.


File: gcc.info, Node: RTL, Next: Machine Desc, Prev: Passes, Up: Top

RTL Representation
******************

Most of the work of the compiler is done on an intermediate
representation called register transfer language. In this language,
the instructions to be output are described, pretty much one by one,
in an algebraic form that describes what the instruction does.

RTL is inspired by Lisp lists. It has both an internal form, made
up of structures that point at other structures, and a textual form
that is used in the machine description and in printed debugging
dumps. The textual form uses nested parentheses to indicate the
pointers in the internal form.

* Menu:

* RTL Objects:: Expressions vs vectors vs strings vs integers.
* Accessors:: Macros to access expression operands or vector elts.
* Flags:: Other flags in an RTL expression.
* Machine Modes:: Describing the size and format of a datum.
* Constants:: Expressions with constant values.
* Regs and Memory:: Expressions representing register contents or memory.
* Arithmetic:: Expressions representing arithmetic on other expressions.
* Comparisons:: Expressions representing comparison of expressions.
* Bit Fields:: Expressions representing bit-fields in memory or reg.
* Conversions:: Extending, truncating, floating or fixing.
* RTL Declarations:: Declaring volatility, constancy, etc.
* Side Effects:: Expressions for storing in registers, etc.
* Incdec:: Embedded side-effects for autoincrement addressing.
* Assembler:: Representing 'asm' with operands.
* Insns:: Expression types for entire insns.
* Calls:: RTL representation of function call insns.
* Sharing:: Some expressions are unique; others *must* be copied.


File: gcc.info, Node: RTL Objects, Next: Accessors, Prev: RTL, Up: RTL

RTL Object Types
================

RTL uses four kinds of objects: expressions, integers, strings and
vectors. Expressions are the most important ones. An RTL expression
("RTX", for short) is a C structure, but it is usually referred to
with a pointer; a type that is given the typedef name 'rtx'.

An integer is simply an 'int'; their written form uses decimal
digits.

A string is a sequence of characters. In core it is represented as
a 'char *' in usual C fashion, and it is written in C syntax as well.
However, strings in RTL may never be null. If you write an empty
string in a machine description, it is represented in core as a null
pointer rather than as a pointer to a null character. In certain
contexts, these null pointers instead of strings are valid. Within
RTL code, strings are most commonly found inside 'symbol_ref'
expressions, but they appear in other contexts in the RTL expressions
that make up machine descriptions.

A vector contains an arbitrary number of pointers to expressions.
The number of elements in the vector is explicitly present in the
vector. The written form of a vector consists of square brackets
('[...]') surrounding the elements, in sequence and with whitespace
separating them. Vectors of length zero are not created; null
pointers are used instead.

Expressions are classified by "expression codes" (also called RTX
codes). The expression code is a name defined in 'rtl.def', which is
also (in upper case) a C enumeration constant. The possible expression
codes and their meanings are machine-independent. The code of an RTX
can be extracted with the macro 'GET_CODE (X)' and altered with
'PUT_CODE (X, NEWCODE)'.

The expression code determines how many operands the expression
contains, and what kinds of objects they are. In RTL, unlike Lisp,
you cannot tell by looking at an operand what kind of object it is.
Instead, you must know from its context--from the expression code of
the containing expression. For example, in an expression of code
'subreg', the first operand is to be regarded as an expression and the
second operand as an integer. In an expression of code 'plus', there
are two operands, both of which are to be regarded as expressions. In
a 'symbol_ref' expression, there is one operand, which is to be
regarded as a string.

Expressions are written as parentheses containing the name of the
expression type, its flags and machine mode if any, and then the
operands of the expression (separated by spaces).

Expression code names in the 'md' file are written in lower case,
but when they appear in C code they are written in upper case. In this
manual, they are shown as follows: 'const_int'.

In a few contexts a null pointer is valid where an expression is
normally wanted. The written form of this is '(nil)'.


File: gcc.info, Node: Accessors, Next: Flags, Prev: RTL Objects, Up: RTL

Access to Operands
==================

For each expression type 'rtl.def' specifies the number of contained
objects and their kinds, with four possibilities: 'e' for expression
(actually a pointer to an expression), 'i' for integer, 's' for
string, and 'E' for vector of expressions. The sequence of letters
for an expression code is called its "format". Thus, the format of
'subreg' is 'ei'.

A few other format characters are used occasionally:

'u'
'u' is equivalent to 'e' except that it is printed differently in
debugging dumps. It is used for pointers to insns.

'n'
'n' is equivalent to 'i' except that it is printed differently in
debugging dumps. It is used for the line number or code number
of a 'note' insn.

'S'
'S' indicates a string which is optional. In the RTL objects in
core, 'S' is equivalent to 's', but when the object is read, from
an 'md' file, the string value of this operand may be omitted.
An omitted string is taken to be the null string.

'V'
'V' indicates a vector which is optional. In the RTL objects in
core, 'V' is equivalent to 'E', but when the object is read from
an 'md' file, the vector value of this operand may be omitted.
An omitted vector is effectively the same as a vector of no
elements.

'0'
'0' means a slot whose contents do not fit any normal category.
'0' slots are not printed at all in dumps, and are often used in
special ways by small parts of the compiler.

There are macros to get the number of operands, the format, and the
class of an expression code:

'GET_RTX_LENGTH (CODE)'
Number of operands of an RTX of code CODE.

'GET_RTX_FORMAT (CODE)'
The format of an RTX of code CODE, as a C string.

'GET_RTX_CLASS (CODE)'
A single character representing the type of RTX operation that
code CODE performs.

The following classes are defined:

'o'
An RTX code that represents an actual object, such as 'reg'
or 'mem'. 'subreg' is not in this class.

'<'
An RTX code for a comparison. The codes in this class are
'NE', 'EQ', 'LE', 'LT', 'GE', 'GT', 'LEU', 'LTU', 'GEU',
'GTU'.

'1'
An RTX code for a unary arithmetic operation, such as 'neg'.

'c'
An RTX code for a commutative binary operation, other than
'NE' and 'EQ' (which have class '<').

'2'
An RTX code for a noncommutative binary operation, such as
'MINUS'.

'b'
An RTX code for a bitfield operation ('ZERO_EXTRACT' and
'SIGN_EXTRACT').

'3'
An RTX code for other three input operations, such as
'IF_THEN_ELSE'.

'i'
An RTX code for a machine insn ('INSN', 'JUMP_INSN', and
'CALL_INSN').

'm'
An RTX code for something that matches in insns, such as
'MATCH_DUP'.

'x'
All other RTX codes.

Operands of expressions are accessed using the macros 'XEXP',
'XINT' and 'XSTR'. Each of these macros takes two arguments: an
expression-pointer (RTX) and an operand number (counting from zero).
Thus,

XEXP (X, 2)

accesses operand 2 of expression X, as an expression.

XINT (X, 2)

accesses the same operand as an integer. 'XSTR', used in the same
fashion, would access it as a string.

Any operand can be accessed as an integer, as an expression or as a
string. You must choose the correct method of access for the kind of
value actually stored in the operand. You would do this based on the
expression code of the containing expression. That is also how you
would know how many operands there are.

For example, if X is a 'subreg' expression, you know that it has
two operands which can be correctly accessed as 'XEXP (X, 0)' and
'XINT (X, 1)'. If you did 'XINT (X, 0)', you would get the address of
the expression operand but cast as an integer; that might occasionally
be useful, but it would be cleaner to write '(int) XEXP (X, 0)'.
'XEXP (X, 1)' would also compile without error, and would return the
second, integer operand cast as an expression pointer, which would
probably result in a crash when accessed. Nothing stops you from
writing 'XEXP (X, 28)' either, but this will access memory past the
end of the expression with unpredictable results.

Access to operands which are vectors is more complicated. You can
use the macro 'XVEC' to get the vector-pointer itself, or the macros
'XVECEXP' and 'XVECLEN' to access the elements and length of a vector.

'XVEC (EXP, IDX)'
Access the vector-pointer which is operand number IDX in EXP.

'XVECLEN (EXP, IDX)'
Access the length (number of elements) in the vector which is in
operand number IDX in EXP. This value is an 'int'.

'XVECEXP (EXP, IDX, ELTNUM)'
Access element number ELTNUM in the vector which is in operand
number IDX in EXP. This value is an RTX.

It is up to you to make sure that ELTNUM is not negative and is
less than 'XVECLEN (EXP, IDX)'.

All the macros defined in this section expand into lvalues and
therefore can be used to assign the operands, lengths and vector
elements as well as to access them.


File: gcc.info, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL

Flags in an RTL Expression
==========================

RTL expressions contain several flags (one-bit bit-fields) that are
used in certain types of expression. Most often they are accessed
with the following macros:

'MEM_VOLATILE_P (X)'
In 'mem' expressions, nonzero for volatile memory references.
Stored in the 'volatil' field and printed as '/v'.

'MEM_IN_STRUCT_P (X)'
In 'mem' expressions, nonzero for reference to an entire
structure, union or array, or to a component of one. Zero for
references to a scalar variable or through a pointer to a scalar.
Stored in the 'in_struct' field and printed as '/s'.

'REG_LOOP_TEST_P'
In 'reg' expressions, nonzero if this register's entire life is
contained in the exit test code for some loop. Stored in the
'in_struct' field and printed as '/s'.

'REG_USERVAR_P (X)'
In a 'reg', nonzero if it corresponds to a variable present in
the user's source code. Zero for temporaries generated
internally by the compiler. Stored in the 'volatil' field and
printed as '/v'.

'REG_FUNCTION_VALUE_P (X)'
Nonzero in a 'reg' if it is the place in which this function's
value is going to be returned. (This happens only in a hard
register.) Stored in the 'integrated' field and printed as '/i'.

The same hard register may be used also for collecting the values
of functions called by this one, but 'REG_FUNCTION_VALUE_P' is
zero in this kind of use.

'RTX_UNCHANGING_P (X)'
Nonzero in a 'reg' or 'mem' if the value is not changed. (This
flag is not set for memory references via pointers to constants.
Such pointers only guarantee that the object will not be changed
explicitly by the current function. The object might be changed
by other functions or by aliasing.) Stored in the 'unchanging'
field and printed as '/u'.

'RTX_INTEGRATED_P (INSN)'
Nonzero in an insn if it resulted from an in-line function call.
Stored in the 'integrated' field and printed as '/i'. This may
be deleted; nothing currently depends on it.

'SYMBOL_REF_USED (X)'
In a 'symbol_ref', indicates that X has been used. This is
normally only used to ensure that X is only declared external
once. Stored in the 'used' field.

'SYMBOL_REF_FLAG (X)'
In a 'symbol_ref', this is used as a flag for machine-specific
purposes. Stored in the 'volatil' field and printed as '/v'.

'LABEL_OUTSIDE_LOOP_P'
In 'label_ref' expressions, nonzero if this is a reference to a
label that is outside the innermost loop containing the reference
to the label. Stored in the 'in_struct' field and printed as
'/s'.

'INSN_DELETED_P (INSN)'
In an insn, nonzero if the insn has been deleted. Stored in the
'volatil' field and printed as '/v'.

'INSN_ANNULLED_BRANCH_P (INSN)'
In an 'insn' in the delay slot of a branch insn, indicates that an
annulling branch should be used. See the discussion under
'sequence' below. Stored in the 'unchanging' field and printed
as '/u'.

'INSN_FROM_TARGET_P (INSN)'
In an 'insn' in a delay slot of a branch, indicates that the insn
is from the target of the branch. If the branch insn has
'INSN_ANNULLED_BRANCH_P' set, this insn should only be executed if
the branch is taken. For annulled branches with this bit clear,
the insn should be executed only if the branch is not taken.
Stored in the 'in_struct' field and printed as '/s'.

'CONSTANT_POOL_ADDRESS_P (X)'
Nonzero in a 'symbol_ref' if it refers to part of the current
function's "constants pool". These are addresses close to the
beginning of the function, and GNU CC assumes they can be
addressed directly (perhaps with the help of base registers).
Stored in the 'unchanging' field and printed as '/u'.

'CONST_CALL_P (X)'
In a 'call_insn', indicates that the insn represents a call to a
const function. Stored in the 'unchanging' field and printed as
'/u'.

'LABEL_PRESERVE_P (X)'
In a 'code_label', indicates that the label can never be deleted.
Labels referenced by a a non-local goto will have this bit set.
Stored in the 'in_struct' field and printed as '/s'.

'SCHED_GROUP_P (INSN)'
During instruction scheduling, in an insn, indicates that the
previous insn must be scheduled together with this insn. This is
used to ensure that certain groups of instructions will not be
split up by the instruction scheduling pass, for example, 'use'
insns before a 'call_insn' may not be separated from the
'call_insn'. Stored in the 'in_struct' field and printed as '/s'.

These are the fields which the above macros refer to:

'used'
Normally, this flag is used only momentarily, at the end of RTL
generation for a function, to count the number of times an
expression appears in insns. Expressions that appear more than
once are copied, according to the rules for shared structure
(*note Sharing::.).

In a 'symbol_ref', it indicates that an external declaration for
the symbol has already been written.

In a 'reg', it is used by the leaf register renumbering code to
ensure that each register is only renumbered once.

'volatil'
This flag is used in 'mem','symbol_ref' and 'reg' expressions and
in insns. In RTL dump files, it is printed as '/v'.

In a 'mem' expression, it is 1 if the memory reference is
volatile. Volatile memory references may not be deleted,
reordered or combined.

In a 'symbol_ref' expression, it is used for machine-specific
purposes.

In a 'reg' expression, it is 1 if the value is a user-level
variable. 0 indicates an internal compiler temporary.

In an insn, 1 means the insn has been deleted.

'in_struct'
In 'mem' expressions, it is 1 if the memory datum referred to is
all or part of a structure or array; 0 if it is (or might be) a
scalar variable. A reference through a C pointer has 0 because
the pointer might point to a scalar variable. This information
allows the compiler to determine something about possible cases
of aliasing.

In an insn in the delay slot of a branch, 1 means that this insn
is from the target of the branch.

During instruction scheduling, in an insn, 1 means that this insn
must be scheduled as part of a group together with the previous
insn.

In 'reg' expressions, it is 1 if the register has its entire life
contained within the test expression of some loopl.

In 'label_ref' expressions, 1 means that the referenced label is
outside the innermost loop containing the insn in which the
'label_ref' was found.

In 'code_label' expressions, it is 1 if the label may never be
deleted. This is used for labels which are the target of
non-local gotos.

In an RTL dump, this flag is represented as '/s'.

'unchanging'
In 'reg' and 'mem' expressions, 1 means that the value of the
expression never changes.

In an insn, 1 means that this is an annulling branch.

In a 'symbol_ref' expression, 1 means that this symbol addresses
something in the per-function constants pool.

In a 'call_insn', 1 means that this instruction is a call to a
const function.

In an RTL dump, this flag is represented as '/u'.

'integrated'
In some kinds of expressions, including insns, this flag means the
rtl was produced by procedure integration.

In a 'reg' expression, this flag indicates the register
containing the value to be returned by the current function. On
machines that pass parameters in registers, the same register
number may be used for parameters as well, but this flag is not
set on such uses.


File: gcc.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL

Machine Modes
=============

A machine mode describes a size of data object and the
representation used for it. In the C code, machine modes are
represented by an enumeration type, 'enum machine_mode', defined in
'machmode.def'. Each RTL expression has room for a machine mode and
so do certain kinds of tree expressions (declarations and types, to be
precise).

In debugging dumps and machine descriptions, the machine mode of an
RTL expression is written after the expression code with a colon to
separate them. The letters 'mode' which appear at the end of each
machine mode name are omitted. For example, '(reg:SI 38)' is a 'reg'
expression with machine mode 'SImode'. If the mode is 'VOIDmode', it
is not written at all.

Here is a table of machine modes. The term "byte" below refers to
an object of 'BITS_PER_UNIT' bits (*note Storage Layout::.).

'QImode'
"Quarter-Integer" mode represents a single byte treated as an
integer.

'HImode'
"Half-Integer" mode represents a two-byte integer.

'PSImode'
"Partial Single Integer" mode represents an integer which occupies
four bytes but which doesn't really use all four. On some
machines, this is the right mode to use for pointers.

'SImode'
"Single Integer" mode represents a four-byte integer.

'PDImode'
"Partial Double Integer" mode represents an integer which occupies
eight bytes but which doesn't really use all eight. On some
machines, this is the right mode to use for certain pointers.

'DImode'
"Double Integer" mode represents an eight-byte integer.

'TImode'
"Tetra Integer" (?) mode represents a sixteen-byte integer.

'SFmode'
"Single Floating" mode represents a single-precision (four byte)
floating point number.

'DFmode'
"Double Floating" mode represents a double-precision (eight byte)
floating point number.

'XFmode'
"Extended Floating" mode represents a triple-precision (twelve
byte) floating point number. This mode is used for IEEE extended
floating point.

'TFmode'
"Tetra Floating" mode represents a quadruple-precision (sixteen
byte) floating point number.

'CCmode'
"Condition Code" mode represents the value of a condition code,
which is a machine-specific set of bits used to represent the
result of a comparison operation. Other machine-specific modes
may also be used for the condition code. These modes are not
used on machines that use 'cc0' (see *note Condition Code::.).

'BLKmode'
"Block" mode represents values that are aggregates to which none
of the other modes apply. In RTL, only memory references can
have this mode, and only if they appear in string-move or vector
instructions. On machines which have no such instructions,
'BLKmode' will not appear in RTL.

'VOIDmode'
Void mode means the absence of a mode or an unspecified mode.
For example, RTL expressions of code 'const_int' have mode
'VOIDmode' because they can be taken to have whatever mode the
context requires. In debugging dumps of RTL, 'VOIDmode' is
expressed by the absence of any mode.

'SCmode, DCmode, XCmode, TCmode'
These modes stand for a complex number represented as a pair of
floating point values. The values are in 'SFmode', 'DFmode',
'XFmode', and 'TFmode', respectively. Since C does not support
complex numbers, these machine modes are only partially
implemented.

The machine description defines 'Pmode' as a C macro which expands
into the machine mode used for addresses. Normally this is the mode
whose size is 'BITS_PER_WORD', 'SImode' on 32-bit machines.

The only modes which a machine description must support are
'QImode', and the modes corresponding to 'BITS_PER_WORD',
'FLOAT_TYPE_SIZE' and 'DOUBLE_TYPE_SIZE'. The compiler will attempt
to use 'DImode' for 8-byte structures and unions, but this can be
prevented by overriding the definition of 'MAX_FIXED_MODE_SIZE'.
Alternatively, you can have the compiler use 'TImode' for 16-byte
structures and unions. Likewise, you can arrange for the C type
'short int' to avoid using 'HImode'.

Very few explicit references to machine modes remain in the
compiler and these few references will soon be removed. Instead, the
machine modes are divided into mode classes. These are represented by
the enumeration type 'enum mode_class' defined in 'machmode.h'. The
possible mode classes are:

'MODE_INT'
Integer modes. By default these are 'QImode', 'HImode',
'SImode', 'DImode', and 'TImode'.

'MODE_PARTIAL_INT'
The "partial integer" modes, 'PSImode' and 'PDImode'.

'MODE_FLOAT'
floating point modes. By default these are 'SFmode', 'DFmode',
'XFmode' and 'TFmode'.

'MODE_COMPLEX_INT'
Complex integer modes. (These are not currently implemented).

'MODE_COMPLEX_FLOAT'
Complex floating point modes. By default these are 'SCmode',
'DCmode', 'XCmode', and 'TCmode'.

'MODE_FUNCTION'
Algol or Pascal function variables including a static chain.
(These are not currently implemented).

'MODE_CC'
Modes representing condition code values. These are 'CCmode' plus
any modes listed in the 'EXTRA_CC_MODES' macro. *Note Jump
Patterns::, also see *Note Condition Code::.

'MODE_RANDOM'
This is a catchall mode class for modes which don't fit into the
above classes. Currently 'VOIDmode' and 'BLKmode' are in
'MODE_RANDOM'.

Here are some C macros that relate to machine modes:

'GET_MODE (X)'
Returns the machine mode of the RTX X.

'PUT_MODE (X, NEWMODE)'
Alters the machine mode of the RTX X to be NEWMODE.

'NUM_MACHINE_MODES'
Stands for the number of machine modes available on the target
machine. This is one greater than the largest numeric value of
any machine mode.

'GET_MODE_NAME (M)'
Returns the name of mode M as a string.

'GET_MODE_CLASS (M)'
Returns the mode class of mode M.

'GET_MODE_WIDER_MODE (M)'
Returns the next wider natural mode. E.g.,
'GET_WIDER_MODE(QImode)' returns 'HImode'.

'GET_MODE_SIZE (M)'
Returns the size in bytes of a datum of mode M.

'GET_MODE_BITSIZE (M)'
Returns the size in bits of a datum of mode M.

'GET_MODE_MASK (M)'
Returns a bitmask containing 1 for all bits in a word that fit
within mode M. This macro can only be used for modes whose
bitsize is less than or equal to 'HOST_BITS_PER_INT'.

'GET_MODE_ALIGNMENT (M))'
Return the required alignment, in bits, for an object of mode M.

'GET_MODE_UNIT_SIZE (M)'
Returns the size in bytes of the subunits of a datum of mode M.
This is the same as 'GET_MODE_SIZE' except in the case of complex
modes. For them, the unit size is the size of the real or
imaginary part.

'GET_MODE_NUNITS (M)'
Returns the number of units contained in a mode, i.e.,
'GET_MODE_SIZE' divided by 'GET_MODE_UNIT_SIZE'.

'GET_CLASS_NARROWEST_MODE (C)'
Returns the narrowest mode in mode class C.

The global variables 'byte_mode' and 'word_mode' contain modes
whose classes are 'MODE_INT' and whose bitsizes are 'BITS_PER_UNIT' or
'BITS_PER_WORD', respectively. On 32-bit machines, these are 'QImode'
and 'SImode', respectively.


File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL
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File: gcc.info, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL

Registers and Memory
====================

Here are the RTL expression types for describing access to machine
registers and to main memory.

'(reg:M N)'
For small values of the integer N (less than
'FIRST_PSEUDO_REGISTER'), this stands for a reference to machine
register number N: a "hard register". For larger values of N, it
stands for a temporary value or "pseudo register". The
compiler's strategy is to generate code assuming an unlimited
number of such pseudo registers, and later convert them into hard
registers or into memory references.

M is the machine mode of the reference. It is necessary because
machines can generally refer to each register in more than one
mode. For example, a register may contain a full word but there
may be instructions to refer to it as a half word or as a single
byte, as well as instructions to refer to it as a floating point
number of various precisions.

Even for a register that the machine can access in only one mode,
the mode must always be specified.

The symbol 'FIRST_PSEUDO_REGISTER' is defined by the machine
description, since the number of hard registers on the machine is
an invariant characteristic of the machine. Note, however, that
not all of the machine registers must be general registers. All
the machine registers that can be used for storage of data are
given hard register numbers, even those that can be used only in
certain instructions or can hold only certain types of data.

A hard register may be accessed in various modes throughout one
function, but each pseudo register is given a natural mode and is
accessed only in that mode. When it is necessary to describe an
access to a pseudo register using a nonnatural mode, a 'subreg'
expression is used.

A 'reg' expression with a machine mode that specifies more than
one word of data may actually stand for several consecutive
registers. If in addition the register number specifies a
hardware register, then it actually represents several
consecutive hardware registers starting with the specified one.

Each pseudo register number used in a function's RTL code is
represented by a unique 'reg' expression.

Some pseudo register numbers, those within the range of
'FIRST_VIRTUAL_REGISTER' to 'LAST_VIRTUAL_REGISTER' only appear
during the RTL generation phase and are eliminated before the
optimization phases. These represent locations in the stack
frame that cannot be determined until RTL generation for the
function has been completed. The following virtual register
numbers are defined:

'VIRTUAL_INCOMING_ARGS_REGNUM'
This points to the first word of the incoming arguments
passed on the stack. Normally these arguments are placed
there by the caller, but the callee may have pushed some
arguments that were previously passed in registers.

When RTL generation is complete, this virtual register is
replaced by the sum of the register given by
'ARG_POINTER_REGNUM' and the value of 'FIRST_PARM_OFFSET'.

'VIRTUAL_STACK_VARS_REGNUM'
If 'FRAME_GROWS_DOWNWARDS' is defined, this points to
immediately above the first variable on the stack.
Otherwise, it points to the first variable on the stack.

It is replaced with the sum of the register given by
'FRAME_POINTER_REGNUM' and the value 'STARTING_FRAME_OFFSET'.

'VIRTUAL_STACK_DYNAMIC_REGNUM'
This points to the location of dynamically allocated memory
on the stack immediately after the stack pointer has been
adjusted by the amount of memory desired.

It is replaced by the sum of the register given by
'STACK_POINTER_REGNUM' and the value 'STACK_DYNAMIC_OFFSET'.

'VIRTUAL_OUTGOING_ARGS_REGNUM'
This points to the location in the stack at which outgoing
arguments should be written when the stack is pre-pushed
(arguments pushed using push insns should always use
'STACK_POINTER_REGNUM').

It is replaced by the sum of the register given by
'STACK_POINTER_REGNUM' and the value 'STACK_POINTER_OFFSET'.

'(subreg:M REG WORDNUM)'
'subreg' expressions are used to refer to a register in a machine
mode other than its natural one, or to refer to one register of a
multi-word 'reg' that actually refers to several registers.

Each pseudo-register has a natural mode. If it is necessary to
operate on it in a different mode--for example, to perform a
fullword move instruction on a pseudo-register that contains a
single byte--the pseudo-register must be enclosed in a 'subreg'.
In such a case, WORDNUM is zero.

Usually M is at least as narrow as the mode of REG, in which case
it is restricting consideration to only the bits of REG that are
in M. However, sometimes M is wider than the mode of REG. These
'subreg' expressions are often called "paradoxical". They are
used in cases where we want to refer to an object in a wider mode
but do not care what value the additional bits have. The reload
pass ensures that paradoxical references are only made to hard
registers.

The other use of 'subreg' is to extract the individual registers
of a multi-register value. Machine modes such as 'DImode' and
'TImode' can indicate values longer than a word, values which
usually require two or more consecutive registers. To access one
of the registers, use a 'subreg' with mode 'SImode' and a WORDNUM
that says which register.

The compilation parameter 'WORDS_BIG_ENDIAN', if set to 1, says
that word number zero is the most significant part; otherwise, it
is the least significant part.

Between the combiner pass and the reload pass, it is possible to
have a paradoxical 'subreg' which contains a 'mem' instead of a
'reg' as its first operand. After the reload pass, it is also
possible to have a non-paradoxical 'subreg' which contains a
'mem'; this usually occurs when the 'mem' is a stack slot which
replaced a pseudo register.

Note that it is not valid to access a 'DFmode' value in 'SFmode'
using a 'subreg'. On some machines the most significant part of a
'DFmode' value does not have the same format as a single-precision
floating value.

It is also not valid to access a single word of a multi-word
value in a hard register when less registers can hold the value
than would be expected from its size. For example, some 32-bit
machines have floating-point registers that can hold an entire
'DFmode' value. If register 10 were such a register '(subreg:SI
(reg:DF 10) 1)' would be invalid because there is no way to
convert that reference to a single machine register. The reload
pass prevents 'subreg' expressions such as these from being
formed.

The first operand of a 'subreg' expression is customarily accessed
with the 'SUBREG_REG' macro and the second operand is customarily
accessed with the 'SUBREG_WORD' macro.

'(scratch:M)'
This represents a scratch register that will be required for the
execution of a single instruction and not used subsequently. It
is converted into a 'reg' by either the local register allocator
or the reload pass.

'scratch' is usually present inside a 'clobber' operation (*note
Side Effects::.).

'(cc0)'
This refers to the machine's condition code register. It has no
operands and may not have a machine mode. There are two ways to
use it:

* To stand for a complete set of condition code flags. This
is best on most machines, where each comparison sets the
entire series of flags.

With this technique, '(cc0)' may be validly used in only two
contexts: as the destination of an assignment (in test and
compare instructions) and in comparison operators comparing
against zero ('const_int' with value zero; that is to say,
'const0_rtx').

* To stand for a single flag that is the result of a single
condition. This is useful on machines that have only a
single flag bit, and in which comparison instructions must
specify the condition to test.

With this technique, '(cc0)' may be validly used in only two
contexts: as the destination of an assignment (in test and
compare instructions) where the source is a comparison
operator, and as the first operand of 'if_then_else' (in a
conditional branch).

There is only one expression object of code 'cc0'; it is the
value of the variable 'cc0_rtx'. Any attempt to create an
expression of code 'cc0' will return 'cc0_rtx'.

Instructions can set the condition code implicitly. On many
machines, nearly all instructions set the condition code based on
the value that they compute or store. It is not necessary to
record these actions explicitly in the RTL because the machine
description includes a prescription for recognizing the
instructions that do so (by means of the macro
'NOTICE_UPDATE_CC'). *Note Condition Code::. Only instructions
whose sole purpose is to set the condition code, and instructions
that use the condition code, need mention '(cc0)'.

On some machines, the condition code register is given a register
number and a 'reg' is used instead of '(cc0)'. This is usually
the preferable approach if only a small subset of instructions
modify the condition code. Other machines store condition codes
in general registers; in such cases a pseudo register should be
used.

Some machines, such as the Sparc and RS/6000, have two sets of
arithmetic instructions, one that sets and one that does not set
the condition code. This is best handled by normally generating
the instruction that does not set the condition code, and making
a pattern that both performs the arithmetic and sets the
condition code register (which would not be '(cc0)' in this
case). For examples, search for 'addcc' and 'andcc' in
'sparc.md'.

'(pc)'
This represents the machine's program counter. It has no
operands and may not have a machine mode. '(pc)' may be validly
used only in certain specific contexts in jump instructions.

There is only one expression object of code 'pc'; it is the value
of the variable 'pc_rtx'. Any attempt to create an expression of
code 'pc' will return 'pc_rtx'.

All instructions that do not jump alter the program counter
implicitly by incrementing it, but there is no need to mention
this in the RTL.

'(mem:M ADDR)'
This RTX represents a reference to main memory at an address
represented by the expression ADDR. M specifies how large a unit
of memory is accessed.


File: gcc.info, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL

RTL Expressions for Arithmetic
==============================

Unless otherwise specified, all the operands of arithmetic
expressions must be valid for mode M. An operand is valid for mode M
if it has mode M, or if it is a 'const_int' or 'const_double' and M is
a mode of class 'MODE_INT'.

For commutative binary operations, constants should be placed in the
second operand.

'(plus:M X Y)'
Represents the sum of the values represented by X and Y carried
out in machine mode M.

'(lo_sum:M X Y)'
Like 'plus', except that it represents that sum of X and the
low-order bits of Y. The number of low order bits is
machine-dependent but is normally the number of bits in a 'Pmode'
item minus the number of bits set by the 'high' code (*note
Constants::.).

M should be 'Pmode'.

'(minus:M X Y)'
Like 'plus' but represents subtraction.

'(compare:M X Y)'
Represents the result of subtracting Y from X for purposes of
comparison. The result is computed without overflow, as if with
infinite precision.

Of course, machines can't really subtract with infinite precision.
However, they can pretend to do so when only the sign of the
result will be used, which is the case when the result is stored
in the condition code. And that is the only way this kind of
expression may validly be used: as a value to be stored in the
condition codes.

The mode M is not related to the modes of X and Y, but instead is
the mode of the condition code value. If '(cc0)' is used, it is
'VOIDmode'. Otherwise it is some mode in class 'MODE_CC', often
'CCmode'. *Note Condition Code::.

Normally, X and Y must have the same mode. Otherwise, 'compare'
is valid only if the mode of X is in class 'MODE_INT' and Y is a
'const_int' or 'const_double' with mode 'VOIDmode'. The mode of X
determines what mode the comparison is to be done in; thus it
must not be 'VOIDmode'.

If one of the operands is a constant, it should be placed in the
second operand and the comparison code adjusted as appropriate.

A 'compare' specifying two 'VOIDmode' constants is not valid
since there is no way to know in what mode the comparison is to be
performed; the comparison must either be folded during the
compilation or the first operand must be loaded into a register
while its mode is still known.

'(neg:M X)'
Represents the negation (subtraction from zero) of the value
represented by X, carried out in mode M.

'(mult:M X Y)'
Represents the signed product of the values represented by X and
Y carried out in machine mode M.

Some machines support a multiplication that generates a product
wider than the operands. Write the pattern for this as

(mult:M (sign_extend:M X) (sign_extend:M Y))

where M is wider than the modes of X and Y, which need not be the
same.

Write patterns for unsigned widening multiplication similarly
using 'zero_extend'.

'(div:M X Y)'
Represents the quotient in signed division of X by Y, carried out
in machine mode M. If M is a floating point mode, it represents
the exact quotient; otherwise, the integerized quotient.

Some machines have division instructions in which the operands and
quotient widths are not all the same; you should represent such
instructions using 'truncate' and 'sign_extend' as in,

(truncate:M1 (div:M2 X (sign_extend:M2 Y)))

'(udiv:M X Y)'
Like 'div' but represents unsigned division.

'(mod:M X Y)'
'(umod:M X Y)'
Like 'div' and 'udiv' but represent the remainder instead of the
quotient.

'(smin:M X Y)'
'(smax:M X Y)'
Represents the smaller (for 'smin') or larger (for 'smax') of X
and Y, interpreted as signed integers in mode M.

'(umin:M X Y)'
'(umax:M X Y)'
Like 'smin' and 'smax', but the values are interpreted as unsigned
integers.

'(not:M X)'
Represents the bitwise complement of the value represented by X,
carried out in mode M, which must be a fixed-point machine mode.

'(and:M X Y)'
Represents the bitwise logical-and of the values represented by X
and Y, carried out in machine mode M, which must be a fixed-point
machine mode.

'(ior:M X Y)'
Represents the bitwise inclusive-or of the values represented by X
and Y, carried out in machine mode M, which must be a fixed-point
mode.

'(xor:M X Y)'
Represents the bitwise exclusive-or of the values represented by X
and Y, carried out in machine mode M, which must be a fixed-point
mode.

'(ashift:M X C)'
Represents the result of arithmetically shifting X left by C
places. X have mode M, a fixed-point machine mode. C be a
fixed-point mode or be a constant with mode 'VOIDmode'; which
mode is determined by the mode called for in the machine
description entry for the left-shift instruction. For example,
on the Vax, the mode of C is 'QImode' regardless of M.

'(lshift:M X C)'
Like 'ashift' but for logical left shift. 'ashift' and 'lshift'
are identical operations; we customarily use 'ashift' for both.

'(lshiftrt:M X C)'
'(ashiftrt:M X C)'
Like 'lshift' and 'ashift' but for right shift. Unlike the case
for left shift, these two operations are distinct.

'(rotate:M X C)'
'(rotatert:M X C)'
Similar but represent left and right rotate. If C is a constant,
use 'rotate'.

'(abs:M X)'
Represents the absolute value of X, computed in mode M.

'(sqrt:M X)'
Represents the square root of X, computed in mode M. Most often
M will be a floating point mode.

'(ffs:M X)'
Represents one plus the index of the least significant 1-bit in
X, represented as an integer of mode M. (The value is zero if X
is zero.) The mode of X need not be M; depending on the target
machine, various mode combinations may be valid.


File: gcc.info, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL

Comparison Operations
=====================

Comparison operators test a relation on two operands and are
considered to represent a machine-dependent nonzero value described
by, but not necessarily equal to, 'STORE_FLAG_VALUE' (*note Misc::.)
if the relation holds, or zero if it does not. The mode of the
comparison operation is independent of the mode of the data being
compared. If the comparison operation is being tested (e.g., the first
operand of an 'if_then_else'), the mode must be 'VOIDmode'. If the
comparison operation is producing data to be stored in some variable,
the mode must be in class 'MODE_INT'. All comparison operations
producing data must use the same mode, which is machine-specific.

There are two ways that comparison operations may be used. The
comparison operators may be used to compare the condition codes
'(cc0)' against zero, as in '(eq (cc0) (const_int 0))'. Such a
construct actually refers to the result of the preceding instruction
in which the condition codes were set. The instructing setting the
condition code must be adjacent to the instruction using the condition
code; only 'note' insns may separate them.

Alternatively, a comparison operation may directly compare two data
objects. The mode of the comparison is determined by the operands;
they must both be valid for a common machine mode. A comparison with
both operands constant would be invalid as the machine mode could not
be deduced from it, but such a comparison should never exist in RTL
due to constant folding.

In the example above, if '(cc0)' were last set to '(compare X Y)',
the comparison operation is identical to '(eq X Y)'. Usually only one
style of comparisons is supported on a particular machine, but the
combine pass will try to merge the operations to produce the 'eq' shown
in case it exists in the context of the particular insn involved.

Inequality comparisons come in two flavors, signed and unsigned.
Thus, there are distinct expression codes 'gt' and 'gtu' for signed and
unsigned greater-than. These can produce different results for the
same pair of integer values: for example, 1 is signed greater-than -1
but not unsigned greater-than, because -1 when regarded as unsigned is
actually '0xffffffff' which is greater than 1.

The signed comparisons are also used for floating point values.
Floating point comparisons are distinguished by the machine modes of
the operands.

'(eq:M X Y)'
1 if the values represented by X and Y are equal, otherwise 0.

'(ne:M X Y)'
1 if the values represented by X and Y are not equal, otherwise 0.

'(gt:M X Y)'
1 if the X is greater than Y. If they are fixed-point, the
comparison is done in a signed sense.

'(gtu:M X Y)'
Like 'gt' but does unsigned comparison, on fixed-point numbers
only.

'(lt:M X Y)'
'(ltu:M X Y)'
Like 'gt' and 'gtu' but test for "less than".

'(ge:M X Y)'
'(geu:M X Y)'
Like 'gt' and 'gtu' but test for "greater than or equal".

'(le:M X Y)'
'(leu:M X Y)'
Like 'gt' and 'gtu' but test for "less than or equal".

'(if_then_else COND THEN ELSE)'
This is not a comparison operation but is listed here because it
is always used in conjunction with a comparison operation. To be
precise, COND is a comparison expression. This expression
represents a choice, according to COND, between the value
represented by THEN and the one represented by ELSE.

On most machines, 'if_then_else' expressions are valid only to
express conditional jumps.

'(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)'
Similar to 'if_then_else', but more general. Each of TEST1,
TEST2, ... is performed in turn. The result of this expression is
the VALUE corresponding to the first non-zero test, or DEFAULT if
none of the tests are non-zero expressions.

This is currently not valid for instruction patterns and is
supported only for insn attributes. *Note Insn Attributes::.


File: gcc.info, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL

Bit Fields
==========

Special expression codes exist to represent bit-field instructions.
These types of expressions are lvalues in RTL; they may appear on the
left side of an assignment, indicating insertion of a value into the
specified bit field.

'(sign_extract:M LOC SIZE POS)'
This represents a reference to a sign-extended bit field
contained or starting in LOC (a memory or register reference).
The bit field is SIZE bits wide and starts at bit POS. The
compilation option 'BITS_BIG_ENDIAN' says which end of the memory
unit POS counts from.

If LOC is in memory, its mode must be a single-byte integer mode.
If LOC is in a register, the mode to use is specified by the
operand of the 'insv' or 'extv' pattern (*note Standard Names::.)
and is usually a full-word integer mode.

The mode of POS is machine-specific and is also specified in the
'insv' or 'extv' pattern.

The mode M is the same as the mode that would be used for LOC if
it were a register.

'(zero_extract:M LOC SIZE POS)'
Like 'sign_extract' but refers to an unsigned or zero-extended
bit field. The same sequence of bits are extracted, but they are
filled to an entire word with zeros instead of by sign-extension.


File: gcc.info, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL

Conversions
===========

All conversions between machine modes must be represented by
explicit conversion operations. For example, an expression which is
the sum of a byte and a full word cannot be written as '(plus:SI
(reg:QI 34) (reg:SI 80))' because the 'plus' operation requires two
operands of the same machine mode. Therefore, the byte-sized operand
is enclosed in a conversion operation, as in

(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))

The conversion operation is not a mere placeholder, because there
may be more than one way of converting from a given starting mode to
the desired final mode. The conversion operation code says how to do
it.

For all conversion operations, X must not be 'VOIDmode' because the
mode in which to do the conversion would not be known. The conversion
must either be done at compile-time or X must be placed into a
register.

'(sign_extend:M X)'
Represents the result of sign-extending the value X to machine
mode M. M must be a fixed-point mode and X a fixed-point value
of a mode narrower than M.

'(zero_extend:M X)'
Represents the result of zero-extending the value X to machine
mode M. M must be a fixed-point mode and X a fixed-point value
of a mode narrower than M.

'(float_extend:M X)'
Represents the result of extending the value X to machine mode M.
M must be a floating point mode and X a floating point value of
a mode narrower than M.

'(truncate:M X)'
Represents the result of truncating the value X to machine mode
M. M must be a fixed-point mode and X a fixed-point value of a
mode wider than M.

'(float_truncate:M X)'
Represents the result of truncating the value X to machine mode
M. M must be a floating point mode and X a floating point value
of a mode wider than M.

'(float:M X)'
Represents the result of converting fixed point value X, regarded
as signed, to floating point mode M.

'(unsigned_float:M X)'
Represents the result of converting fixed point value X, regarded
as unsigned, to floating point mode M.

'(fix:M X)'
When M is a fixed point mode, represents the result of converting
floating point value X to mode M, regarded as signed. How
rounding is done is not specified, so this operation may be used
validly in compiling C code only for integer-valued operands.

'(unsigned_fix:M X)'
Represents the result of converting floating point value X to
fixed point mode M, regarded as unsigned. How rounding is done
is not specified.

'(fix:M X)'
When M is a floating point mode, represents the result of
converting floating point value X (valid for mode M) to an
integer, still represented in floating point mode M, by rounding
towards zero.


File: gcc.info, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL

Declarations
============

Declaration expression codes do not represent arithmetic operations
but rather state assertions about their operands.

'(strict_low_part (subreg:M (reg:N R) 0))'
This expression code is used in only one context: operand 0 of a
'set' expression. In addition, the operand of this expression
must be a non-paradoxical 'subreg' expression.

The presence of 'strict_low_part' says that the part of the
register which is meaningful in mode N, but is not part of mode
M, is not to be altered. Normally, an assignment to such a
subreg is allowed to have undefined effects on the rest of the
register when M is less than a word.


File: gcc.info, Node: Side Effects, Next: Incdec, Prev: RTL Declarations, Up: RTL

Side Effect Expressions
=======================

The expression codes described so far represent values, not actions.
But machine instructions never produce values; they are meaningful
only for their side effects on the state of the machine. Special
expression codes are used to represent side effects.

The body of an instruction is always one of these side effect codes;
the codes described above, which represent values, appear only as the
operands of these.

'(set LVAL X)'
Represents the action of storing the value of X into the place
represented by LVAL. LVAL must be an expression representing a
place that can be stored in: 'reg' (or 'subreg' or
'strict_low_part'), 'mem', 'pc' or 'cc0'.

If LVAL is a 'reg', 'subreg' or 'mem', it has a machine mode;
then X must be valid for that mode.

If LVAL is a 'reg' whose machine mode is less than the full width
of the register, then it means that the part of the register
specified by the machine mode is given the specified value and the
rest of the register receives an undefined value. Likewise, if
LVAL is a 'subreg' whose machine mode is narrower than the mode
of the register, the rest of the register can be changed in an
undefined way.

If LVAL is a 'strict_low_part' of a 'subreg', then the part of
the register specified by the machine mode of the 'subreg' is
given the value X and the rest of the register is not changed.

If LVAL is '(cc0)', it has no machine mode, and X may be either a
'compare' expression or a value that may have any mode. The
latter case represents a "test" instruction. The expression
'(set (cc0) (reg:M N))' is equivalent to '(set (cc0) (compare
(reg:M N) (const_int 0)))'. Use the former expression to save
space during the compilation.

If LVAL is '(pc)', we have a jump instruction, and the
possibilities for X are very limited. It may be a 'label_ref'
expression (unconditional jump). It may be an 'if_then_else'
(conditional jump), in which case either the second or the third
operand must be '(pc)' (for the case which does not jump) and the
other of the two must be a 'label_ref' (for the case which does
jump). X may also be a 'mem' or '(plus:SI (pc) Y)', where Y may
be a 'reg' or a 'mem'; these unusual patterns are used to
represent jumps through branch tables.

If LVAL is neither '(cc0)' nor '(pc)', the mode of LVAL must not
be 'VOIDmode' and the mode of X must be valid for the mode of
LVAL.

LVAL is customarily accessed with the 'SET_DEST' macro and X with
the 'SET_SRC' macro.

'(return)'
As the sole expression in a pattern, represents a return from the
current function, on machines where this can be done with one
instruction, such as Vaxes. On machines where a multi-instruction
"epilogue" must be executed in order to return from the function,
returning is done by jumping to a label which precedes the
epilogue, and the 'return' expression code is never used.

Inside an 'if_then_else' expression, represents the value to be
placed in 'pc' to return to the caller.

Note that an insn pattern of '(return)' is logically equivalent to
'(set (pc) (return))', but the latter form is never used.

'(call FUNCTION NARGS)'
Represents a function call. FUNCTION is a 'mem' expression whose
address is the address of the function to be called. NARGS is an
expression which can be used for two purposes: on some machines
it represents the number of bytes of stack argument; on others,
it represents the number of argument registers.

Each machine has a standard machine mode which FUNCTION must
have. The machine description defines macro 'FUNCTION_MODE' to
expand into the requisite mode name. The purpose of this mode is
to specify what kind of addressing is allowed, on machines where
the allowed kinds of addressing depend on the machine mode being
addressed.

'(clobber X)'
Represents the storing or possible storing of an unpredictable,
undescribed value into X, which must be a 'reg', 'scratch' or
'mem' expression.

One place this is used is in string instructions that store
standard values into particular hard registers. It may not be
worth the trouble to describe the values that are stored, but it
is essential to inform the compiler that the registers will be
altered, lest it attempt to keep data in them across the string
instruction.

If X is '(mem:BLK (const_int 0))', it means that all memory
locations must be presumed clobbered.

Note that the machine description classifies certain hard
registers as "call-clobbered". All function call instructions
are assumed by default to clobber these registers, so there is no
need to use 'clobber' expressions to indicate this fact. Also,
each function call is assumed to have the potential to alter any
memory location, unless the function is declared 'const'.

If the last group of expressions in a 'parallel' are each a
'clobber' expression whose arguments are 'reg' or 'match_scratch'
(*note RTL Template::.) expressions, the combiner phase can add
the appropriate 'clobber' expressions to an insn it has
constructed when doing so will cause a pattern to be matched.

This feature can be used, for example, on a machine that whose
multiply and add instructions don't use an MQ register but which
has an add-accumulate instruction that does clobber the MQ
register. Similarly, a combined instruction might require a
temporary register while the constituent instructions might not.

When a 'clobber' expression for a register appears inside a
'parallel' with other side effects, the register allocator
guarantees that the register is unoccupied both before and after
that insn. However, the reload phase may allocate a register
used for one of the inputs unless the '&' constraint is specified
for the selected alternative (*note Modifiers::.). You can
clobber either a specific hard register, a pseudo register, or a
'scratch' expression; in the latter two cases, GNU CC will
allocate a hard register that is available there for use as a
temporary.

For instructions that require a temporary register, you should use
'scratch' instead of a pseudo-register because this will allow the
combiner phase to add the 'clobber' when required. You do this by
coding ('clobber' ('match_scratch' ...)). If you do clobber a
pseudo register, use one which appears nowhere else--generate a
new one each time. Otherwise, you may confuse CSE.

There is one other known use for clobbering a pseudo register in a
'parallel': when one of the input operands of the insn is also
clobbered by the insn. In this case, using the same pseudo
register in the clobber and elsewhere in the insn produces the
expected results.

'(use X)'
Represents the use of the value of X. It indicates that the
value in X at this point in the program is needed, even though it
may not be apparent why this is so. Therefore, the compiler will
not attempt to delete previous instructions whose only effect is
to store a value in X. X must be a 'reg' expression.

During the delayed branch scheduling phase, X may be an insn.
This indicates that X previously was located at this place in the
code and its data dependencies need to be taken into account.
These 'use' insns will be deleted before the delayed branch
scheduling phase exits.

'(parallel [X0 X1 ...])'
Represents several side effects performed in parallel. The square
brackets stand for a vector; the operand of 'parallel' is a
vector of expressions. X0, X1 and so on are individual side
effect expressions--expressions of code 'set', 'call', 'return',
'clobber' or 'use'.

"In parallel" means that first all the values used in the
individual side-effects are computed, and second all the actual
side-effects are performed. For example,

(parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
(set (mem:SI (reg:SI 1)) (reg:SI 1))])

says unambiguously that the values of hard register 1 and the
memory location addressed by it are interchanged. In both places
where '(reg:SI 1)' appears as a memory address it refers to the
value in register 1 *before* the execution of the insn.

It follows that it is *incorrect* to use 'parallel' and expect
the result of one 'set' to be available for the next one. For
example, people sometimes attempt to represent a jump-if-zero
instruction this way:

(parallel [(set (cc0) (reg:SI 34))
(set (pc) (if_then_else
(eq (cc0) (const_int 0))
(label_ref ...)
(pc)))])

But this is incorrect, because it says that the jump condition
depends on the condition code value *before* this instruction,
not on the new value that is set by this instruction.

Peephole optimization, which takes place together with final
assembly code output, can produce insns whose patterns consist of
a 'parallel' whose elements are the operands needed to output the
resulting assembler code--often 'reg', 'mem' or constant
expressions. This would not be well-formed RTL at any other
stage in compilation, but it is ok then because no further
optimization remains to be done. However, the definition of the
macro 'NOTICE_UPDATE_CC', if any, must deal with such insns if
you define any peephole optimizations.

'(sequence [INSNS ...])'
Represents a sequence of insns. Each of the INSNS that appears
in the vector is suitable for appearing in the chain of insns, so
it must be an 'insn', 'jump_insn', 'call_insn', 'code_label',
'barrier' or 'note'.

A 'sequence' RTX is never placed in an actual insn during RTL
generation. It represents the sequence of insns that result from
a 'define_expand' *before* those insns are passed to 'emit_insn'
to insert them in the chain of insns. When actually inserted,
the individual sub-insns are separated out and the 'sequence' is
forgotten.

After delay-slot scheduling is completed, an insn and all the
insns that reside in its delay slots are grouped together into a
'sequence'. The insn requiring the delay slot is the first insn
in the vector; subsequent insns are to be placed in the delay
slot.

'INSN_ANNULLED_BRANCH_P' is set on an insn in a delay slot to
indicate that a branch insn should be used that will
conditionally annul the effect of the insns in the delay slots.
In such a case, 'INSN_FROM_TARGET_P' indicates that the insn is
from the target of the branch and should be executed only if the
branch is taken; otherwise the insn should be executed only if
the branch is not taken. *Note Delay Slots::.

These expression codes appear in place of a side effect, as the
body of an insn, though strictly speaking they do not always describe
side effects as such:

'(asm_input S)'
Represents literal assembler code as described by the string S.

'(unspec [OPERANDS ...] INDEX)'
'(unspec_volatile [OPERANDS ...] INDEX)'
Represents a machine-specific operation on OPERANDS. INDEX
selects between multiple machine-specific operations.
'unspec_volatile' is used for volatile operations and operations
that may trap; 'unspec' is used for other operations.

These codes may appear inside a 'pattern' of an insn, inside a
'parallel', or inside an expression.

'(addr_vec:M [LR0 LR1 ...])'
Represents a table of jump addresses. The vector elements LR0,
etc., are 'label_ref' expressions. The mode M specifies how much
space is given to each address; normally M would be 'Pmode'.

'(addr_diff_vec:M BASE [LR0 LR1 ...])'
Represents a table of jump addresses expressed as offsets from
BASE. The vector elements LR0, etc., are 'label_ref' expressions
and so is BASE. The mode M specifies how much space is given to
each address-difference.


File: gcc.info, Node: Incdec, Next: Assembler, Prev: Side Effects, Up: RTL

Embedded Side-Effects on Addresses
==================================

Four special side-effect expression codes appear as memory
addresses.

'(pre_dec:M X)'
Represents the side effect of decrementing X by a standard amount
and represents also the value that X has after being decremented.
X must be a 'reg' or 'mem', but most machines allow only a
'reg'. M must be the machine mode for pointers on the machine in
use. The amount X is decremented by is the length in bytes of
the machine mode of the containing memory reference of which this
expression serves as the address. Here is an example of its use:

(mem:DF (pre_dec:SI (reg:SI 39)))

This says to decrement pseudo register 39 by the length of a
'DFmode' value and use the result to address a 'DFmode' value.

'(pre_inc:M X)'
Similar, but specifies incrementing X instead of decrementing it.

'(post_dec:M X)'
Represents the same side effect as 'pre_dec' but a different
value. The value represented here is the value X has before
being decremented.

'(post_inc:M X)'
Similar, but specifies incrementing X instead of decrementing it.

These embedded side effect expressions must be used with care.
Instruction patterns may not use them. Until the 'flow' pass of the
compiler, they may occur only to represent pushes onto the stack. The
'flow' pass finds cases where registers are incremented or decremented
in one instruction and used as an address shortly before or after;
these cases are then transformed to use pre- or post-increment or
-decrement.

If a register used as the operand of these expressions is used in
another address in an insn, the original value of the register is used.
Uses of the register outside of an address are not permitted within the
same insn as a use in an embedded side effect expression because such
insns behave differently on different machines and hence must be

treated as ambiguous and disallowed.

An instruction that can be represented with an embedded side effect
could also be represented using 'parallel' containing an additional
'set' to describe how the address register is altered. This is not
done because machines that allow these operations at all typically
allow them wherever a memory address is called for. Describing them as
additional parallel stores would require doubling the number of entries
in the machine description.


File: gcc.info, Node: Assembler, Next: Insns, Prev: IncDec, Up: RTL
File: gcc.info, Node: Insns, Next: Calls, Prev: Assembler, Up: RTL

Insns
=====

The RTL representation of the code for a function is a doubly-linked
chain of objects called "insns". Insns are expressions with special
codes that are used for no other purpose. Some insns are actual
instructions; others represent dispatch tables for 'switch'
statements; others represent labels to jump to or various sorts of
declarative information.

In addition to its own specific data, each insn must have a unique
id-number that distinguishes it from all other insns in the current
function (after delayed branch scheduling, copies of an insn with the
same id-number may be present in multiple places in a function, but
these copies will always be identical and will only appear inside a
'sequence'), and chain pointers to the preceding and following insns.
These three fields occupy the same position in every insn, independent
of the expression code of the insn. They could be accessed with
'XEXP' and 'XINT', but instead three special macros are always used:

'INSN_UID (I)'
Accesses the unique id of insn I.

'PREV_INSN (I)'
Accesses the chain pointer to the insn preceding I. If I is the
first insn, this is a null pointer.

'NEXT_INSN (I)'
Accesses the chain pointer to the insn following I. If I is the
last insn, this is a null pointer.

The first insn in the chain is obtained by calling 'get_insns'; the

last insn is the result of calling 'get_last_insn'. Within the chain
delimited by these insns, the 'NEXT_INSN' and 'PREV_INSN' pointers
must always correspond: if INSN is not the first insn,

NEXT_INSN (PREV_INSN (INSN)) == INSN

is always true and if INSN is not the last insn,

PREV_INSN (NEXT_INSN (INSN)) == INSN

is always true.

After delay slot scheduling, some of the insns in the chain might be
'sequence' expressions, which contain a vector of insns. The value of
'NEXT_INSN' in all but the last of these insns is the next insn in the
vector; the value of 'NEXT_INSN' of the last insn in the vector is the
same as the value of 'NEXT_INSN' for the 'sequence' in which it is
contained. Similar rules apply for 'PREV_INSN'.

This means that the above invariants are not necessarily true for
insns inside 'sequence' expressions. Specifically, if INSN is the
first insn in a 'sequence', 'NEXT_INSN (PREV_INSN (INSN))' is the insn
containing the 'sequence' expression, as is the value of 'PREV_INSN
(NEXT_INSN (INSN))' is INSN is the last insn in the 'sequence'
expression. You can use these expressions to find the containing
'sequence' expression.

Every insn has one of the following six expression codes:

'insn'
The expression code 'insn' is used for instructions that do not
jump and do not do function calls. 'sequence' expressions are
always contained in insns with code 'insn' even if one of those
insns should jump or do function calls.

Insns with code 'insn' have four additional fields beyond the
three mandatory ones listed above. These four are described in a
table below.

'jump_insn'
The expression code 'jump_insn' is used for instructions that may
jump (or, more generally, may contain 'label_ref' expressions).
If there is an instruction to return from the current function,
it is recorded as a 'jump_insn'.

'jump_insn' insns have the same extra fields as 'insn' insns,
accessed in the same way and in addition contains a field
'JUMP_LABEL' which is defined once jump optimization has
completed.

For simple conditional and unconditional jumps, this field
contains the 'code_label' to which this insn will (possibly
conditionally) branch. In a more complex jump, 'JUMP_LABEL'
records one of the labels that the insn refers to; the only way
to find the others is to scan the entire body of the insn.

Return insns count as jumps, but since they do not refer to any
labels, they have zero in the 'JUMP_LABEL' field.

'call_insn'
The expression code 'call_insn' is used for instructions that may
do function calls. It is important to distinguish these
instructions because they imply that certain registers and memory
locations may be altered unpredictably.

A 'call_insn' insn may be preceded by insns that contain a single
'use' expression and be followed by insns the contain a single
'clobber' expression. If so, these 'use' and 'clobber'
expressions are treated as being part of the function call.
There must not even be a 'note' between the 'call_insn' and the
'use' or 'clobber' insns for this special treatment to take
place. This is somewhat of a kludge and will be removed in a
later version of GNU CC.

'call_insn' insns have the same extra fields as 'insn' insns,
accessed in the same way.

'code_label'
A 'code_label' insn represents a label that a jump insn can jump
to. It contains two special fields of data in addition to the
three standard ones. 'CODE_LABEL_NUMBER' is used to hold the
"label number", a number that identifies this label uniquely
among all the labels in the compilation (not just in the current
function). Ultimately, the label is represented in the assembler
output as an assembler label, usually of the form 'LN' where N is
the label number.

When a 'code_label' appears in an RTL expression, it normally
appears within a 'label_ref' which represents the address of the
label, as a number.

The field 'LABEL_NUSES' is only defined once the jump optimization
phase is completed and contains the number of times this label is
referenced in the current function.

'barrier'
Barriers are placed in the instruction stream when control cannot
flow past them. They are placed after unconditional jump
instructions to indicate that the jumps are unconditional and
after calls to 'volatile' functions, which do not return (e.g.,
'exit'). They contain no information beyond the three standard
fields.

'note'
'note' insns are used to represent additional debugging and
declarative information. They contain two nonstandard fields, an
integer which is accessed with the macro 'NOTE_LINE_NUMBER' and a
string accessed with 'NOTE_SOURCE_FILE'.

If 'NOTE_LINE_NUMBER' is positive, the note represents the
position of a source line and 'NOTE_SOURCE_FILE' is the source
file name that the line came from. These notes control
generation of line number data in the assembler output.

Otherwise, 'NOTE_LINE_NUMBER' is not really a line number but a
code with one of the following values (and 'NOTE_SOURCE_FILE'
must contain a null pointer):

'NOTE_INSN_DELETED'
Such a note is completely ignorable. Some passes of the
compiler delete insns by altering them into notes of this
kind.

'NOTE_INSN_BLOCK_BEG'
'NOTE_INSN_BLOCK_END'
These types of notes indicate the position of the beginning
and end of a level of scoping of variable names. They
control the output of debugging information.

'NOTE_INSN_LOOP_BEG'
'NOTE_INSN_LOOP_END'
These types of notes indicate the position of the beginning
and end of a 'while' or 'for' loop. They enable the loop
optimizer to find loops quickly.

'NOTE_INSN_LOOP_CONT'
Appears at the place in a loop that 'continue' statements
jump to.

'NOTE_INSN_LOOP_VTOP'
This note indicates the place in a loop where the exit test
begins for those loops in which the exit test has been
duplicated. This position becomes another virtual start of
the loop when considering loop invariants.

'NOTE_INSN_FUNCTION_END'
Appears near the end of the function body, just before the
label that 'return' statements jump to (on machine where a
single instruction does not suffice for returning). This
note may be deleted by jump optimization.

'NOTE_INSN_SETJMP'
Appears following each call to 'setjmp' or a related
function.

These codes are printed symbolically when they appear in
debugging dumps.

The machine mode of an insn is normally 'VOIDmode', but some phases
use the mode for various purposes; for example, the reload pass sets
it to 'HImode' if the insn needs reloading but not register
elimination and 'QImode' if both are required. The common
subexpression elimination pass sets the mode of an insn to 'QImode'
when it is the first insn in a block that has already been processed.

Here is a table of the extra fields of 'insn', 'jump_insn' and
'call_insn' insns:

'PATTERN (I)'
An expression for the side effect performed by this insn. This
must be one of the following codes: 'set', 'call', 'use',
'clobber', 'return', 'asm_input', 'asm_output', 'addr_vec',
'addr_diff_vec', 'trap_if', 'unspec', 'unspec_volatile', or
'parallel'. If it is a 'parallel', each element of the
'parallel' must be one these codes, except that 'parallel'
expressions cannot be nested and 'addr_vec' and 'addr_diff_vec'
are not permitted inside a 'parallel' expression.

'INSN_CODE (I)'
An integer that says which pattern in the machine description
matches this insn, or -1 if the matching has not yet been
attempted.

Such matching is never attempted and this field remains -1 on an
insn whose pattern consists of a single 'use', 'clobber',
'asm_input', 'addr_vec' or 'addr_diff_vec' expression.

Matching is also never attempted on insns that result from an
'asm' statement. These contain at least one 'asm_operands'
expression. The function 'asm_noperands' returns a non-negative
value for such insns.

In the debugging output, this field is printed as a number
followed by a symbolic representation that locates the pattern in
the 'md' file as some small positive or negative offset from a
named pattern.

'LOG_LINKS (I)'
A list (chain of 'insn_list' expressions) giving information about
dependencies between instructions within a basic block. Neither
a jump nor a label may come between the related insns.

'REG_NOTES (I)'
A list (chain of 'expr_list' and 'insn_list' expressions) giving
miscellaneous information about the insn. It is often information
pertaining to the registers used in this insn.

The 'LOG_LINKS' field of an insn is a chain of 'insn_list'
expressions. Each of these has two operands: the first is an insn,
and the second is another 'insn_list' expression (the next one in the
chain). The last 'insn_list' in the chain has a null pointer as
second operand. The significant thing about the chain is which insns
appear in it (as first operands of 'insn_list' expressions). Their
order is not significant.

This list is originally set up by the flow analysis pass; it is a
null pointer until then. Flow only adds links for those data
dependencies which can be used for instruction combination. For each
insn, the flow analysis pass adds a link to insns which store into
registers values that are used for the first time in this insn. The
instruction scheduling pass adds extra links so that every dependence
will be represented. Links represent data dependencies,
antidependencies and output dependencies; the machine mode of the link
distinguishes these three types: antidependencies have mode
'REG_DEP_ANTI', output dependencies have mode 'REG_DEP_OUTPUT', and
data dependencies have mode 'VOIDmode'.

The 'REG_NOTES' field of an insn is a chain similar to the
'LOG_LINKS' field but it includes 'expr_list' expressions in addition
to 'insn_list' expressions. There are several kinds of register
notes, which are distinguished by the machine mode, which in a
register note is really understood as being an 'enum reg_note'. The
first operand OP of the note is data whose meaning depends on the kind
of note.

The macro 'REG_NOTE_KIND (X)' returns the the kind of register
note. Its counterpart, the macro 'PUT_REG_NOTE_KIND (X, NEWKIND)'
sets the register note type of X to be NEWKIND.

Register notes are of three classes: They may say something about an
input to an insn, they may say something about an output of an insn, or
they may create a linkage between two insns. There are also a set of
values that are only used in 'LOG_LINKS'.

These register notes annotate inputs to an insn:

'REG_DEAD'
The value in OP dies in this insn; that is to say, altering the
value immediately after this insn would not affect the future
behavior of the program.

This does not necessarily mean that the register OP has no useful
value after this insn since it may also be an output of the insn.
In such a case, however, a 'REG_DEAD' note would be redundant
and is usually not present until after the reload pass, but no
code relies on this fact.

'REG_INC'
The register OP is incremented (or decremented; at this level
there is no distinction) by an embedded side effect inside this
insn. This means it appears in a 'post_inc', 'pre_inc',
'post_dec' or 'pre_dec' expression.

'REG_NONNEG'
The register OP is known to have a nonnegative value when this
insn is reached. This is used so that decrement and branch until
zero instructions, such as the m68k dbra, can be matched.

The 'REG_NONNEG' note is added to insns only if the machine
description contains a pattern named
'decrement_and_branch_until_zero'.

'REG_NO_CONFLICT'
This insn does not cause a conflict between OP and the item being
set by this insn even though it might appear that it does. In
other words, if the destination register and OP could otherwise
be assigned the same register, this insn does not prevent that
assignment.

Insns with this note are usually part of a block that begins with
a 'clobber' insn specifying a multi-word pseudo register (which
will be the output of the block), a group of insns that each set
one word of the value and have the 'REG_NO_CONFLICT' note
attached, and a final insn that copies the output to itself with
an attached 'REG_EQUAL' note giving the expression being
computed. This block is encapsulated with 'REG_LIBCALL' and
'REG_RETVAL' notes on the first and last insns, respectively.

'REG_LABEL'
This insn uses OP, a 'code_label', but is not a 'jump_insn'. The
presence of this note allows jump optimization to be aware that
OP is, in fact, being used.

The following notes describe attributes of outputs of an insn:

'REG_EQUIV'
'REG_EQUAL'
This note is only valid on an insn that sets only one register and
indicates that that register will be equal to OP at run time; the
scope of this equivalence differs between the two types of notes.
The value which the insn explicitly copies into the register may
look different from OP, but they will be equal at run time. If
the output of the single 'set' is a 'strict_low_part' expression,
the note refers to the register that is contained in 'SUBREG_REG'
of the 'subreg' expression.

For 'REG_EQUIV', the register is equivalent to OP throughout the
entire function, and could validly be replaced in all its
occurrences by OP. ("Validly" here refers to the data flow of
the program; simple replacement may make some insns invalid.) For
example, when a constant is loaded into a register that is never
assigned any other value, this kind of note is used.

When a parameter is copied into a pseudo-register at entry to a
function, a note of this kind records that the register is
equivalent to the stack slot where the parameter was passed.
Although in this case the register may be set by other insns, it
is still valid to replace the register by the stack slot
throughout the function.

In the case of 'REG_EQUAL', the register that is set by this insn
will be equal to OP at run time at the end of this insn but not
necessarily elsewhere in the function. In this case, OP is
typically an arithmetic expression. For example, when a sequence
of insns such as a library call is used to perform an arithmetic
operation, this kind of note is attached to the insn that
produces or copies the final value.

These two notes are used in different ways by the compiler passes.
'REG_EQUAL' is used by passes prior to register allocation (such
as common subexpression elimination and loop optimization) to
tell them how to think of that value. 'REG_EQUIV' notes are used
by register allocation to indicate that there is an available
substitute expression (either a constant or a 'mem' expression
for the location of a parameter on the stack) that may be used in
place of a register if insufficient registers are available.

Except for stack homes for parameters, which are indicated by a
'REG_EQUIV' note and are not useful to the early optimization
passes and pseudo registers that are equivalent to a memory
location throughout there entire life, which is not detected
until later in the compilation, all equivalences are initially
indicated by an attached 'REG_EQUAL' note. In the early stages
of register allocation, a 'REG_EQUAL' note is changed into a
'REG_EQUIV' note if OP is a constant and the insn represents the
only set of its destination register.

Thus, compiler passes prior to register allocation need only
check for 'REG_EQUAL' notes and passes subsequent to register
allocation need only check for 'REG_EQUIV' notes.

'REG_UNUSED'
The register OP being set by this insn will not be used in a
subsequent insn. This differs from a 'REG_DEAD' note, which
indicates that the value in an input will not be used
subsequently. These two notes are independent; both may be
present for the same register.

'REG_WAS_0'
The single output of this insn contained zero before this insn.
OP is the insn that set it to zero. You can rely on this note if
it is present and OP has not been deleted or turned into a 'note';
its absence implies nothing.

These notes describe linkages between insns. They occur in pairs:
one insn has one of a pair of notes that points to a second insn,
which has the inverse note pointing back to the first insn.

'REG_RETVAL'
This insn copies the value of a multi-insn sequence (for example,
a library call), and OP is the first insn of the sequence (for a
library call, the first insn that was generated to set up the
arguments for the library call).

Loop optimization uses this note to treat such a sequence as a
single operation for code motion purposes and flow analysis uses
this note to delete such sequences whose results are dead.

A 'REG_EQUAL' note will also usually be attached to this insn to
provide the expression being computed by the sequence.

'REG_LIBCALL'
This is the inverse of 'REG_RETVAL': it is placed on the first
insn of a multi-insn sequence, and it points to the last one.

'REG_CC_SETTER'
'REG_CC_USER'
On machines that use 'cc0', the insns which set and use 'cc0' set
and use 'cc0' are adjacent. However, when branch delay slot
filling is done, this may no longer be true. In this case a
'REG_CC_USER' note will be placed on the insn setting 'cc0' to
point to the insn using 'cc0' and a 'REG_CC_SETTER' note will be
placed on the insn using 'cc0' to point to the insn setting 'cc0'.

These values are only used in the 'LOG_LINKS' field, and indicate
the type of dependency that each link represents. Links which indicate
a data dependence (a read after write dependence) do not use any code,
they simply have mode 'VOIDmode', and are printed without any
descriptive text.

'REG_DEP_ANTI'
This indicates an anti dependence (a write after read dependence).

'REG_DEP_OUTPUT'
This indicates an output dependence (a write after write
dependence).

For convenience, the machine mode in an 'insn_list' or 'expr_list'
is printed using these symbolic codes in debugging dumps.

The only difference between the expression codes 'insn_list' and
'expr_list' is that the first operand of an 'insn_list' is assumed to
be an insn and is printed in debugging dumps as the insn's unique id;
the first operand of an 'expr_list' is printed in the ordinary way as
an expression.


File: gcc.info, Node: Calls, Next: Sharing, Prev: Insns, Up: RTL

RTL Representation of Function-Call Insns
=========================================

Insns that call subroutines have the RTL expression code
'call_insn'. These insns must satisfy special rules, and their bodies
must use a special RTL expression code, 'call'.

A 'call' expression has two operands, as follows:

(call (mem:FM ADDR) NBYTES)

Here NBYTES is an operand that represents the number of bytes of
argument data being passed to the subroutine, FM is a machine mode
(which must equal as the definition of the 'FUNCTION_MODE' macro in
the machine description) and ADDR represents the address of the
subroutine.

For a subroutine that returns no value, the 'call' expression as
shown above is the entire body of the insn, except that the insn might
also contain 'use' or 'clobber' expressions.

For a subroutine that returns a value whose mode is not 'BLKmode',
the value is returned in a hard register. If this register's number is
R, then the body of the call insn looks like this:

(set (reg:M R)
(call (mem:FM ADDR) NBYTES))

This RTL expression makes it clear (to the optimizer passes) that the
appropriate register receives a useful value in this insn.

When a subroutine returns a 'BLKmode' value, it is handled by
passing to the subroutine the address of a place to store the value.
So the call insn itself does not "return" any value, and it has the
same RTL form as a call that returns nothing.

On some machines, the call instruction itself clobbers some
register, for example to contain the return address. 'call_insn' insns
on these machines should have a body which is a 'parallel' that
contains both the 'call' expression and 'clobber' expressions that
indicate which registers are destroyed. Similarly, if the call
instruction requires some register other than the stack pointer that
is not explicitly mentioned it its RTL, a 'use' subexpression should
mention that register.

Functions that are called are assumed to modify all registers
listed in the configuration macro 'CALL_USED_REGISTERS' (*note
Register Basics::.) and, with the exception of 'const' functions and
library calls, to modify all of memory.

Insns containing just 'use' expressions directly precede the
'call_insn' insn to indicate which registers contain inputs to the
function. Similarly, if registers other than those in
'CALL_USED_REGISTERS' are clobbered by the called function, insns
containing a single 'clobber' follow immediately after the call to
indicate which registers.


File: gcc.info, Node: Sharing, Prev: Calls, Up: RTL

Structure Sharing Assumptions
=============================

The compiler assumes that certain kinds of RTL expressions are
unique; there do not exist two distinct objects representing the same
value. In other cases, it makes an opposite assumption: that no RTL
expression object of a certain kind appears in more than one place in
the containing structure.

These assumptions refer to a single function; except for the RTL
objects that describe global variables and external functions, and a
few standard objects such as small integer constants, no RTL objects
are common to two functions.

* Each pseudo-register has only a single 'reg' object to represent
it, and therefore only a single machine mode.

* For any symbolic label, there is only one 'symbol_ref' object
referring to it.

* There is only one 'const_int' expression with value 0, only one
with value 1, and only one with value -1. Some other integer
values are also stored uniquely.

* There is only one 'pc' expression.

* There is only one 'cc0' expression.

* There is only one 'const_double' expression with value 0 for each
floating point mode. Likewise for values 1 and 2.

* No 'label_ref' or 'scratch' appears in more than one place in the
RTL structure; in other words, it is safe to do a tree-walk of all
the insns in the function and assume that each time a 'label_ref'
or 'scratch' is seen it is distinct from all others that are seen.

* Only one 'mem' object is normally created for each static
variable or stack slot, so these objects are frequently shared in
all the places they appear. However, separate but equal objects
for these variables are occasionally made.

* When a single 'asm' statement has multiple output operands, a
distinct 'asm_operands' expression is made for each output
operand. However, these all share the vector which contains the
sequence of input operands. This sharing is used later on to
test whether two 'asm_operands' expressions come from the same
statement, so all optimizations must carefully preserve the
sharing if they copy the vector at all.

* No RTL object appears in more than one place in the RTL structure
except as described above. Many passes of the compiler rely on
this by assuming that they can modify RTL objects in place
without unwanted side-effects on other insns.

* During initial RTL generation, shared structure is freely
introduced. After all the RTL for a function has been generated,
all shared structure is copied by 'unshare_all_rtl' in
'emit-rtl.c', after which the above rules are guaranteed to be
followed.

* During the combiner pass, shared structure within an insn can
exist temporarily. However, the shared structure is copied
before the combiner is finished with the insn. This is done by
calling 'copy_rtx_if_shared', which is a subroutine of
'unshare_all_rtl'.


File: gcc.info, Node: Machine Desc, Next: Target Macros, Prev: RTL, Up: Top

Machine Descriptions
********************

A machine description has two parts: a file of instruction patterns
('.md' file) and a C header file of macro definitions.

The '.md' file for a target machine contains a pattern for each
instruction that the target machine supports (or at least each
instruction that is worth telling the compiler about). It may also
contain comments. A semicolon causes the rest of the line to be a
comment, unless the semicolon is inside a quoted string.

See the next chapter for information on the C header file.

* Menu:

* Patterns:: How to write instruction patterns.
* Example:: An explained example of a 'define_insn' pattern.
* RTL Template:: The RTL template defines what insns match a pattern.
* Output Template:: The output template says how to make assembler code
from such an insn.
* Output Statement:: For more generality, write C code to output
the assembler code.
* Constraints:: When not all operands are general operands.
* Standard Names:: Names mark patterns to use for code generation.
* Pattern Ordering:: When the order of patterns makes a difference.
* Dependent Patterns:: Having one pattern may make you need another.
* Jump Patterns:: Special considerations for patterns for jump insns.
* Insn Canonicalizations::Canonicalization of Instructions
* Peephole Definitions::Defining machine-specific peephole optimizations.
* Expander Definitions::Generating a sequence of several RTL insns
for a standard operation.
* Insn Splitting:: Splitting Instructions into Multiple Instructions
* Insn Attributes:: Specifying the value of attributes for generated insns.


File: gcc.info, Node: Patterns, Next: Example, Prev: Machine Desc, Up: Machine Desc

Everything about Instruction Patterns
=====================================

Each instruction pattern contains an incomplete RTL expression,
with pieces to be filled in later, operand constraints that restrict
how the pieces can be filled in, and an output pattern or C code to
generate the assembler output, all wrapped up in a 'define_insn'
expression.

A 'define_insn' is an RTL expression containing four or five
operands:

1. An optional name. The presence of a name indicate that this
instruction pattern can perform a certain standard job for the
RTL-generation pass of the compiler. This pass knows certain
names and will use the instruction patterns with those names, if
the names are defined in the machine description.

The absence of a name is indicated by writing an empty string
where the name should go. Nameless instruction patterns are never
used for generating RTL code, but they may permit several simpler
insns to be combined later on.

Names that are not thus known and used in RTL-generation have
no effect; they are equivalent to no name at all.

2. The "RTL template" (*note RTL Template::.) is a vector of
incomplete RTL expressions which show what the instruction should
look like. It is incomplete because it may contain
'match_operand', 'match_operator', and 'match_dup' expressions
that stand for operands of the instruction.

If the vector has only one element, that element is the
template for the instruction pattern. If the vector has multiple
elements, then the instruction pattern is a 'parallel' expression
containing the elements described.

3. A condition. This is a string which contains a C expression that
is the final test to decide whether an insn body matches this
pattern.

For a named pattern, the condition (if present) may not depend
on the data in the insn being matched, but only the
target-machine-type flags. The compiler needs to test these
conditions during initialization in order to learn exactly which
named instructions are available in a particular run.

For nameless patterns, the condition is applied only when
matching an individual insn, and only after the insn has matched
the pattern's recognition template. The insn's operands may be
found in the vector 'operands'.

4. The "output template": a string that says how to output matching
insns as assembler code. '%' in this string specifies where to
substitute the value of an operand. *Note Output Template::.

When simple substitution isn't general enough, you can specify
a piece of C code to compute the output. *Note Output
Statement::.

5. Optionally, a vector containing the values of attributes for
insns matching this pattern. *Note Insn Attributes::.


File: gcc.info, Node: Example, Next: RTL Template, Prev: Patterns, Up: Machine Desc

Example of 'define_insn'
========================

Here is an actual example of an instruction pattern, for the
68000/68020.

(define_insn "tstsi"
[(set (cc0)
(match_operand:SI 0 "general_operand" "rm"))]
""
"*
{ if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
return \"tstl %0\";
return \"cmpl #0,%0\"; }")

This is an instruction that sets the condition codes based on the
value of a general operand. It has no condition, so any insn whose
RTL description has the form shown may be handled according to this
pattern. The name 'tstsi' means "test a 'SImode' value" and tells the
RTL generation pass that, when it is necessary to test such a value,
an insn to do so can be constructed using this pattern.

The output control string is a piece of C code which chooses which
output template to return based on the kind of operand and the specific
type of CPU for which code is being generated.

'"rm"' is an operand constraint. Its meaning is explained below.


File: gcc.info, Node: RTL Template, Next: Output Template, Prev: Example, Up: Machine Desc

RTL Template for Generating and Recognizing Insns
=================================================

The RTL template is used to define which insns match the particular
pattern and how to find their operands. For named patterns, the RTL
template also says how to construct an insn from specified operands.

Construction involves substituting specified operands into a copy
of the template. Matching involves determining the values that serve
as the operands in the insn being matched. Both of these activities
are controlled by special expression types that direct matching and
substitution of the operands.

'(match_operand:M N PREDICATE CONSTRAINT)'
This expression is a placeholder for operand number N of the
insn. When constructing an insn, operand number N will be
substituted at this point. When matching an insn, whatever
appears at this position in the insn will be taken as operand
number N; but it must satisfy PREDICATE or this instruction
pattern will not match at all.

Operand numbers must be chosen consecutively counting from zero in
each instruction pattern. There may be only one 'match_operand'
expression in the pattern for each operand number. Usually
operands are numbered in the order of appearance in
'match_operand' expressions.

PREDICATE is a string that is the name of a C function that
accepts two arguments, an expression and a machine mode. During
matching, the function will be called with the putative operand
as the expression and M as the mode argument (if M is not
specified, 'VOIDmode' will be used, which normally causes
PREDICATE to accept any mode). If it returns zero, this
instruction pattern fails to match. PREDICATE may be an empty
string; then it means no test is to be done on the operand, so
anything which occurs in this position is valid.

Most of the time, PREDICATE will reject modes other than M--but
not always. For example, the predicate 'address_operand' uses M
as the mode of memory ref that the address should be valid for.
Many predicates accept 'const_int' nodes even though their mode is
'VOIDmode'.

CONSTRAINT controls reloading and the choice of the best register
class to use for a value, as explained later (*note
Constraints::.).

People are often unclear on the difference between the constraint
and the predicate. The predicate helps decide whether a given
insn matches the pattern. The constraint plays no role in this
decision; instead, it controls various decisions in the case of
an insn which does match.

On CISC machines, PREDICATE is most often '"general_operand"'.
This function checks that the putative operand is either a
constant, a register or a memory reference, and that it is valid
for mode M.

For an operand that must be a register, PREDICATE should be
'"register_operand"'. It would be valid to use
'"general_operand"', since the reload pass would copy any
non-register operands through registers, but this would make GNU
CC do extra work, it would prevent invariant operands (such as
constant) from being removed from loops, and it would prevent the
register allocator from doing the best possible job. On RISC
machines, it is usually most efficient to allow PREDICATE to
accept only objects that the constraints allow.

For an operand that must be a constant, either use
'"immediate_operand"' for PREDICATE, or make the instruction
pattern's extra condition require a constant, or both. You cannot
expect the constraints to do this work! If the constraints allow
only constants, but the predicate allows something else, the
compiler will crash when that case arises.

'(match_scratch:M N CONSTRAINT)'
This expression is also a placeholder for operand number N and
indicates that operand must be a 'scratch' or 'reg' expression.

When matching patterns, this is completely equivalent to

(match_operand:M N "scratch_operand" PRED)

but, when generating RTL, it produces a ('scratch':M) expression.

If the last few expressions in a 'parallel' are 'clobber'
expressions whose operands are either a hard register or
'match_scratch', the combiner can add them when necessary. *Note
Side Effects::.

'(match_dup N)'
This expression is also a placeholder for operand number N. It
is used when the operand needs to appear more than once in the
insn.

In construction, 'match_dup' behaves exactly like
'match_operand': the operand is substituted into the insn being
constructed. But in matching, 'match_dup' behaves differently.
It assumes that operand number N has already been determined by a
'match_operand' appearing earlier in the recognition template,
and it matches only an identical-looking expression.

'(match_operator:M N PREDICATE [OPERANDS...])'
This pattern is a kind of placeholder for a variable RTL
expression code.

When constructing an insn, it stands for an RTL expression whose
expression code is taken from that of operand N, and whose
operands are constructed from the patterns OPERANDS.

When matching an expression, it matches an expression if the
function PREDICATE returns nonzero on that expression *and* the
patterns OPERANDS match the operands of the expression.

Suppose that the function 'commutative_operator' is defined as
follows, to match any expression whose operator is one of the
commutative arithmetic operators of RTL and whose mode is MODE:

int
commutative_operator (x, mode)
rtx x;
enum machine_mode mode;
{
enum rtx_code code = GET_CODE (x);
if (GET_MODE (x) != mode)
return 0;
return GET_RTX_CLASS (code) == 'c' || code == EQ || code == NE;
}

Then the following pattern will match any RTL expression
consisting of a commutative operator applied to two general
operands:

(match_operator:SI 3 "commutative_operator"
[(match_operand:SI 1 "general_operand" "g")
(match_operand:SI 2 "general_operand" "g")])

Here the vector '[OPERANDS...]' contains two patterns because the
expressions to be matched all contain two operands.

When this pattern does match, the two operands of the commutative
operator are recorded as operands 1 and 2 of the insn. (This is
done by the two instances of 'match_operand'.) Operand 3 of the
insn will be the entire commutative expression: use 'GET_CODE
(operands[3])' to see which commutative operator was used.

The machine mode M of 'match_operator' works like that of
'match_operand': it is passed as the second argument to the
predicate function, and that function is solely responsible for
deciding whether the expression to be matched "has" that mode.

When constructing an insn, argument 3 of the gen-function will
specify the operation (i.e. the expression code) for the
expression to be made. It should be an RTL expression, whose
expression code is copied into a new expression whose operands
are arguments 1 and 2 of the gen-function. The subexpressions of
argument 3 are not used; only its expression code matters.

When 'match_operator' is used in a pattern for matching an insn,
it usually best if the operand number of the 'match_operator' is
higher than that of the actual operands of the insn. This
improves register allocation because the register allocator often
looks at operands 1 and 2 of insns to see if it can do register
tying.

There is no way to specify constraints in 'match_operator'. The
operand of the insn which corresponds to the 'match_operator'
never has any constraints because it is never reloaded as a whole.
However, if parts of its OPERANDS are matched by 'match_operand'
patterns, those parts may have constraints of their own.

'(address (match_operand:M N "address_operand" ""))'
This complex of expressions is a placeholder for an operand number
N in a "load address" instruction: an operand which specifies a
memory location in the usual way, but for which the actual operand
value used is the address of the location, not the contents of the
location.

'address' expressions never appear in RTL code, only in machine
descriptions. And they are used only in machine descriptions
that do not use the operand constraint feature. When operand
constraints are in use, the letter 'p' in the constraint serves
this purpose.

M is the machine mode of the *memory location being addressed*,
not the machine mode of the address itself. That mode is always
the same on a given target machine (it is 'Pmode', which normally
is 'SImode'), so there is no point in mentioning it; thus, no
machine mode is written in the 'address' expression. If some day
support is added for machines in which addresses of different
kinds of objects appear differently or are used differently (such
as the PDP-10), different formats would perhaps need different
machine modes and these modes might be written in the 'address'
expression.


File: gcc.info, Node: Output Template, Next: Output Statement, Prev: RTL Template, Up: Machine
Desc

Output Templates and Operand Substitution
=========================================

The "output template" is a string which specifies how to output the
assembler code for an instruction pattern. Most of the template is a
fixed string which is output literally. The character '%' is used to
specify where to substitute an operand; it can also be used to
identify places where different variants of the assembler require
different syntax.

In the simplest case, a '%' followed by a digit N says to output
operand N at that point in the string.

'%' followed by a letter and a digit says to output an operand in an
alternate fashion. Four letters have standard, built-in meanings
described below. The machine description macro 'PRINT_OPERAND' can
define additional letters with nonstandard meanings.

'%cDIGIT' can be used to substitute an operand that is a constant
value without the syntax that normally indicates an immediate operand.

'%nDIGIT' is like '%cDIGIT' except that the value of the constant
is negated before printing.

'%aDIGIT' can be used to substitute an operand as if it were a
memory reference, with the actual operand treated as the address.
This may be useful when outputting a "load address" instruction,
because often the assembler syntax for such an instruction requires
you to write the operand as if it were a memory reference.

'%lDIGIT' is used to substitute a 'label_ref' into a jump
instruction.

'%' followed by a punctuation character specifies a substitution
that does not use an operand. Only one case is standard: '%%' outputs
a '%' into the assembler code. Other nonstandard cases can be defined
in the 'PRINT_OPERAND' macro. You must also define which punctuation
characters are valid with the 'PRINT_OPERAND_PUNCT_VALID_P' macro.

The template may generate multiple assembler instructions. Write
the text for the instructions, with '\;' between them.

When the RTL contains two operands which are required by constraint
to match each other, the output template must refer only to the
lower-numbered operand. Matching operands are not always identical,
and the rest of the compiler arranges to put the proper RTL expression
for printing into the lower-numbered operand.

One use of nonstandard letters or punctuation following '%' is to
distinguish between different assembler languages for the same
machine; for example, Motorola syntax versus MIT syntax for the 68000.
Motorola syntax requires periods in most opcode names, while MIT
syntax does not. For example, the opcode 'movel' in MIT syntax is
'move.l' in Motorola syntax. The same file of patterns is used for
both kinds of output syntax, but the character sequence '%.' is used
in each place where Motorola syntax wants a period. The
'PRINT_OPERAND' macro for Motorola syntax defines the sequence to
output a period; the macro for MIT syntax defines it to do nothing.


File: gcc.info, Node: Output Statement, Next: Constraints, Prev: Output Template, Up: Machine Desc

C Statements for Generating Assembler Output
============================================

Often a single fixed template string cannot produce correct and
efficient assembler code for all the cases that are recognized by a
single instruction pattern. For example, the opcodes may depend on
the kinds of operands; or some unfortunate combinations of operands
may require extra machine instructions.

If the output control string starts with a '@', then it is actually
a series of templates, each on a separate line. (Blank lines and
leading spaces and tabs are ignored.) The templates correspond to the
pattern's constraint alternatives (*note Multi-Alternative::.). For
example, if a target machine has a two-address add instruction 'addr'
to add into a register and another 'addm' to add a register to memory,
you might write this pattern:

(define_insn "addsi3"
[(set (match_operand:SI 0 "general_operand" "r,m")
(plus:SI (match_operand:SI 1 "general_operand" "0,0")
(match_operand:SI 2 "general_operand" "g,r")))]
""
"@
addr %1,%0
addm %1,%0")

If the output control string starts with a '*', then it is not an
output template but rather a piece of C program that should compute a
template. It should execute a 'return' statement to return the
template-string you want. Most such templates use C string literals,
which require doublequote characters to delimit them. To include these
doublequote characters in the string, prefix each one with '\'.

The operands may be found in the array 'operands', whose C data type
is 'rtx []'.

It is very common to select different ways of generating assembler
code based on whether an immediate operand is within a certain range.
Be careful when doing this, because the result of 'INTVAL' is an
integer on the host machine. If the host machine has more bits in an
'int' than the target machine has in the mode in which the constant
will be used, then some of the bits you get from 'INTVAL' will be
superfluous. For proper results, you must carefully disregard the
values of those bits.

It is possible to output an assembler instruction and then go on to
output or compute more of them, using the subroutine
'output_asm_insn'. This receives two arguments: a template-string and
a vector of operands. The vector may be 'operands', or it may be
another array of 'rtx' that you declare locally and initialize
yourself.

When an insn pattern has multiple alternatives in its constraints,
often the appearance of the assembler code is determined mostly by
which alternative was matched. When this is so, the C code can test
the variable 'which_alternative', which is the ordinal number of the
alternative that was actually satisfied (0 for the first, 1 for the
second alternative, etc.).

For example, suppose there are two opcodes for storing zero,
'clrreg' for registers and 'clrmem' for memory locations. Here is how
a pattern could use 'which_alternative' to choose between them:

(define_insn ""
[(set (match_operand:SI 0 "general_operand" "r,m")
(const_int 0))]
""
"*
return (which_alternative == 0
? \"clrreg %0\" : \"clrmem %0\");
")

The example above, where the assembler code to generate was
*solely* determined by the alternative, could also have been specified
as follows, having the output control string start with a '@':

(define_insn ""
[(set (match_operand:SI 0 "general_operand" "r,m")
(const_int 0))]
""
"@
clrreg %0
clrmem %0")


File: gcc.info, Node: Constraints, Next: Standard Names, Prev: Output Statement, Up: Machine Desc

Operand Constraints
===================

Each 'match_operand' in an instruction pattern can specify a
constraint for the type of operands allowed. Constraints can say
whether an operand may be in a register, and which kinds of register;
whether the operand can be a memory reference, and which kinds of
address; whether the operand may be an immediate constant, and which
possible values it may have. Constraints can also require two
operands to match.

* Menu:

* Simple Constraints:: Basic use of constraints.
* Multi-Alternative:: When an insn has two alternative constraint-patterns.
* Class Preferences:: Constraints guide which hard register to put things in.
* Modifiers:: More precise control over effects of constraints.
* No Constraints:: Describing a clean machine without constraints.



Assembler Instructions as Expressions
=====================================

The RTX code 'asm_operands' represents a value produced by a
user-specified assembler instruction. It is used to represent an
'asm' statement with arguments. An 'asm' statement with a single
output operand, like this:

asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));

is represented using a single 'asm_operands' RTX which represents the
value that is stored in 'outputvar':

(set RTX-FOR-OUTPUTVAR
(asm_operands "foo %1,%2,%0" "a" 0
[RTX-FOR-ADDITION-RESULT RTX-FOR-*Z]
[(asm_input:M1 "g")
(asm_input:M2 "di")]))

Here the operands of the 'asm_operands' RTX are the assembler template
string, the output-operand's constraint, the index-number of the
output operand among the output operands specified, a vector of input
operand RTX's, and a vector of input-operand modes and constraints.
The mode M1 is the mode of the sum 'x+y'; M2 is that of '*z'.

When an 'asm' statement has multiple output values, its insn has
several such 'set' RTX's inside of a 'parallel'. Each 'set' contains
a 'asm_operands'; all of these share the same assembler template and
vectors, but each contains the constraint for the respective output
operand. They are also distinguished by the output-operand index
number, which is 0, 1, ... for successive output operands.



Constant Expression Types
=========================

The simplest RTL expressions are those that represent constant
values.

'(const_int I)'
This type of expression represents the integer value I. I is
customarily accessed with the macro 'INTVAL' as in 'INTVAL
(EXP)', which is equivalent to 'XINT (EXP, 0)'.

Keep in mind that the result of 'INTVAL' is an integer on the host
machine. If the host machine has more bits in an 'int' than the
target machine has in the mode in which the constant will be
used, then some of the bits you get from 'INTVAL' will be
superfluous. In many cases, for proper results, you must
carefully disregard the values of those bits.

There is only one expression object for the integer value zero;
it is the value of the variable 'const0_rtx'. Likewise, the only
expression for integer value one is found in 'const1_rtx', the
only expression for integer value two is found in 'const2_rtx',
and the only expression for integer value negative one is found in
'constm1_rtx'. Any attempt to create an expression of code
'const_int' and value zero, one, two or negative one will return
'const0_rtx', 'const1_rtx', 'const2_rtx' or 'constm1_rtx' as
appropriate.

Similarly, there is only one object for the integer whose value is
'STORE_FLAG_VALUE'. It is found in 'const_true_rtx'. If
'STORE_FLAG_VALUE' is one, 'const_true_rtx' and 'const1_rtx' will
point to the same object. If 'STORE_FLAG_VALUE' is -1,
'const_true_rtx' and 'constm1_rtx' will point to the same object.

'(const_double:M ADDR I0 I1 ...)'
Represents either a floating-point constant of mode M or an
integer constant that is too large to fit into 'HOST_BITS_PER_INT'
bits but small enough to fit within twice that number of bits
(GNU CC does not provide a mechanism to represent even larger
constants). In the latter case, M will be 'VOIDmode'.

ADDR is used to contain the 'mem' expression that corresponds to
the location in memory that at which the constant can be found.
If it has not been allocated a memory location, but is on the
chain of all 'const_double' expressions in this compilation
(maintained using an undisplayed field), ADDR contains
'const0_rtx'. If it is not on the chain, ADDR contains
'cc0_rtx'. ADDR is customarily accessed with the macro
'CONST_DOUBLE_MEM' and the chain field via 'CONST_DOUBLE_CHAIN'.

If M is 'VOIDmode', the bits of the value are stored in I0 and
I1. I0 is customarily accessed with the macro 'CONST_DOUBLE_LOW'
and I1 with 'CONST_DOUBLE_HIGH'.

If the constant is floating point (either single or double
precision), then the number of integers used to store the value
depends on the size of 'REAL_VALUE_TYPE' (*note
Cross-compilation::.). The integers represent a 'double'. To
convert them to a 'double', do

union real_extract u;
bcopy (&CONST_DOUBLE_LOW (x), &u, sizeof u);

and then refer to 'u.d'.

The macro 'CONST0_RTX (MODE)' refers to an expression with value
0 in mode MODE. If mode MODE is of mode class 'MODE_INT', it
returns 'const0_rtx'. Otherwise, it returns a 'CONST_DOUBLE'
expression in mode MODE. Similarly, the macro 'CONST1_RTX
(MODE)' refers to an expression with value 1 in mode MODE and
similarly for 'CONST2_RTX'.

'(const_string STR)'
Represents a constant string with value STR. Currently this is
used only for insn attributes (*note Insn Attributes::.) since
constant strings in C are placed in memory.

'(symbol_ref:MODE SYMBOL)'
Represents the value of an assembler label for data. SYMBOL is a
string that describes the name of the assembler label. If it
starts with a '*', the label is the rest of SYMBOL not including
the '*'. Otherwise, the label is SYMBOL, usually prefixed with
'_'.

The 'symbol_ref' contains a mode, which is usually 'Pmode'.
Usually that is the only mode for which a symbol is directly
valid.

'(label_ref LABEL)'
Represents the value of an assembler label for code. It contains
one operand, an expression, which must be a 'code_label' that
appears in the instruction sequence to identify the place where
the label should go.

The reason for using a distinct expression type for code label
references is so that jump optimization can distinguish them.

'(const:M EXP)'
Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, EXP, is an expression that
contains only constants ('const_int', 'symbol_ref' and
'label_ref' expressions) combined with 'plus' and 'minus'.
However, not all combinations are valid, since the assembler
cannot do arbitrary arithmetic on relocatable symbols.

M should be 'Pmode'.

'(high:M EXP)'
Represents the high-order bits of EXP, usually a 'symbol_ref'.
The number of bits is machine-dependent and is normally the
number of bits specified in an instruction that initializes the
high order bits of a register. It is used with 'lo_sum' to
represent the typical two-instruction sequence used in RISC
machines to reference a global memory location.

M should be 'Pmode'.


File: gcc.info, Node: Simple Constraints, Next: Multi-Alternative, Prev: Constraints, Up: Constraints

Simple Constraints
------------------

The simplest kind of constraint is a string full of letters, each of
which describes one kind of operand that is permitted. Here are the
letters that are allowed:

'm'
A memory operand is allowed, with any kind of address that the
machine supports in general.

'o'
A memory operand is allowed, but only if the address is
"offsettable". This means that adding a small integer (actually,
the width in bytes of the operand, as determined by its machine
mode) may be added to the address and the result is also a valid
memory address.

For example, an address which is constant is offsettable; so is an
address that is the sum of a register and a constant (as long as a
slightly larger constant is also within the range of
address-offsets supported by the machine); but an autoincrement
or autodecrement address is not offsettable. More complicated
indirect/indexed addresses may or may not be offsettable
depending on the other addressing modes that the machine supports.

Note that in an output operand which can be matched by another
operand, the constraint letter 'o' is valid only when accompanied
by both '<' (if the target machine has predecrement addressing)
and '>' (if the target machine has preincrement addressing).

'V'
A memory operand that is not offsettable. In other words,
anything that would fit the 'm' constraint but not the 'o'
constraint.

'<'
A memory operand with autodecrement addressing (either
predecrement or postdecrement) is allowed.

'>'
A memory operand with autoincrement addressing (either
preincrement or postincrement) is allowed.

'r'
A register operand is allowed provided that it is in a general
register.

'd', 'a', 'f', ...
Other letters can be defined in machine-dependent fashion to
stand for particular classes of registers. 'd', 'a' and 'f' are
defined on the 68000/68020 to stand for data, address and floating
point registers.

'i'
An immediate integer operand (one with constant value) is allowed.
This includes symbolic constants whose values will be known only
at assembly time.

'n'
An immediate integer operand with a known numeric value is
allowed. Many systems cannot support assembly-time constants for
operands less than a word wide. Constraints for these operands
should use 'n' rather than 'i'.

'I', 'J', 'K', ... 'P'
Other letters in the range 'I' through 'P' may be defined in a
machine-dependent fashion to permit immediate integer operands
with explicit integer values in specified ranges. For example,
on the 68000, 'I' is defined to stand for the range of values 1
to 8. This is the range permitted as a shift count in the shift
instructions.

'E'
An immediate floating operand (expression code 'const_double') is
allowed, but only if the target floating point format is the same
as that of the host machine (on which the compiler is running).

'F'
An immediate floating operand (expression code 'const_double') is
allowed.

'G', 'H'
'G' and 'H' may be defined in a machine-dependent fashion to
permit immediate floating operands in particular ranges of values.

's'
An immediate integer operand whose value is not an explicit
integer is allowed.

This might appear strange; if an insn allows a constant operand
with a value not known at compile time, it certainly must allow
any known value. So why use 's' instead of 'i'? Sometimes it
allows better code to be generated.

For example, on the 68000 in a fullword instruction it is
possible to use an immediate operand; but if the immediate value
is between -128 and 127, better code results from loading the
value into a register and using the register. This is because
the load into the register can be done with a 'moveq'
instruction. We arrange for this to happen by defining the
letter 'K' to mean "any integer outside the range -128 to 127",
and then specifying 'Ks' in the operand constraints.

'g'
Any register, memory or immediate integer operand is allowed,
except for registers that are not general registers.

'X'
Any operand whatsoever is allowed, even if it does not satisfy
'general_operand'. This is normally used in the constraint of a
'match_scratch' when certain alternatives will not actually
require a scratch register.

'0', '1', '2', ... '9'
An operand that matches the specified operand number is allowed.
If a digit is used together with letters within the same
alternative, the digit should come last.

This is called a "matching constraint" and what it really means is
that the assembler has only a single operand that fills two roles
considered separate in the RTL insn. For example, an add insn
has two input operands and one output operand in the RTL, but on
most machines an add instruction really has only two operands,
one of them an input-output operand.

Matching constraints work only in circumstances like that add
insn. More precisely, the two operands that match must include
one input-only operand and one output-only operand. Moreover,
the digit must be a smaller number than the number of the operand
that uses it in the constraint.

For operands to match in a particular case usually means that they
are identical-looking RTL expressions. But in a few special cases
specific kinds of dissimilarity are allowed. For example, '*x'
as an input operand will match '*x++' as an output operand. For
proper results in such cases, the output template should always
use the output-operand's number when printing the operand.

'p'
An operand that is a valid memory address is allowed. This is
for "load address" and "push address" instructions.

'p' in the constraint must be accompanied by 'address_operand' as
the predicate in the 'match_operand'. This predicate interprets
the mode specified in the 'match_operand' as the mode of the
memory reference for which the address would be valid.

'Q', 'R', 'S', ... 'U'
Letters in the range 'Q' through 'U' may be defined in a
machine-dependent fashion to stand for arbitrary operand types.
The machine description macro 'EXTRA_CONSTRAINT' is passed the
operand as its first argument and the constraint letter as its
second operand.

A typical use for this would be to distinguish certain types of
memory references that affect other insn operands.

Do not define these constraint letters to accept register
references ('reg'); the reload pass does not expect this and
would not handle it properly.

In order to have valid assembler code, each operand must satisfy
its constraint. But a failure to do so does not prevent the pattern
from applying to an insn. Instead, it directs the compiler to modify
the code so that the constraint will be satisfied. Usually this is
done by copying an operand into a register.

Contrast, therefore, the two instruction patterns that follow:

(define_insn ""
[(set (match_operand:SI 0 "general_operand" "r")
(plus:SI (match_dup 0)
(match_operand:SI 1 "general_operand" "r")))]
""
"...")

which has two operands, one of which must appear in two places, and

(define_insn ""
[(set (match_operand:SI 0 "general_operand" "r")
(plus:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "r")))]
""
"...")

which has three operands, two of which are required by a constraint to
be identical. If we are considering an insn of the form

(insn N PREV NEXT
(set (reg:SI 3)
(plus:SI (reg:SI 6) (reg:SI 109)))
...)

the first pattern would not apply at all, because this insn does not
contain two identical subexpressions in the right place. The pattern
would say, "That does not look like an add instruction; try other
patterns." The second pattern would say, "Yes, that's an add
instruction, but there is something wrong with it." It would direct
the reload pass of the compiler to generate additional insns to make
the constraint true. The results might look like this:

(insn N2 PREV N
(set (reg:SI 3) (reg:SI 6))
...)

(insn N N2 NEXT
(set (reg:SI 3)
(plus:SI (reg:SI 3) (reg:SI 109)))
...)

It is up to you to make sure that each operand, in each pattern, has
constraints that can handle any RTL expression that could be present
for that operand. (When multiple alternatives are in use, each
pattern must, for each possible combination of operand expressions,
have at least one alternative which can handle that combination of
operands.) The constraints don't need to *allow* any possible
operand--when this is the case, they do not constrain--but they must
at least point the way to reloading any possible operand so that it
will fit.

* If the constraint accepts whatever operands the predicate permits,
there is no problem: reloading is never necessary for this
operand.

For example, an operand whose constraints permit everything except
registers is safe provided its predicate rejects registers.

An operand whose predicate accepts only constant values is safe
provided its constraints include the letter 'i'. If any possible
constant value is accepted, then nothing less than 'i' will do;
if the predicate is more selective, then the constraints may also
be more selective.

* Any operand expression can be reloaded by copying it into a
register. So if an operand's constraints allow some kind of
register, it is certain to be safe. It need not permit all
classes of registers; the compiler knows how to copy a register
into another register of the proper class in order to make an
instruction valid.

* A nonoffsettable memory reference can be reloaded by copying the
address into a register. So if the constraint uses the letter
'o', all memory references are taken care of.

* A constant operand can be reloaded by allocating space in memory
to hold it as preinitialized data. Then the memory reference can
be used in place of the constant. So if the constraint uses the
letters 'o' or 'm', constant operands are not a problem.

* If the constraint permits a constant and a pseudo register used
in an insn was not allocated to a hard register and is equivalent
to a constant, the register will be replaced with the constant.
If the predicate does not permit a constant and the insn is
re-recognized for some reason, the compiler will crash. Thus the
predicate must always recognize any objects allowed by the
constraint.

If the operand's predicate can recognize registers, but the
constraint does not permit them, it can make the compiler crash. When
this operand happens to be a register, the reload pass will be
stymied, because it does not know how to copy a register temporarily
into memory.


File: gcc.info, Node: Multi-Alternative, Next: Class Preferences, Prev: Simple Constraints, Up: Constraints

Multiple Alternative Constraints
--------------------------------

Sometimes a single instruction has multiple alternative sets of
possible operands. For example, on the 68000, a logical-or
instruction can combine register or an immediate value into memory, or
it can combine any kind of operand into a register; but it cannot
combine one memory location into another.

These constraints are represented as multiple alternatives. An
alternative can be described by a series of letters for each operand.
The overall constraint for an operand is made from the letters for
this operand from the first alternative, a comma, the letters for this
operand from the second alternative, a comma, and so on until the last
alternative. Here is how it is done for fullword logical-or on the
68000:

(define_insn "iorsi3"
[(set (match_operand:SI 0 "general_operand" "=m,d")
(ior:SI (match_operand:SI 1 "general_operand" "%0,0")
(match_operand:SI 2 "general_operand" "dKs,dmKs")))]
...)

The first alternative has 'm' (memory) for operand 0, '0' for
operand 1 (meaning it must match operand 0), and 'dKs' for operand 2.
The second alternative has 'd' (data register) for operand 0, '0' for
operand 1, and 'dmKs' for operand 2. The '=' and '%' in the
constraints apply to all the alternatives; their meaning is explained
in the next section (*note Class Preferences::.).

If all the operands fit any one alternative, the instruction is
valid. Otherwise, for each alternative, the compiler counts how many
instructions must be added to copy the operands so that that
alternative applies. The alternative requiring the least copying is
chosen. If two alternatives need the same amount of copying, the one
that comes first is chosen. These choices can be altered with the '?'
and '!' characters:

'?'
Disparage slightly the alternative that the '?' appears in, as a
choice when no alternative applies exactly. The compiler regards
this alternative as one unit more costly for each '?' that appears
in it.

'!'
Disparage severely the alternative that the '!' appears in. This
alternative can still be used if it fits without reloading, but
if reloading is needed, some other alternative will be used.

When an insn pattern has multiple alternatives in its constraints,
often the appearance of the assembler code is determined mostly by
which alternative was matched. When this is so, the C code for
writing the assembler code can use the variable 'which_alternative',
which is the ordinal number of the alternative that was actually
satisfied (0 for the first, 1 for the second alternative, etc.).
*Note Output Statement::.


File: gcc.info, Node: Class Preferences, Next: Modifiers, Prev: Multi-Alternative, Up: Constraints

Register Class Preferences
--------------------------

The operand constraints have another function: they enable the
compiler to decide which kind of hardware register a pseudo register
is best allocated to. The compiler examines the constraints that
apply to the insns that use the pseudo register, looking for the
machine-dependent letters such as 'd' and 'a' that specify classes of
registers. The pseudo register is put in whichever class gets the
most "votes". The constraint letters 'g' and 'r' also vote: they vote
in favor of a general register. The machine description says which
registers are considered general.

Of course, on some machines all registers are equivalent, and no
register classes are defined. Then none of this complexity is
relevant.


File: gcc.info, Node: Modifiers, Next: No Constraints, Prev: Class Preferences, Up: Constraints

Constraint Modifier Characters
------------------------------

'='
Means that this operand is write-only for this instruction: the
previous value is discarded and replaced by output data.

'+'
Means that this operand is both read and written by the
instruction.

When the compiler fixes up the operands to satisfy the
constraints, it needs to know which operands are inputs to the
instruction and which are outputs from it. '=' identifies an
output; '+' identifies an operand that is both input and output;
all other operands are assumed to be input only.

'&'
Means (in a particular alternative) that this operand is written
before the instruction is finished using the input operands.
Therefore, this operand may not lie in a register that is used as
an input operand or as part of any memory address.

'&' applies only to the alternative in which it is written. In
constraints with multiple alternatives, sometimes one alternative
requires '&' while others do not. See, for example, the 'movdf'
insn of the 68000.

'&' does not obviate the need to write '='.

'%'
Declares the instruction to be commutative for this operand and
the following operand. This means that the compiler may
interchange the two operands if that is the cheapest way to make
all operands fit the constraints. This is often used in patterns
for addition instructions that really have only two operands: the
result must go in one of the arguments. Here for example, is how
the 68000 halfword-add instruction is defined:

(define_insn "addhi3"
[(set (match_operand:HI 0 "general_operand" "=m,r")
(plus:HI (match_operand:HI 1 "general_operand" "%0,0")
(match_operand:HI 2 "general_operand" "di,g")))]
...)

'#'
Says that all following characters, up to the next comma, are to
be ignored as a constraint. They are significant only for
choosing register preferences.

'*'
Says that the following character should be ignored when choosing
register preferences. '*' has no effect on the meaning of the
constraint as a constraint, and no effect on reloading.

Here is an example: the 68000 has an instruction to sign-extend a
halfword in a data register, and can also sign-extend a value by
copying it into an address register. While either kind of
register is acceptable, the constraints on an address-register
destination are less strict, so it is best if register allocation
makes an address register its goal. Therefore, '*' is used so
that the 'd' constraint letter (for data register) is ignored
when computing register preferences.

(define_insn "extendhisi2"
[(set (match_operand:SI 0 "general_operand" "=*d,a")
(sign_extend:SI
(match_operand:HI 1 "general_operand" "0,g")))]
...)


File: gcc.info, Node: No Constraints, Prev: Modifiers, Up: Constraints

Not Using Constraints
---------------------

Some machines are so clean that operand constraints are not
required. For example, on the Vax, an operand valid in one context is
valid in any other context. On such a machine, every operand
constraint would be 'g', excepting only operands of "load address"
instructions which are written as if they referred to a memory
location's contents but actual refer to its address. They would have
constraint 'p'.

For such machines, instead of writing 'g' and 'p' for all the
constraints, you can choose to write a description with empty
constraints. Then you write '""' for the constraint in every
'match_operand'. Address operands are identified by writing an
'address' expression around the 'match_operand', not by their
constraints.

When the machine description has just empty constraints, certain
parts of compilation are skipped, making the compiler faster. However,
few machines actually do not need constraints; all machine descriptions
now in existence use constraints.


File: gcc.info, Node: Standard Names, Next: Pattern Ordering, Prev: Constraints, Up: Machine Desc

Standard Names for Patterns Used in Generation
==============================================

Here is a table of the instruction names that are meaningful in the
RTL generation pass of the compiler. Giving one of these names to an
instruction pattern tells the RTL generation pass that it can use the
pattern in to accomplish a certain task.

'movM'
Here M stands for a two-letter machine mode name, in lower case.
This instruction pattern moves data with that machine mode from
operand 1 to operand 0. For example, 'movsi' moves full-word
data.

If operand 0 is a 'subreg' with mode M of a register whose own
mode is wider than M, the effect of this instruction is to store
the specified value in the part of the register that corresponds
to mode M. The effect on the rest of the register is undefined.

This class of patterns is special in several ways. First of all,
each of these names *must* be defined, because there is no other
way to copy a datum from one place to another.

Second, these patterns are not used solely in the RTL generation
pass. Even the reload pass can generate move insns to copy
values from stack slots into temporary registers. When it does
so, one of the operands is a hard register and the other is an
operand that can need to be reloaded into a register.

Therefore, when given such a pair of operands, the pattern must
generate RTL which needs no reloading and needs no temporary
registers--no registers other than the operands. For example, if
you support the pattern with a 'define_expand', then in such a
case the 'define_expand' mustn't call 'force_reg' or any other
such function which might generate new pseudo registers.

This requirement exists even for subword modes on a RISC machine
where fetching those modes from memory normally requires several
insns and some temporary registers. Look in 'spur.md' to see how
the requirement can be satisfied.

During reload a memory reference with an invalid address may be
passed as an operand. Such an address will be replaced with a
valid address later in the reload pass. In this case, nothing
may be done with the address except to use it as it stands. If
it is copied, it will not be replaced with a valid address. No
attempt should be made to make such an address into a valid
address and no routine (such as 'change_address') that will do so
may be called. Note that 'general_operand' will fail when
applied to such an address.

The global variable 'reload_in_progress' (which must be explicitly
declared if required) can be used to determine whether such
special handling is required.

The variety of operands that have reloads depends on the rest of
the machine description, but typically on a RISC machine these
can only be pseudo registers that did not get hard registers,
while on other machines explicit memory references will get
optional reloads.

If a scratch register is required to move an object to or from
memory, it can be allocated using 'gen_reg_rtx' prior to reload.
But this is impossible during and after reload. If there are
cases needing scratch registers after reload, you must define
'SECONDARY_INPUT_RELOAD_CLASS' and/or
'SECONDARY_OUTPUT_RELOAD_CLASS' to detect them, and provide
patterns 'reload_inM' or 'reload_outM' to handle them. *Note
Register Classes::.

The constraints on a 'moveM' must permit moving any hard register
to any other hard register provided that 'HARD_REGNO_MODE_OK'
permits mode M in both registers and 'REGISTER_MOVE_COST' applied
to their classes returns a value of 2.

It is obligatory to support floating point 'moveM' instructions
into and out of any registers that can hold fixed point values,
because unions and structures (which have modes 'SImode' or
'DImode') can be in those registers and they may have floating
point members.

There may also be a need to support fixed point 'moveM'
instructions in and out of floating point registers.
Unfortunately, I have forgotten why this was so, and I don't know
whether it is still true. If 'HARD_REGNO_MODE_OK' rejects fixed
point values in floating point registers, then the constraints of
the fixed point 'moveM' instructions must be designed to avoid
ever trying to reload into a floating point register.

'reload_inM'
'reload_outM'
Like 'movM', but used when a scratch register is required to move
between operand 0 and operand 1. Operand 2 describes the scratch
register. See the discussion of the 'SECONDARY_RELOAD_CLASS'
macro in *note Register Classes::..

'movstrictM'
Like 'movM' except that if operand 0 is a 'subreg' with mode M of
a register whose natural mode is wider, the 'movstrictM'
instruction is guaranteed not to alter any of the register except
the part which belongs to mode M.

'addM3'
Add operand 2 and operand 1, storing the result in operand 0.
All operands must have mode M. This can be used even on
two-address machines, by means of constraints requiring operands
1 and 0 to be the same location.

'subM3', 'mulM3'
'divM3', 'udivM3', 'modM3', 'umodM3'
'sminM3', 'smaxM3', 'uminM3', 'umaxM3'
'andM3', 'iorM3', 'xorM3'
Similar, for other arithmetic operations.

'mulhisi3'
Multiply operands 1 and 2, which have mode 'HImode', and store a
'SImode' product in operand 0.

'mulqihi3', 'mulsidi3'
Similar widening-multiplication instructions of other widths.

'umulqihi3', 'umulhisi3', 'umulsidi3'
Similar widening-multiplication instructions that do unsigned
multiplication.

'divmodM4'
Signed division that produces both a quotient and a remainder.
Operand 1 is divided by operand 2 to produce a quotient stored in
operand 0 and a remainder stored in operand 3.

For machines with an instruction that produces both a quotient
and a remainder, provide a pattern for 'divmodM4' but do not
provide patterns for 'divM3' and 'modM3'. This allows
optimization in the relatively common case when both the quotient
and remainder are computed.

If an instruction that just produces a quotient or just a
remainder exists and is more efficient than the instruction that
produces both, write the output routine of 'divmodM4' to call
'find_reg_note' and look for a 'REG_UNUSED' note on the quotient
or remainder and generate the appropriate instruction.

'udivmodM4'
Similar, but does unsigned division.

'ashlM3'
Arithmetic-shift operand 1 left by a number of bits specified by
operand 2, and store the result in operand 0. Operand 2 has mode
'SImode', not mode M.

'ashrM3', 'lshlM3', 'lshrM3', 'rotlM3', 'rotrM3'
Other shift and rotate instructions.

Logical and arithmetic left shift are the same. Machines that do
not allow negative shift counts often have only one instruction
for shifting left. On such machines, you should define a pattern
named 'ashlM3' and leave 'lshlM3' undefined.

'negM2'
Negate operand 1 and store the result in operand 0.

'absM2'
Store the absolute value of operand 1 into operand 0.

'sqrtM2'
Store the square root of operand 1 into operand 0.

'ffsM2'
Store into operand 0 one plus the index of the least significant
1-bit of operand 1. If operand 1 is zero, store zero. M is the
mode of operand 0; operand 1's mode is specified by the
instruction pattern, and the compiler will convert the operand to
that mode before generating the instruction.

'one_cmplM2'
Store the bitwise-complement of operand 1 into operand 0.

'cmpM'
Compare operand 0 and operand 1, and set the condition codes.
The RTL pattern should look like this:

(set (cc0) (compare (match_operand:M 0 ...)
(match_operand:M 1 ...)))

'tstM'
Compare operand 0 against zero, and set the condition codes. The
RTL pattern should look like this:

(set (cc0) (match_operand:M 0 ...))

'tstM' patterns should not be defined for machines that do not
use '(cc0)'. Doing so would confuse the optimizer since it would
no longer be clear which 'set' operations were comparisons. The
'cmpM' patterns should be used instead.

'movstrM'
Block move instruction. The addresses of the destination and
source strings are the first two operands, and both are in mode
'Pmode'. The number of bytes to move is the third operand, in
mode M.

The fourth operand is the known shared alignment of the source and
destination, in the form of a 'const_int' rtx. Thus, if the
compiler knows that both source and destination are word-aligned,
it may provide the value 4 for this operand.

These patterns need not give special consideration to the
possibility that the source and destination strings might overlap.

'cmpstrM'
Block compare instruction, with five operands. Operand 0 is the
output; it has mode M. The remaining four operands are like the
operands of 'movstrM'. The two memory blocks specified are
compared byte by byte in lexicographic order. The effect of the
instruction is to store a value in operand 0 whose sign indicates
the result of the comparison.

'floatMN2'
Convert signed integer operand 1 (valid for fixed point mode M) to
floating point mode N and store in operand 0 (which has mode N).

'floatunsMN2'
Convert unsigned integer operand 1 (valid for fixed point mode M)
to floating point mode N and store in operand 0 (which has mode
N).

'fixMN2'
Convert operand 1 (valid for floating point mode M) to fixed
point mode N as a signed number and store in operand 0 (which has
mode N). This instruction's result is defined only when the
value of operand 1 is an integer.

'fixunsMN2'
Convert operand 1 (valid for floating point mode M) to fixed
point mode N as an unsigned number and store in operand 0 (which
has mode N). This instruction's result is defined only when the
value of operand 1 is an integer.

'ftruncM2'
Convert operand 1 (valid for floating point mode M) to an integer
value, still represented in floating point mode M, and store it
in operand 0 (valid for floating point mode M).

'fix_truncMN2'
Like 'fixMN2' but works for any floating point value of mode M by
converting the value to an integer.

'fixuns_truncMN2'
Like 'fixunsMN2' but works for any floating point value of mode M
by converting the value to an integer.

'truncMN'
Truncate operand 1 (valid for mode M) to mode N and store in
operand 0 (which has mode N). Both modes must be fixed point or
both floating point.

'extendMN'
Sign-extend operand 1 (valid for mode M) to mode N and store in
operand 0 (which has mode N). Both modes must be fixed point or
both floating point.

'zero_extendMN'
Zero-extend operand 1 (valid for mode M) to mode N and store in
operand 0 (which has mode N). Both modes must be fixed point.

'extv'
Extract a bit field from operand 1 (a register or memory
operand), where operand 2 specifies the width in bits and operand
3 the starting bit, and store it in operand 0. Operand 0 must
have mode 'word_mode'. Operand 1 may have mode 'byte_mode' or
'word_mode'; often 'word_mode' is allowed only for registers.
Operands 2 and 3 must be valid for 'word_mode'.

The RTL generation pass generates this instruction only with
constants for operands 2 and 3.

The bit-field value is sign-extended to a full word integer
before it is stored in operand 0.

'extzv'
Like 'extv' except that the bit-field value is zero-extended.

'insv'
Store operand 3 (which must be valid for 'word_mode') into a bit
field in operand 0, where operand 1 specifies the width in bits
and operand 2 the starting bit. Operand 0 may have mode
'byte_mode' or 'word_mode'; often 'word_mode' is allowed only for
registers. Operands 1 and 2 must be valid for 'word_mode'.

The RTL generation pass generates this instruction only with
constants for operands 1 and 2.

'sCOND'
Store zero or nonzero in the operand according to the condition
codes. Value stored is nonzero iff the condition COND is true.
COND is the name of a comparison operation expression code, such
as 'eq', 'lt' or 'leu'.

You specify the mode that the operand must have when you write the
'match_operand' expression. The compiler automatically sees
which mode you have used and supplies an operand of that mode.

The value stored for a true condition must have 1 as its low bit,
or else must be negative. Otherwise the instruction is not
suitable and you should omit it from the machine description.
You describe to the compiler exactly which value is stored by
defining the macro 'STORE_FLAG_VALUE' (*note Misc::.). If a
description cannot be found that can be used for all the 'sCOND'
patterns, you should omit those operations from the machine
description.

These operations may fail, but should do so only in relatively
uncommon cases; if they would fail for common cases involving
integer comparisons, it is best to omit these patterns.

If these operations are omitted, the compiler will usually
generate code that copies the constant one to the target and
branches around an assignment of zero to the target. If this
code is more efficient than the potential instructions used for
the 'sCOND' pattern followed by those required to convert the
result into a 1 or a zero in 'SImode', you should omit the
'sCOND' operations from the machine description.

'bCOND'
Conditional branch instruction. Operand 0 is a 'label_ref' that
refers to the label to jump to. Jump if the condition codes meet
condition COND.

Some machines do not follow the model assumed here where a
comparison instruction is followed by a conditional branch
instruction. In that case, the 'cmpM' (and 'tstM') patterns
should simply store the operands away and generate all the
required insns in a 'define_expand' (*note Expander
Definitions::.) for the conditional branch operations. All calls
to expand 'vCOND' patterns are immediately preceded by calls to
expand either a 'cmpM' pattern or a 'tstM' pattern.

Machines that use a pseudo register for the condition code value,
or where the mode used for the comparison depends on the
condition being tested, should also use the above mechanism.
*Note Jump Patterns::

The above discussion also applies to 'sCOND' patterns.

'call'
Subroutine call instruction returning no value. Operand 0 is the
function to call; operand 1 is the number of bytes of arguments
pushed (in mode 'SImode', except it is normally a 'const_int');
operand 2 is the number of registers used as operands.

On most machines, operand 2 is not actually stored into the RTL
pattern. It is supplied for the sake of some RISC machines which
need to put this information into the assembler code; they can
put it in the RTL instead of operand 1.

Operand 0 should be a 'mem' RTX whose address is the address of
the function. Note, however, that this address can be a
'symbol_ref' expression even if it would not be a legitimate
memory address on the target machine. If it is also not a valid
argument for a call instruction, the pattern for this operation
should be a 'define_expand' (*note Expander Definitions::.) that
places the address into a register and uses that register in the
call instruction.

'call_value'
Subroutine call instruction returning a value. Operand 0 is the
hard register in which the value is returned. There are three
more operands, the same as the three operands of the 'call'
instruction (but with numbers increased by one).

Subroutines that return 'BLKmode' objects use the 'call' insn.

'call_pop', 'call_value_pop'
Similar to 'call' and 'call_value', except used if defined and if
'RETURN_POPS_ARGS' is non-zero. They should emit a 'parallel'
that contains both the function call and a 'set' to indicate the
adjustment made to the frame pointer.

For machines where 'RETURN_POPS_ARGS' can be non-zero, the use of
these patterns increases the number of functions for which the
frame pointer can be eliminated, if desired.

'return'
Subroutine return instruction. This instruction pattern name
should be defined only if a single instruction can do all the
work of returning from a function.

Like the 'movM' patterns, this pattern is also used after the RTL
generation phase. In this case it is to support machines where
multiple instructions are usually needed to return from a
function, but some class of functions only requires one
instruction to implement a return. Normally, the applicable
functions are those which do not need to save any registers or
allocate stack space.

For such machines, the condition specified in this pattern should
only be true when 'reload_completed' is non-zero and the
function's epilogue would only be a single instruction. For
machines with register windows, the routine 'leaf_function_p' may
be used to determine if a register window push is required.

Machines that have conditional return instructions should define
patterns such as

(define_insn ""
[(set (pc)
(if_then_else (match_operator 0 "comparison_operator"
[(cc0) (const_int 0)])
(return)
(pc)))]
"CONDITION"
"...")

where CONDITION would normally be the same condition specified on
the named 'return' pattern.

'nop'
No-op instruction. This instruction pattern name should always
be defined to output a no-op in assembler code. '(const_int 0)'
will do as an RTL pattern.

'indirect_jump'
An instruction to jump to an address which is operand zero. This
pattern name is mandatory on all machines.

'casesi'
Instruction to jump through a dispatch table, including bounds
checking. This instruction takes five operands:

1. The index to dispatch on, which has mode 'SImode'.

2. The lower bound for indices in the table, an integer
constant.

3. The total range of indices in the table--the largest index
minus the smallest one (both inclusive).

4. A label that precedes the table itself.

5. A label to jump to if the index has a value outside the
bounds. (If the machine-description macro
'CASE_DROPS_THROUGH' is defined, then an out-of-bounds index
drops through to the code following the jump table instead
of jumping to this label. In that case, this label is not
actually used by the 'casesi' instruction, but it is always
provided as an operand.)

The table is a 'addr_vec' or 'addr_diff_vec' inside of a
'jump_insn'. The number of elements in the table is one plus the
difference between the upper bound and the lower bound.

'tablejump'
Instruction to jump to a variable address. This is a low-level
capability which can be used to implement a dispatch table when
there is no 'casesi' pattern.

This pattern requires two operands: the address or offset, and a
label which should immediately precede the jump table. If the
macro 'CASE_VECTOR_PC_RELATIVE' is defined then the first operand
is an offset which counts from the address of the table;
otherwise, it is an absolute address to jump to.

The 'tablejump' insn is always the last insn before the jump
table it uses. Its assembler code normally has no need to use the
second operand, but you should incorporate it in the RTL pattern
so that the jump optimizer will not delete the table as
unreachable code.


File: gcc.info, Node: Pattern Ordering, Next: Dependent Patterns, Prev: Standard Names, Up: Machine Desc

When the Order of Patterns Matters
==================================

Sometimes an insn can match more than one instruction pattern.
Then the pattern that appears first in the machine description is the
one used. Therefore, more specific patterns (patterns that will match
fewer things) and faster instructions (those that will produce better
code when they do match) should usually go first in the description.

In some cases the effect of ordering the patterns can be used to
hide a pattern when it is not valid. For example, the 68000 has an
instruction for converting a fullword to floating point and another
for converting a byte to floating point. An instruction converting an
integer to floating point could match either one. We put the pattern
to convert the fullword first to make sure that one will be used
rather than the other. (Otherwise a large integer might be generated
as a single-byte immediate quantity, which would not work.) Instead of
using this pattern ordering it would be possible to make the pattern
for convert-a-byte smart enough to deal properly with any constant
value.


File: gcc.info, Node: Dependent Patterns, Next: Jump Patterns, Prev: Pattern Ordering, Up: Machine Desc

Interdependence of Patterns
===========================

Every machine description must have a named pattern for each of the
conditional branch names 'bCOND'. The recognition template must
always have the form

(set (pc)
(if_then_else (COND (cc0) (const_int 0))
(label_ref (match_operand 0 "" ""))
(pc)))

In addition, every machine description must have an anonymous pattern
for each of the possible reverse-conditional branches. Their templates
look like

(set (pc)
(if_then_else (COND (cc0) (const_int 0))
(pc)
(label_ref (match_operand 0 "" ""))))

They are necessary because jump optimization can turn
direct-conditional branches into reverse-conditional branches.

It is often convenient to use the 'match_operator' construct to
reduce the number of patterns that must be specified for branches. For
example,

(define_insn ""
[(set (pc)
(if_then_else (match_operator 0 "comparison_operator"
[(cc0) (const_int 0)])
(pc)
(label_ref (match_operand 1 "" ""))))]
"CONDITION"
"...")

In some cases machines support instructions identical except for the
machine mode of one or more operands. For example, there may be
"sign-extend halfword" and "sign-extend byte" instructions whose
patterns are

(set (match_operand:SI 0 ...)
(extend:SI (match_operand:HI 1 ...)))

(set (match_operand:SI 0 ...)
(extend:SI (match_operand:QI 1 ...)))

Constant integers do not specify a machine mode, so an instruction to
extend a constant value could match either pattern. The pattern it
actually will match is the one that appears first in the file. For
correct results, this must be the one for the widest possible mode
('HImode', here). If the pattern matches the 'QImode' instruction,
the results will be incorrect if the constant value does not actually
fit that mode.

Such instructions to extend constants are rarely generated because
they are optimized away, but they do occasionally happen in
nonoptimized compilations.

If a constraint in a pattern allows a constant, the reload pass may
replace a register with a constant permitted by the constraint in some
cases. Similarly for memory references. You must ensure that the
predicate permits all objects allowed by the constraints to prevent the
compiler from crashing.

Because of this substitution, you should not provide separate
patterns for increment and decrement instructions. Instead, they
should be generated from the same pattern that supports
register-register add insns by examining the operands and generating
the appropriate machine instruction.


File: gcc.info, Node: Jump Patterns, Next: Insn Canonicalizations, Prev: Dependent Patterns, Up: Machine Desc
File: gcc.info, Node: Insn Canonicalizations, Next: Peephole Definitions, Prev: Jump Patterns, Up: Machine Desc

Canonicalization of Instructions
================================

There are often cases where multiple RTL expressions could
represent an operation performed by a single machine instruction.
This situation is most commonly encountered with logical, branch, and
multiply-accumulate instructions. In such cases, the compiler
attempts to convert these multiple RTL expressions into a single
canonical form to reduce the number of insn patterns required.

In addition to algebraic simplifications, following
canonicalizations are performed:

* For commutative and comparison operators, a constant is always
made the second operand. If a machine only supports a constant
as the second operand, only patterns that match a constant in the
second operand need be supplied.

For these operators, if only one operand is a 'neg', 'not',
'mult', 'plus', or 'minus' expression, it will be the first
operand.

* For the 'compare' operator, a constant is always the second
operand on machines where 'cc0' is used (*note Jump Patterns::.).
On other machines, there are rare cases where the compiler might
want to construct a 'compare' with a constant as the first
operand. However, these cases are not common enough for it to be
worthwhile to provide a pattern matching a constant as the first
operand unless the machine actually has such an instruction.

An operand of 'neg', 'not', 'mult', 'plus', or 'minus' is made
the first operand under the same conditions as above.

* '(minus X (const_int N))' is converted to '(plus X (const_int
-N))'.

* Within address computations (i.e., inside 'mem'), a left shift is
converted into the appropriate multiplication by a power of two.

De'Morgan's Law is used to move bitwise negation inside a bitwise
logical-and or logical-or operation. If this results in only one
operand being a 'not' expression, it will be the first one.

A machine that has an instruction that performs a bitwise
logical-and of one operand with the bitwise negation of the other
should specify the pattern for that instruction as

(define_insn ""
[(set (match_operand:M 0 ...)
(and:M (not:M (match_operand:M 1 ...))
(match_operand:M 2 ...)))]
"..."
"...")

Similarly, a pattern for a "NAND" instruction should be written

(define_insn ""
[(set (match_operand:M 0 ...)
(ior:M (not:M (match_operand:M 1 ...))
(not:M (match_operand:M 2 ...))))]
"..."
"...")

In both cases, it is not necessary to include patterns for the
many logically equivalent RTL expressions.

* The only possible RTL expressions involving both bitwise
exclusive-or and bitwise negation are '(xor:M X) Y)' and '(not:M
(xor:M X Y))'.

* The sum of three items, one of which is a constant, will only
appear in the form

(plus:M (plus:M X Y) CONSTANT)

* On machines that do not use 'cc0', '(compare X (const_int 0))'
will be converted to X.

* Equality comparisons of a group of bits (usually a single bit)
with zero will be written using 'zero_extract' rather than the
equivalent 'and' or 'sign_extract' operations.


File: gcc.info, Node: Peephole Definitions, Next: Expander Definitions, Prev: Insn Canonicalizations, Up: Machine Desc

Defining Machine-Specific Peephole Optimizers
=============================================

In addition to instruction patterns the 'md' file may contain
definitions of machine-specific peephole optimizations.

The combiner does not notice certain peephole optimizations when
the data flow in the program does not suggest that it should try them.
For example, sometimes two consecutive insns related in purpose can
be combined even though the second one does not appear to use a
register computed in the first one. A machine-specific peephole
optimizer can detect such opportunities.

A definition looks like this:

(define_peephole
[INSN-PATTERN-1
INSN-PATTERN-2
...]
"CONDITION"
"TEMPLATE"
"OPTIONAL INSN-ATTRIBUTES")

The last string operand may be omitted if you are not using any
machine-specific information in this machine description. If present,
it must obey the same rules as in a 'define_insn'.

In this skeleton, INSN-PATTERN-1 and so on are patterns to match
consecutive insns. The optimization applies to a sequence of insns
when INSN-PATTERN-1 matches the first one, INSN-PATTERN-2 matches the
next, and so on.

Each of the insns matched by a peephole must also match a
'define_insn'. Peepholes are checked only at the last stage just
before code generation, and only optionally. Therefore, any insn which
would match a peephole but no 'define_insn' will cause a crash in code
generation in an unoptimized compilation, or at various optimization
stages.

The operands of the insns are matched with 'match_operands',
'match_operator', and 'match_dup', as usual. What is not usual is
that the operand numbers apply to all the insn patterns in the
definition. So, you can check for identical operands in two insns by
using 'match_operand' in one insn and 'match_dup' in the other.

The operand constraints used in 'match_operand' patterns do not have
any direct effect on the applicability of the peephole, but they will
be validated afterward, so make sure your constraints are general
enough to apply whenever the peephole matches. If the peephole matches
but the constraints are not satisfied, the compiler will crash.

It is safe to omit constraints in all the operands of the peephole;
or you can write constraints which serve as a double-check on the
criteria previously tested.

Once a sequence of insns matches the patterns, the CONDITION is
checked. This is a C expression which makes the final decision
whether to perform the optimization (we do so if the expression is
nonzero). If CONDITION is omitted (in other words, the string is
empty) then the optimization is applied to every sequence of insns
that matches the patterns.

The defined peephole optimizations are applied after register
allocation is complete. Therefore, the peephole definition can check
which operands have ended up in which kinds of registers, just by
looking at the operands.

The way to refer to the operands in CONDITION is to write
'operands[I]' for operand number I (as matched by '(match_operand I
...)'). Use the variable 'insn' to refer to the last of the insns
being matched; use 'prev_nonnote_insn' to find the preceding insns.

When optimizing computations with intermediate results, you can use
CONDITION to match only when the intermediate results are not used
elsewhere. Use the C expression 'dead_or_set_p (INSN, OP)', where
INSN is the insn in which you expect the value to be used for the last
time (from the value of 'insn', together with use of
'prev_nonnote_insn'), and OP is the intermediate value (from
'operands[I]').

Applying the optimization means replacing the sequence of insns
with one new insn. The TEMPLATE controls ultimate output of assembler
code for this combined insn. It works exactly like the template of a
'define_insn'. Operand numbers in this template are the same ones
used in matching the original sequence of insns.

The result of a defined peephole optimizer does not need to match
any of the insn patterns in the machine description; it does not even
have an opportunity to match them. The peephole optimizer definition
itself serves as the insn pattern to control how the insn is output.

Defined peephole optimizers are run as assembler code is being
output, so the insns they produce are never combined or rearranged in
any way.

Here is an example, taken from the 68000 machine description:

(define_peephole
[(set (reg:SI 15) (plus:SI (reg:SI 15) (const_int 4)))
(set (match_operand:DF 0 "register_operand" "f")
(match_operand:DF 1 "register_operand" "ad"))]
"FP_REG_P (operands[0]) && ! FP_REG_P (operands[1])"
"*
{
rtx xoperands[2];
xoperands[1] = gen_rtx (REG, SImode, REGNO (operands[1]) + 1);
#ifdef MOTOROLA
output_asm_insn (\"move.l %1,(sp)\", xoperands);
output_asm_insn (\"move.l %1,-(sp)\", operands);
return \"fmove.d (sp)+,%0\";
#else
output_asm_insn (\"movel %1,[email protected]\", xoperands);
output_asm_insn (\"movel %1,[email protected]\", operands);
return \"fmoved [email protected]+,%0\";
#endif
}
")

The effect of this optimization is to change

jbsr _foobar
addql #4,sp
movel d1,[email protected]
movel d0,[email protected]
fmoved [email protected]+,fp0

into

jbsr _foobar
movel d1,[email protected]
movel d0,[email protected]
fmoved [email protected]+,fp0

INSN-PATTERN-1 and so on look *almost* like the second operand of
'define_insn'. There is one important difference: the second operand
of 'define_insn' consists of one or more RTX's enclosed in square
brackets. Usually, there is only one: then the same action can be
written as an element of a 'define_peephole'. But when there are
multiple actions in a 'define_insn', they are implicitly enclosed in a
'parallel'. Then you must explicitly write the 'parallel', and the
square brackets within it, in the 'define_peephole'. Thus, if an insn
pattern looks like this,

(define_insn "divmodsi4"
[(set (match_operand:SI 0 "general_operand" "=d")
(div:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "dmsK")))
(set (match_operand:SI 3 "general_operand" "=d")
(mod:SI (match_dup 1) (match_dup 2)))]
"TARGET_68020"
"divsl%.l %2,%3:%0")

then the way to mention this insn in a peephole is as follows:

(define_peephole
[...
(parallel
[(set (match_operand:SI 0 "general_operand" "=d")
(div:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "dmsK")))
(set (match_operand:SI 3 "general_operand" "=d")
(mod:SI (match_dup 1) (match_dup 2)))])
...]
...)


File: gcc.info, Node: Expander Definitions, Next: Insn Splitting, Prev: Peephole Definitions, Up: Machine Desc

Defining RTL Sequences for Code Generation
==========================================

On some target machines, some standard pattern names for RTL
generation cannot be handled with single insn, but a sequence of RTL
insns can represent them. For these target machines, you can write a
'define_expand' to specify how to generate the sequence of RTL.

A 'define_expand' is an RTL expression that looks almost like a
'define_insn'; but, unlike the latter, a 'define_expand' is used only
for RTL generation and it can produce more than one RTL insn.

A 'define_expand' RTX has four operands:

* The name. Each 'define_expand' must have a name, since the only
use for it is to refer to it by name.

* The RTL template. This is just like the RTL template for a
'define_peephole' in that it is a vector of RTL expressions each
being one insn.

* The condition, a string containing a C expression. This
expression is used to express how the availability of this
pattern depends on subclasses of target machine, selected by
command-line options when GNU CC is run. This is just like the
condition of a 'define_insn' that has a standard name.

* The preparation statements, a string containing zero or more C
statements which are to be executed before RTL code is generated
from the RTL template.

Usually these statements prepare temporary registers for use as
internal operands in the RTL template, but they can also generate
RTL insns directly by calling routines such as 'emit_insn', etc.
Any such insns precede the ones that come from the RTL template.

Every RTL insn emitted by a 'define_expand' must match some
'define_insn' in the machine description. Otherwise, the compiler

will crash when trying to generate code for the insn or trying to
optimize it.

The RTL template, in addition to controlling generation of RTL
insns, also describes the operands that need to be specified when this
pattern is used. In particular, it gives a predicate for each operand.

A true operand, which needs to be specified in order to generate
RTL from the pattern, should be described with a 'match_operand' in
its first occurrence in the RTL template. This enters information on
the operand's predicate into the tables that record such things. GNU
CC uses the information to preload the operand into a register if that
is required for valid RTL code. If the operand is referred to more
than once, subsequent references should use 'match_dup'.

The RTL template may also refer to internal "operands" which are
temporary registers or labels used only within the sequence made by the
'define_expand'. Internal operands are substituted into the RTL
template with 'match_dup', never with 'match_operand'. The values of
the internal operands are not passed in as arguments by the compiler
when it requests use of this pattern. Instead, they are computed
within the pattern, in the preparation statements. These statements
compute the values and store them into the appropriate elements of
'operands' so that 'match_dup' can find them.

There are two special macros defined for use in the preparation
statements: 'DONE' and 'FAIL'. Use them with a following semicolon,
as a statement.

'DONE'
Use the 'DONE' macro to end RTL generation for the pattern. The
only RTL insns resulting from the pattern on this occasion will be
those already emitted by explicit calls to 'emit_insn' within the
preparation statements; the RTL template will not be generated.

'FAIL'
Make the pattern fail on this occasion. When a pattern fails, it
means that the pattern was not truly available. The calling
routines in the compiler will try other strategies for code
generation using other patterns.

Failure is currently supported only for binary (addition,
multiplication, shifting, etc.) and bitfield ('extv', 'extzv',
and 'insv') operations.

Here is an example, the definition of left-shift for the SPUR chip:

(define_expand "ashlsi3"
[(set (match_operand:SI 0 "register_operand" "")
(ashift:SI
(match_operand:SI 1 "register_operand" "")
(match_operand:SI 2 "nonmemory_operand" "")))]
""
"
{
if (GET_CODE (operands[2]) != CONST_INT
|| (unsigned) INTVAL (operands[2]) > 3)
FAIL;
}")

This example uses 'define_expand' so that it can generate an RTL insn
for shifting when the shift-count is in the supported range of 0 to 3
but fail in other cases where machine insns aren't available. When it
fails, the compiler tries another strategy using different patterns
(such as, a library call).

If the compiler were able to handle nontrivial condition-strings in
patterns with names, then it would be possible to use a 'define_insn'
in that case. Here is another case (zero-extension on the 68000)
which makes more use of the power of 'define_expand':

(define_expand "zero_extendhisi2"
[(set (match_operand:SI 0 "general_operand" "")
(const_int 0))
(set (strict_low_part
(subreg:HI
(match_dup 0)
0))
(match_operand:HI 1 "general_operand" ""))]
""
"operands[1] = make_safe_from (operands[1], operands[0]);")

Here two RTL insns are generated, one to clear the entire output
operand and the other to copy the input operand into its low half.
This sequence is incorrect if the input operand refers to [the old
value of] the output operand, so the preparation statement makes sure
this isn't so. The function 'make_safe_from' copies the 'operands[1]'
into a temporary register if it refers to 'operands[0]'. It does this
by emitting another RTL insn.

Finally, a third example shows the use of an internal operand.
Zero-extension on the SPUR chip is done by 'and'-ing the result
against a halfword mask. But this mask cannot be represented by a
'const_int' because the constant value is too large to be legitimate
on this machine. So it must be copied into a register with
'force_reg' and then the register used in the 'and'.

(define_expand "zero_extendhisi2"
[(set (match_operand:SI 0 "register_operand" "")
(and:SI (subreg:SI
(match_operand:HI 1 "register_operand" "")
0)
(match_dup 2)))]
""
"operands[2]
= force_reg (SImode, gen_rtx (CONST_INT,
VOIDmode, 65535)); ")

*Note:* If the 'define_expand' is used to serve a standard binary
or unary arithmetic operation or a bitfield operation, then the last
insn it generates must not be a 'code_label', 'barrier' or 'note'. It
must be an 'insn', 'jump_insn' or 'call_insn'. If you don't need a
real insn at the end, emit an insn to copy the result of the operation
into itself. Such an insn will generate no code, but it can avoid
problems in the compiler.


File: gcc.info, Node: Insn Splitting, Next: Insn Attributes, Prev: Expander Definitions, Up: Machine Desc

Splitting Instructions into Multiple Instructions
=================================================

On machines that have instructions requiring delay slots (*note
Delay Slots::.) or that have instructions whose output is not
available for multiple cycles (*note Function Units::.), the compiler
phases that optimize these cases need to be able to move insns into
one-cycle delay slots. However, some insns may generate more than one
machine instruction. These insns would be unable to be placed into a
delay slot.

It is often possible to write the single insn as a list of
individual insns, each corresponding to one machine instruction. The
disadvantage of doing so is that it will cause the compilation to be
slower and require more space. If the resulting insns are too
complex, it may also suppress some optimizations.

The 'define_split' definition tells the compiler how to split a
complex insn into several simpler insns. This spilling will be
performed if there is a reason to believe that it might improve
instruction or delay slot scheduling. The definition looks like this:

(define_split
[INSN-PATTERN]
"CONDITION"
[NEW-INSN-PATTERN-1
NEW-INSN-PATTERN-2
...]
"PREPARATION STATEMENTS")

INSN-PATTERN is a pattern that needs to be split and CONDITION is
the final condition to be tested, as in a 'define_insn'. Any insn
matched by a 'define_split' must also be matched by a 'define_insn' in
case it does not need to be split.

When an insn matching INSN-PATTERN and satisfying CONDITION is
found, it is replaced in the insn list with the insns given by
NEW-INSN-PATTERN-1, NEW-INSN-PATTERN-2, etc.

The PREPARATION STATEMENTS are similar to those specified for
'define_expand' (*note Expander Definitions::.) and are executed
before the new RTL is generated to prepare for the generated code or
emit some insns whose pattern is not fixed.

As a simple case, consider the following example from the AMD 29000
machine description, which splits a 'sign_extend' from 'HImode' to
'SImode' into a pair of shift insns:

(define_split
[(set (match_operand:SI 0 "gen_reg_operand" "")
(sign_extend:SI (match_operand:HI 1 "gen_reg_operand" "")))]
""
[(set (match_dup 0)
(ashift:SI (match_dup 1)
(const_int 16)))
(set (match_dup 0)
(ashiftrt:SI (match_dup 0)
(const_int 16)))]
"
{ operands[1] = gen_lowpart (SImode, operands[1]); }")


File: gcc.info, Node: Insn Attributes, Prev: Insn Splitting, Up: Machine Desc

Instruction Attributes
======================

In addition to describing the instruction supported by the target
machine, the 'md' file also defines a group of "attributes" and a set
of values for each. Every generated insn is assigned a value for each
attribute. One possible attribute would be the effect that the insn
has on the machine's condition code. This attribute can then be used
by 'NOTICE_UPDATE_CC' to track the condition codes.

* Menu:

* Defining Attributes:: Specifying attributes and their values.
* Expressions:: Valid expressions for attribute values.
* Tagging Insns:: Assigning attribute values to insns.
* Attr Example:: An example of assigning attributes.
* Insn Lengths:: Computing the length of insns.
* Constant Attributes:: Defining attributes that are constant.
* Delay Slots:: Defining delay slots required for a machine.
* Function Units:: Specifying information for insn scheduling.


File: gcc.info, Node: Defining Attributes, Next: Expressions, Prev: Insn Attributes, Up: Insn Attributes

Defining Attributes and their Values
------------------------------------

The 'define_attr' expression is used to define each attribute
required by the target machine. It looks like:

(define_attr NAME LIST-OF-VALUES DEFAULT)

NAME is a string specifying the name of the attribute being defined.

LIST-OF-VALUES is either a string that specifies a comma-separated
list of values that can be assigned to the attribute, or a null string
to indicate that the attribute takes numeric values.

DEFAULT is an attribute expression that gives the value of this
attribute for insns that match patterns whose definition does not
include an explicit value for this attribute. *Note Attr Example::,
for more information on the handling of defaults. *Note Constant
Attributes::, for information on attributes that do not depend on any
particular insn.

For each defined attribute, a number of definitions are written to
the 'insn-attr.h' file. For cases where an explicit set of values is
specified for an attribute, the following are defined:

* A '#define' is written for the symbol 'HAVE_ATTR_NAME'.

* An enumeral class is defined for 'attr_NAME' with elements of the
form 'UPPER-NAME_UPPER-VALUE' where the attribute name and value
are first converted to upper case.

* A function 'get_attr_NAME' is defined that is passed an insn and
returns the attribute value for that insn.

For example, if the following is present in the 'md' file:

(define_attr "type" "branch,fp,load,store,arith" ...)

the following lines will be written to the file 'insn-attr.h'.

#define HAVE_ATTR_type
enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD,
TYPE_STORE, TYPE_ARITH};
extern enum attr_type get_attr_type ();

If the attribute takes numeric values, no 'enum' type will be
defined and the function to obtain the attribute's value will return
'int'.


File: gcc.info, Node: Expressions, Next: Tagging Insns, Prev: Defining Attributes, Up: Insn Attributes

Attribute Expressions
---------------------

RTL expressions used to define attributes use the codes described
above plus a few specific to attribute definitions, to be discussed
below. Attribute value expressions must have one of the following
forms:

'(const_int I)'
The integer I specifies the value of a numeric attribute. I must
be non-negative.

The value of a numeric attribute can be specified either with a
'const_int' or as an integer represented as a string in
'const_string', 'eq_attr' (see below), and 'set_attr' (*note
Tagging Insns::.) expressions.

'(const_string VALUE)'
The string VALUE specifies a constant attribute value. If VALUE
is specified as '"*"', it means that the default value of the
attribute is to be used for the insn containing this expression.
'"*"' obviously cannot be used in the DEFAULT expression of a
'define_attr'.

If the attribute whose value is being specified is numeric, VALUE
must be a string containing a non-negative integer (normally
'const_int' would be used in this case). Otherwise, it must
contain one of the valid values for the attribute.

'(if_then_else TEST TRUE-VALUE FALSE-VALUE)'
TEST specifies an attribute test, whose format is defined below.
The value of this expression is TRUE-VALUE if TEST is true,
otherwise it is FALSE-VALUE.

'(cond [TEST1 VALUE1 ...] DEFAULT)'
The first operand of this expression is a vector containing an
even number of expressions and consisting of pairs of TEST and
VALUE expressions. The value of the 'cond' expression is that of
the VALUE corresponding to the first true TEST expression. If
none of the TEST expressions are true, the value of the 'cond'
expression is that of the DEFAULT expression.

TEST expressions can have one of the following forms:

'(const_int I)'
This test is true if I is non-zero and false otherwise.

'(not TEST)'
'(ior TEST1 TEST2)'
'(and TEST1 TEST2)'
These tests are true if the indicated logical function is true.

'(match_operand:M N PRED CONSTRAINTS)'
This test is true if operand N of the insn whose attribute value
is being determined has mode M (this part of the test is ignored
if M is 'VOIDmode') and the function specified by the string PRED
returns a non-zero value when passed operand N and mode M (this
part of the test is ignored if PRED is the null string).

The CONSTRAINTS operand is ignored and should be the null string.

'(le ARITH1 ARITH2)'
'(leu ARITH1 ARITH2)'
'(lt ARITH1 ARITH2)'
'(ltu ARITH1 ARITH2)'
'(gt ARITH1 ARITH2)'
'(gtu ARITH1 ARITH2)'
'(ge ARITH1 ARITH2)'
'(geu ARITH1 ARITH2)'
'(ne ARITH1 ARITH2)'
'(eq ARITH1 ARITH2)'
These tests are true if the indicated comparison of the two
arithmetic expressions is true. Arithmetic expressions are
formed with 'plus', 'minus', 'mult', 'div', 'mod', 'abs', 'neg',
'and', 'ior', 'xor', 'not', 'lshift', 'ashift', 'lshiftrt', and
'ashiftrt' expressions.

'const_int' and 'symbol_ref' are always valid terms (*note Insn
Lengths::.,for additional forms). 'symbol_ref' is a string
denoting a C expression that yields an 'int' when evaluated by the
'get_attr_...' routine. It should normally be a global variable.

'(eq_attr NAME VALUE)'
NAME is a string specifying the name of an attribute.

VALUE is a string that is either a valid value for attribute
NAME, a comma-separated list of values, or '!' followed by a
value or list. If VALUE does not begin with a '!', this test is
true if the value of the NAME attribute of the current insn is in
the list specified by VALUE. If VALUE begins with a '!', this
test is true if the attribute's value is *not* in the specified
list.

For example,

(eq_attr "type" "load,store")

is equivalent to

(ior (eq_attr "type" "load") (eq_attr "type" "store"))

If NAME specifies an attribute of 'alternative', it refers to the
value of the compiler variable 'which_alternative' (*note Output
Statement::.) and the values must be small integers. For example,

(eq_attr "alternative" "2,3")

is equivalent to

(ior (eq (symbol_ref "which_alternative") (const_int 2))
(eq (symbol_ref "which_alternative") (const_int 3)))

Note that, for most attributes, an 'eq_attr' test is simplified
in cases where the value of the attribute being tested is known
for all insns matching a particular pattern. This is by far the
most common case.


File: gcc.info, Node: Tagging Insns, Next: Attr Example, Prev: Expressions, Up: Insn Attributes

Assigning Attribute Values to Insns
-----------------------------------

The value assigned to an attribute of an insn is primarily
determined by which pattern is matched by that insn (or which
'define_peephole' generated it). Every 'define_insn' and
'define_peephole' can have an optional last argument to specify the
values of attributes for matching insns. The value of any attribute
not specified in a particular insn is set to the default value for
that attribute, as specified in its 'define_attr'. Extensive use of
default values for attributes permits the specification of the values
for only one or two attributes in the definition of most insn
patterns, as seen in the example in the next section.

The optional last argument of 'define_insn' and 'define_peephole'
is a vector of expressions, each of which defines the value for a
single attribute. The most general way of assigning an attribute's
value is to use a 'set' expression whose first operand is an 'attr'
expression giving the name of the attribute being set. The second
operand of the 'set' is an attribute expression (*note Expressions::.)
giving the value of the attribute.

When the attribute value depends on the 'alternative' attribute
(i.e., which is the applicable alternative in the constraint of the
insn), the 'set_attr_alternative' expression can can be used. It
allows the specification of a vector of attribute expressions, one for
each alternative.

When the generality of arbitrary attribute expressions is not
required, the simpler 'set_attr' expression can be used, which allows
specifying a string giving either a single attribute value or a list
of attribute values, one for each alternative.

The form of each of the above specifications is shown below. In
each case, NAME is a string specifying the attribute to be set.

'(set_attr NAME VALUE-STRING)'
VALUE-STRING is either a string giving the desired attribute
value, or a string containing a comma-separated list giving the
values for succeeding alternatives. The number of elements must
match the number of alternatives in the constraint of the insn
pattern.

Note that it may be useful to specify '*' for some alternative, in
which case the attribute will assume its default value for insns
matching that alternative.

'(set_attr_alternative NAME [VALUE1 VALUE2 ...])'
Depending on the alternative of the insn, the value will be one
of the specified values. This is a shorthand for using a 'cond'
with tests on the 'alternative' attribute.

'(set (attr NAME) VALUE)'
The first operand of this 'set' must be the special RTL expression
'attr', whose sole operand is a string giving the name of the
attribute being set. VALUE is the value of the attribute.

The following shows three different ways of representing the same
attribute value specification:

(set_attr "type" "load,store,arith")

(set_attr_alternative "type"
[(const_string "load") (const_string "store")
(const_string "arith")])

(set (attr "type")
(cond [(eq_attr "alternative" "1") (const_string "load")
(eq_attr "alternative" "2") (const_string "store")]
(const_string "arith")))

The 'define_asm_attributes' expression provides a mechanism to
specify the attributes assigned to insns produced from an 'asm'
statement. It has the form:

(define_asm_attributes [ATTR-SETS])

where ATTR-SETS is specified the same as for 'define_insn' and
'define_peephole' expressions.

These values will typically be the "worst case" attribute values.
For example, they might indicate that the condition code will be
clobbered.

A specification for a 'length' attribute is handled specially. To
compute the length of an 'asm' insn, the length specified in the
'define_asm_attributes' expression is multiplied by the number of
machine instructions specified in the 'asm' statement, determined by
counting the number of semicolons and newlines in the string.
Therefore, the value of the 'length' attribute specified in a
'define_asm_attributes' should be the maximum possible length of a
single machine instruction.


File: gcc.info, Node: Attr Example, Next: Insn Lengths, Prev: Tagging Insns, Up: Insn Attributes

Example of Attribute Specifications
-----------------------------------

The judicious use of defaulting is important in the efficient use of
insn attributes. Typically, insns are divided into "types" and an
attribute, customarily called 'type', is used to represent this value.
This attribute is normally used only to define the default value for
other attributes. An example will clarify this usage.

Assume we have a RISC machine with a condition code and in which
only full-word operations are performed in registers. Let us assume
that we can divide all insns into loads, stores, (integer) arithmetic
operations, floating point operations, and branches.

Here we will concern ourselves with determining the effect of an
insn on the condition code and will limit ourselves to the following
possible effects: The condition code can be set unpredictably
(clobbered), not be changed, be set to agree with the results of the
operation, or only changed if the item previously set into the
condition code has been modified.

Here is part of a sample 'md' file for such a machine:

(define_attr "type" "load,store,arith,fp,branch" (const_string "arith"))

(define_attr "cc" "clobber,unchanged,set,change0"
(cond [(eq_attr "type" "load")
(const_string "change0")
(eq_attr "type" "store,branch")
(const_string "unchanged")
(eq_attr "type" "arith")
(if_then_else (match_operand:SI 0 "" "")
(const_string "set")
(const_string "clobber"))]
(const_string "clobber")))

(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,r,m")
(match_operand:SI 1 "general_operand" "r,m,r"))]
""
"@
move %0,%1
load %0,%1
store %0,%1"
[(set_attr "type" "arith,load,store")])

Note that we assume in the above example that arithmetic operations
performed on quantities smaller than a machine word clobber the
condition code since they will set the condition code to a value
corresponding to the full-word result.


File: gcc.info, Node: Insn Lengths, Next: Constant Attributes, Prev: Attr Example, Up: Insn Attributes

Computing the Length of an Insn
-------------------------------

For many machines, multiple types of branch instructions are
provided, each for different length branch displacements. In most
cases, the assembler will choose the correct instruction to use.
However, when the assembler cannot do so, GCC can when a special
attribute, the 'length' attribute, is defined. This attribute must be
defined to have numeric values by specifying a null string in its
'define_attr'.

In the case of the 'length' attribute, two additional forms of
arithmetic terms are allowed in test expressions:

'(match_dup N)'
This refers to the address of operand N of the current insn, which
must be a 'label_ref'.

'(pc)'
This refers to the address of the *current* insn. It might have
been more consistent with other usage to make this the address of
the *next* insn but this would be confusing because the length of
the current insn is to be computed.

For normal insns, the length will be determined by value of the
'length' attribute. In the case of 'addr_vec' and 'addr_diff_vec'
insn patterns, the length will be computed as the number of vectors
multiplied by the size of each vector.

The following macros can be used to refine the length computation:

'FIRST_INSN_ADDRESS'
When the 'length' insn attribute is used, this macro specifies the
value to be assigned to the address of the first insn in a
function. If not specified, 0 is used.

'ADJUST_INSN_LENGTH (INSN, LENGTH)'
If defined, modifies the length assigned to instruction INSN as a
function of the context in which it is used. LENGTH is an lvalue
that contains the initially computed length of the insn and
should be updated with the correct length of the insn. If
updating is required, INSN must not be a varying-length insn.

This macro will normally not be required. A case in which it is
required is the ROMP. On this machine, the size of an 'addr_vec'
insn must be increased by two to compensate for the fact that
alignment may be required.

The routine that returns the value of the 'length' attribute,
'get_attr_length', can be used by the output routine to determine the
form of the branch instruction to be written, as the example below
illustrates.

As an example of the specification of variable-length branches,
consider the IBM 360. If we adopt the convention that a register will
be set to the starting address of a function, we can jump to labels
within 4K of the start using a four-byte instruction. Otherwise, we
need a six-byte sequence to load the address from memory and then
branch to it.

On such a machine, a pattern for a branch instruction might be
specified as follows:

(define_insn "jump"
[(set (pc)
(label_ref (match_operand 0 "" "")))]
""
"*
{
return (get_attr_length (insn) == 4
? \"b %l0\" : \"l r15,=a(%l0); br r15\");
}"
[(set (attr "length") (if_then_else (lt (match_dup 0) (const_int 4096))
(const_int 4)
(const_int 6)))])


File: gcc.info, Node: Constant Attributes, Next: Delay Slots, Prev: Insn Lengths, Up: Insn Attributes

Constant Attributes
-------------------

A special form of DEFINE_ATTR, where the expression for the default
value is a CONST expression, indicates an attribute that is constant
for a given run of the compiler. Constant attributes may be used to
specify which variety of processor is used. For example,

(define_attr "cpu" "m88100,m88110,m88000"
(const
(cond [(symbol_ref "TARGET_88100") (const_string "m88100")
(symbol_ref "TARGET_88110") (const_string "m88110")]
(const_string "m88000"))))

(define_attr "memory" "fast,slow"
(const
(if_then_else (symbol_ref "TARGET_FAST_MEM")
(const_string "fast")
(const_string "slow"))))

The routine generated for constant attributes has no parameters as
it does not depend on any particular insn. RTL expressions used to
define the value of a constant attribute may use the SYMBOL_REF form,
but may not use either the MATCH_OPERAND form or EQ_ATTR forms
involving insn attributes.


File: gcc.info, Node: Delay Slots, Next: Function Units, Prev: Constant Attributes, Up: Insn Attributes

Delay Slot Scheduling
---------------------

The insn attribute mechanism can be used to specify the
requirements for delay slots, if any, on a target machine. An
instruction is said to require a "delay slot" if some instructions
that are physically after the instruction are executed as if they were
located before it. Classic examples are branch and call instructions,
which often execute the following instruction before the branch or
call is performed.

On some machines, conditional branch instructions can optionally
"annul" instructions in the delay slot. This means that the
instruction will not be executed for certain branch outcomes. Both
instructions that annul if the branch is true and instructions that
annul if the branch is false are supported.

Delay slot scheduling differs from instruction scheduling in that
determining whether an instruction needs a delay slot is dependent only
on the type of instruction being generated, not on data flow between
the instructions. See the next section for a discussion of
data-dependent instruction scheduling.

The requirement of an insn needing one or more delay slots is
indicated via the 'define_delay' expression. It has the following
form:

(define_delay TEST
[DELAY-1 ANNUL-TRUE-1 ANNUL-FALSE-1
DELAY-2 ANNUL-TRUE-2 ANNUL-FALSE-2
...])

TEST is an attribute test that indicates whether this
'define_delay' applies to a particular insn. If so, the number of
required delay slots is determined by the length of the vector
specified as the second argument. An insn placed in delay slot N must
satisfy attribute test DELAY-N. ANNUL-TRUE-N is an attribute test
that specifies which insns may be annulled if the branch is true.
Similarly, ANNUL-FALSE-N specifies which insns in the delay slot may
be annulled if the branch is false. If annulling is not supported for
that delay slot, '(nil)' should be coded.

For example, in the common case where branch and call insns require
a single delay slot, which may contain any insn other than a branch or
call, the following would be placed in the 'md' file:

(define_delay (eq_attr "type" "branch,call")
[(eq_attr "type" "!branch,call") (nil) (nil)])

Multiple 'define_delay' expressions may be specified. In this
case, each such expression specifies different delay slot requirements
and there must be no insn for which tests in two 'define_delay'
expressions are both true.

For example, if we have a machine that requires one delay slot for
branches but two for calls, no delay slot can contain a branch or
call insn, and any valid insn in the delay slot for the branch can be
annulled if the branch is true, we might represent this as follows:

(define_delay (eq_attr "type" "branch")
[(eq_attr "type" "!branch,call") (eq_attr "type" "!branch,call") (nil)])

(define_delay (eq_attr "type" "call")
[(eq_attr "type" "!branch,call") (nil) (nil)
(eq_attr "type" "!branch,call") (nil) (nil)])


File: gcc.info, Node: Function Units, Prev: Delay Slots, Up: Insn Attributes

Specifying Function Units
-------------------------

On most RISC machines, there are instructions whose results are not
available for a specific number of cycles. Common cases are
instructions that load data from memory. On many machines, a pipeline
stall will result if the data is referenced too soon after the load
instruction.

In addition, many newer microprocessors have multiple function
units, usually one for integer and one for floating point, and often
will incur pipeline stalls when a result that is needed is not yet
ready.

The descriptions in this section allow the specification of how much
time must elapse between the execution of an instruction and the time
when its result is used. It also allows specification of when the
execution of an instruction will delay execution of similar
instructions due to function unit conflicts.

For the purposes of the specifications in this section, a machine is
divided into "function units", each of which execute a specific class
of instructions. Function units that accept one instruction each
cycle and allow a result to be used in the succeeding instruction
(usually via forwarding) need not be specified. Classic RISC
microprocessors will normally have a single function unit, which we can
call 'memory'. The newer "superscalar" processors will often have
function units for floating point operations, usually at least a
floating point adder and multiplier.

Each usage of a function units by a class of insns is specified
with a 'define_function_unit' expression, which looks like this:

(define_function_unit NAME MULTIPLICITY SIMULTANEITY
TEST READY-DELAY BUSY-DELAY
[CONFLICT-LIST])

NAME is a string giving the name of the function unit.

MULTIPLICITY is an integer specifying the number of identical units
in the processor. If more than one unit is specified, they will be
scheduled independently. Only truly independent units should be
counted; a pipelined unit should be specified as a single unit. (The
only common example of a machine that has multiple function units for a
single instruction class that are truly independent and not pipelined
are the two multiply and two increment units of the CDC 6600.)

SIMULTANEITY specifies the maximum number of insns that can be
executing in each instance of the function unit simultaneously or zero
if the unit is pipelined and has no limit.

All 'define_function_unit' definitions referring to function unit
NAME must have the same name and values for MULTIPLICITY and
SIMULTANEITY.

TEST is an attribute test that selects the insns we are describing
in this definition. Note that an insn may use more than one function
unit and a function unit may be specified in more than one
'define_function_unit'.

READY-DELAY is an integer that specifies the number of cycles after
which the result of the instruction can be used without introducing
any stalls.

BUSY-DELAY is an integer that represents the default cost if an
insn is scheduled for this unit while the unit is active with another
insn. If SIMULTANEITY is zero, this specification is ignored.
Otherwise, a zero value indicates that these insns execute on NAME in
a fully pipelined fashion, even if SIMULTANEITY is non-zero. A
non-zero value indicates that scheduling a new insn on this unit while
another is active will incur a cost. A cost of two indicates a single
cycle delay. For a normal non-pipelined function unit, BUSY-DELAY
will be twice READY-DELAY.

CONFLICT-LIST is an optional list giving detailed conflict costs
for this unit. If specified, it is a list of condition test
expressions which are applied to insns already executing in NAME. For
each insn that is in the list, BUSY-DELAY will be used for the conflict
cost, while a value of zero will be used for insns not in the list.

Typical uses of this vector are where a floating point function
unit can pipeline either single- or double-precision operations, but
not both, or where a memory unit can pipeline loads, but not stores,
etc.

As an example, consider a classic RISC machine where the result of a
load instruction is not available for two cycles (a single "delay"
instruction is required) and where only one load instruction can be
executed simultaneously. This would be specified as:

(define_function_unit "memory" 1 1 (eq_attr "type" "load") 2 4)

For the case of a floating point function unit that can pipeline
either single or double precision, but not both, the following could
be specified:

(define_function_unit
"fp" 1 1 (eq_attr "type" "sp_fp") 4 8 (eq_attr "type" "dp_fp")]
(define_function_unit
"fp" 1 1 (eq_attr "type" "dp_fp") 4 8 (eq_attr "type" "sp_fp")]

*Note:* No code currently exists to avoid function unit conflicts,
only data conflicts. Hence MULTIPLICITY, SIMULTANEITY, BUSY-COST, and
CONFLICT-LIST are currently ignored. When such code is written, it is
possible that the specifications for these values may be changed. It
has recently come to our attention that these specifications may not
allow modeling of some of the newer "superscalar" processors that have
insns using multiple pipelined units. These insns will cause a
potential conflict for the second unit used during their execution and
there is no way of representing that conflict. We welcome any
examples of how function unit conflicts work in such processors and
suggestions for their representation.


File: gcc.info, Node: Target Macros, Next: Config, Prev: Machine Desc, Up: Top

Target Description Macros
*************************

In addition to the file 'MACHINE.md', a machine description
includes a C header file conventionally given the name 'MACHINE.h'.
This header file defines numerous macros that convey the information
about the target machine that does not fit into the scheme of the
'.md' file. The file 'tm.h' should be a link to 'MACHINE.h'. The
header file 'config.h' includes 'tm.h' and most compiler source files
include 'config.h'.

* Menu:

* Driver:: Controlling how the driver runs the compilation passes.
* Run-time Target:: Defining '-m' options like '-m68000' and '-m68020'.
* Storage Layout:: Defining sizes and alignments of data.
* Type Layout:: Defining sizes and properties of basic user data types.
* Registers:: Naming and describing the hardware registers.
* Register Classes:: Defining the classes of hardware registers.
* Stack and Calling:: Defining which way the stack grows and by how much.
* Varargs:: Defining the varargs macros.
* Trampolines:: Code set up at run time to enter a nested function.
* Library Calls:: Controlling how library routines are implicitly called.
* Addressing Modes:: Defining addressing modes valid for memory operands.
* Condition Code:: Defining how insns update the condition code.
* Costs:: Defining relative costs of different operations.
* Sections:: Dividing storage into text, data, and other sections.
* PIC:: Macros for position independent code.
* Assembler Format:: Defining how to write insns and pseudo-ops to output.
* Debugging Info:: Defining the format of debugging output.
* Cross-compilation:: Handling floating point for cross-compilers.
* Misc:: Everything else.



Defining Jump Instruction Patterns
==================================

For most machines, GNU CC assumes that the machine has a condition
code. A comparison insn sets the condition code, recording the
results of both signed and unsigned comparison of the given operands.
A separate branch insn tests the condition code and branches or not
according its value. The branch insns come in distinct signed and
unsigned flavors. Many common machines, such as the Vax, the 68000
and the 32000, work this way.

Some machines have distinct signed and unsigned compare
instructions, and only one set of conditional branch instructions.
The easiest way to handle these machines is to treat them just like
the others until the final stage where assembly code is written. At
this time, when outputting code for the compare instruction, peek
ahead at the following branch using 'next_cc0_user (insn)'. (The
variable 'insn' refers to the insn being output, in the output-writing
code in an instruction pattern.) If the RTL says that is an unsigned
branch, output an unsigned compare; otherwise output a signed compare.
When the branch itself is output, you can treat signed and unsigned
branches identically.

The reason you can do this is that GNU CC always generates a pair of
consecutive RTL insns, possibly separated by 'note' insns, one to set
the condition code and one to test it, and keeps the pair inviolate
until the end.

To go with this technique, you must define the machine-description
macro 'NOTICE_UPDATE_CC' to do 'CC_STATUS_INIT'; in other words, no
compare instruction is superfluous.

Some machines have compare-and-branch instructions and no condition
code. A similar technique works for them. When it is time to
"output" a compare instruction, record its operands in two static
variables. When outputting the branch-on-condition-code instruction
that follows, actually output a compare-and-branch instruction that
uses the remembered operands.

It also works to define patterns for compare-and-branch
instructions. In optimizing compilation, the pair of compare and
branch instructions will be combined according to these patterns. But
this does not happen if optimization is not requested. So you must
use one of the solutions above in addition to any special patterns you
define.

In many RISC machines, most instructions do not affect the condition
code and there may not even be a separate condition code register. On
these machines, the restriction that the definition and use of the
condition code be adjacent insns is not necessary and can prevent

important optimizations. For example, on the IBM RS/6000, there is a
delay for taken branches unless the condition code register is set
three instructions earlier than the conditional branch. The
instruction scheduler cannot perform this optimization if it is not
permitted to separate the definition and use of the condition code
register.

On these machines, do not use '(cc0)', but instead use a register
to represent the condition code. If there is a specific condition code
register in the machine, use a hard register. If the condition code or
comparison result can be placed in any general register, or if there
are multiple condition registers, use a pseudo register.

On some machines, the type of branch instruction generated may
depend on the way the condition code was produced; for example, on the
68k and Sparc, setting the condition code directly from an add or
subtract instruction does not clear the overflow bit the way that a
test instruction does, so a different branch instruction must be used
for some conditional branches. For machines that use '(cc0)', the set
and use of the condition code must be adjacent (separated only by
'note' insns) allowing flags in 'cc_status' to be used. (*Note
Condition Code::.) Also, the comparison and branch insns can be
located from each other by using the functions 'prev_cc0_setter' and
'next_cc0_user'.

However, this is not true on machines that do not use '(cc0)'. On
those machines, no assumptions can be made about the adjacency of the
compare and branch insns and the above methods cannot be used.
Instead, we use the machine mode of the condition code register to
record different formats of the condition code register.

Registers used to store the condition code value should have a mode
that is in class 'MODE_CC'. Normally, it will be 'CCmode'. If
additional modes are required (as for the add example mentioned above
in the Sparc), define the macro 'EXTRA_CC_MODES' to list the
additional modes required (*note Condition Code::.). Also define
'EXTRA_CC_NAMES' to list the names of those modes and 'SELECT_CC_MODE'
to choose a mode given an operand of a compare.

If it is known during RTL generation that a different mode will be
required (for example, if the machine has separate compare instructions
for signed and unsigned quantities, like most IBM processors), they can
be specified at that time.

If the cases that require different modes would be made by
instruction combination, the macro 'SELECT_CC_MODE' determines which
machine mode should be used for the comparison result. The patterns
should be written using that mode. To support the case of the add on
the Sparc discussed above, we have the pattern

(define_insn ""
[(set (reg:CC_NOOV 0)
(compare:CC_NOOV (plus:SI (match_operand:SI 0 "register_operand" "%r")
(match_operand:SI 1 "arith_operand" "rI"))
(const_int 0)))]
""
"...")

The 'SELECT_CC_MODE' macro on the Sparc returns 'CC_NOOVmode' for
comparisons whose argument is a 'plus'.


File: gcc.info, Node: Driver, Next: Run-time Target, Up: Target Macros

Controlling the Compilation Driver, 'gcc'
=========================================

'SWITCH_TAKES_ARG (CHAR)'
A C expression which determines whether the option '-CHAR' takes
arguments. The value should be the number of arguments that
option takes--zero, for many options.

By default, this macro is defined to handle the standard options
properly. You need not define it unless you wish to add
additional options which take arguments.

'WORD_SWITCH_TAKES_ARG (NAME)'
A C expression which determines whether the option '-NAME' takes
arguments. The value should be the number of arguments that
option takes--zero, for many options. This macro rather than
'SWITCH_TAKES_ARG' is used for multi-character option names.

By default, this macro is defined to handle the standard options
properly. You need not define it unless you wish to add
additional options which take arguments.

'SWITCHES_NEED_SPACES'
A string-valued C expression which is nonempty if the linker
needs a space between the '-L' or '-o' option and its argument.

If this macro is not defined, the default value is 0.

'CPP_SPEC'
A C string constant that tells the GNU CC driver program options
to pass to CPP. It can also specify how to translate options you
give to GNU CC into options for GNU CC to pass to the CPP.

Do not define this macro if it does not need to do anything.

'SIGNED_CHAR_SPEC'
A C string constant that tells the GNU CC driver program options
to pass to CPP. By default, this macro is defined to pass the
option '-D__CHAR_UNSIGNED__' to CPP if 'char' will be treated as
'unsigned char' by 'cc1'.

Do not define this macro unless you need to override the default
definition.

'CC1_SPEC'
A C string constant that tells the GNU CC driver program options
to pass to 'cc1'. It can also specify how to translate options
you give to GNU CC into options for GNU CC to pass to the 'cc1'.

Do not define this macro if it does not need to do anything.

'CC1PLUS_SPEC'
A C string constant that tells the GNU CC driver program options
to pass to 'cc1plus'. It can also specify how to translate
options you give to GNU CC into options for GNU CC to pass to the
'cc1plus'.

Do not define this macro if it does not need to do anything.

'ASM_SPEC'
A C string constant that tells the GNU CC driver program options
to pass to the assembler. It can also specify how to translate
options you give to GNU CC into options for GNU CC to pass to the
assembler. See the file 'sun3.h' for an example of this.

Do not define this macro if it does not need to do anything.

'ASM_FINAL_SPEC'
A C string constant that tells the GNU CC driver program how to
run any programs which cleanup after the normal assembler.
Normally, this is not needed. See the file 'mips.h' for an
example of this.

Do not define this macro if it does not need to do anything.

'LINK_SPEC'
A C string constant that tells the GNU CC driver program options
to pass to the linker. It can also specify how to translate
options you give to GNU CC into options for GNU CC to pass to the
linker.

Do not define this macro if it does not need to do anything.

'LIB_SPEC'
Another C string constant used much like 'LINK_SPEC'. The
difference between the two is that 'LIB_SPEC' is used at the end
of the command given to the linker.

If this macro is not defined, a default is provided that loads
the standard C library from the usual place. See 'gcc.c'.

'STARTFILE_SPEC'
Another C string constant used much like 'LINK_SPEC'. The
difference between the two is that 'STARTFILE_SPEC' is used at
the very beginning of the command given to the linker.

If this macro is not defined, a default is provided that loads the
standard C startup file from the usual place. See 'gcc.c'.

'ENDFILE_SPEC'
Another C string constant used much like 'LINK_SPEC'. The
difference between the two is that 'ENDFILE_SPEC' is used at the
very end of the command given to the linker.

Do not define this macro if it does not need to do anything.

'LINK_LIBGCC_SPECIAL'
Define this macro meaning that 'gcc' should find the library
'libgcc.a' by hand, rather than passing the argument '-lgcc' to
tell the linker to do the search.

'RELATIVE_PREFIX_NOT_LINKDIR'
Define this macro to tell 'gcc' that it should only translate a
'-B' prefix into a '-L' linker option if the prefix indicates an
absolute file name.

'STANDARD_EXEC_PREFIX'
Define this macro as a C string constant if you wish to override
the standard choice of '/usr/local/lib/gcc-lib/' as the default
prefix to try when searching for the executable files of the
compiler.

'MD_EXEC_PREFIX'
If defined, this macro is an additional prefix to try after
'STANDARD_EXEC_PREFIX'. 'MD_EXEC_PREFIX' is not searched when
the '-b' option is used, or the compiler is built as a cross
compiler.

'STANDARD_STARTFILE_PREFIX'
Define this macro as a C string constant if you wish to override
the standard choice of '/usr/local/lib/gcc/' as the default
prefix to try when searching for startup files such as 'crt0.o'.

'MD_STARTFILE_PREFIX'
If defined, this macro supplies an additional prefix to try after
the standard prefixes. 'MD_EXEC_PREFIX' is not searched when the
'-b' option is used, or the compiler is built as a cross compiler.

'LOCAL_INCLUDE_DIR'
Define this macro as a C string constant if you wish to override
the standard choice of '/usr/local/include' as the default prefix
to try when searching for local header files. 'LOCAL_INCLUDE_DIR'
comes before 'SYSTEM_INCLUDE_DIR' in the search order.

Cross compilers do not use this macro and do not search either
'/usr/local/include' or its replacement.

'SYSTEM_INCLUDE_DIR'
Define this macro as a C string constant if you wish to specify a
system-specific directory to search for header files before the
standard directory. 'SYSTEM_INCLUDE_DIR' comes before
'STANDARD_INCLUDE_DIR' in the search order.

Cross compilers do not use this macro and do not search the
directory specified.

'STANDARD_INCLUDE_DIR'
Define this macro as a C string constant if you wish to override
the standard choice of '/usr/include' as the default prefix to
try when searching for header files.

Cross compilers do not use this macro and do not search either
'/usr/include' or its replacement.

'INCLUDE_DEFAULTS'
Define this macro if you wish to override the entire default
search path for include files. The default search path includes
'GPLUSPLUS_INCLUDE_DIR', 'GCC_INCLUDE_DIR', 'LOCAL_INCLUDE_DIR',
'SYSTEM_INCLUDE_DIR', and 'STANDARD_INCLUDE_DIR'. In addition,
the macros 'GPLUSPLUS_INCLUDE_DIR' and 'GCC_INCLUDE_DIR' are
defined automatically by 'Makefile', and specify private search
areas for GCC. The directory 'GPLUSPLUS_INCLUDE_DIR' is used
only for C++ programs.

The definition should be an initializer for an array of
structures. Each array element should have two elements: the
directory name (a string constant) and a flag for C++-only
directories. Mark the end of the array with a null element. For
example, here is the definition used for VMS:

#define INCLUDE_DEFAULTS \
{ \
{ "GNU_GXX_INCLUDE:", 1}, \
{ "GNU_CC_INCLUDE:", 0}, \
{ "SYS$SYSROOT:[SYSLIB.]", 0}, \
{ ".", 0}, \
{ 0, 0} \
}

Here is the order of prefixes tried for exec files:

1. Any prefixes specified by the user with '-B'.

2. The environment variable 'GCC_EXEC_PREFIX', if any.

3. The directories specified by the environment variable
'COMPILER_PATH'.

4. The macro 'STANDARD_EXEC_PREFIX'.

5. '/usr/lib/gcc/'.

6. The macro 'MD_EXEC_PREFIX', if any.

Here is the order of prefixes tried for startfiles:

1. Any prefixes specified by the user with '-B'.

2. The environment variable 'GCC_EXEC_PREFIX', if any.

3. The directories specified by the environment variable
'LIBRARY_PATH'.

4. The macro 'STANDARD_EXEC_PREFIX'.

5. '/usr/lib/gcc/'.

6. The macro 'MD_EXEC_PREFIX', if any.

7. The macro 'MD_STARTFILE_PREFIX', if any.

8. The macro 'STANDARD_STARTFILE_PREFIX'.

9. '/lib/'.

10. '/usr/lib/'.


File: gcc.info, Node: Run-time Target, Next: Storage Layout, Prev: Driver, Up: Target Macros

Run-time Target Specification
=============================

'CPP_PREDEFINES'
Define this to be a string constant containing '-D' options to
define the predefined macros that identify this machine and
system. These macros will be predefined unless the '-ansi'
option is specified.

In addition, a parallel set of macros are predefined, whose names
are made by appending '__' at the beginning and at the end. These
'__' macros are permitted by the ANSI standard, so they are
predefined regardless of whether '-ansi' is specified.

For example, on the Sun, one can use the following value:

"-Dmc68000 -Dsun -Dunix"

The result is to define the macros '__mc68000__', '__sun__' and
'__unix__' unconditionally, and the macros 'mc68000', 'sun' and
'unix' provided '-ansi' is not specified.

'STDC_VALUE'
Define the value to be assigned to the built-in macro '__STDC__'.
The default is the value '1'.

'extern int target_flags;'
This declaration should be present.

'TARGET_...'
This series of macros is to allow compiler command arguments to
enable or disable the use of optional features of the target
machine. For example, one machine description serves both the
68000 and the 68020; a command argument tells the compiler
whether it should use 68020-only instructions or not. This
command argument works by means of a macro 'TARGET_68020' that
tests a bit in 'target_flags'.

Define a macro 'TARGET_FEATURENAME' for each such option. Its
definition should test a bit in 'target_flags'; for example:

#define TARGET_68020 (target_flags & 1)

One place where these macros are used is in the
condition-expressions of instruction patterns. Note how
'TARGET_68020' appears frequently in the 68000 machine
description file, 'm68k.md'. Another place they are used is in
the definitions of the other macros in the 'MACHINE.h' file.

'TARGET_SWITCHES'
This macro defines names of command options to set and clear bits
in 'target_flags'. Its definition is an initializer with a
subgrouping for each command option.

Each subgrouping contains a string constant, that defines the
option name, and a number, which contains the bits to set in
'target_flags'. A negative number says to clear bits instead;
the negative of the number is which bits to clear. The actual
option name is made by appending '-m' to the specified name.

One of the subgroupings should have a null string. The number in
this grouping is the default value for 'target_flags'. Any
target options act starting with that value.

Here is an example which defines '-m68000' and '-m68020' with
opposite meanings, and picks the latter as the default:

#define TARGET_SWITCHES \
{ { "68020", 1}, \
{ "68000", -1}, \
{ "", 1}}

'TARGET_OPTIONS'
This macro is similar to 'TARGET_SWITCHES' but defines names of
command options that have values. Its definition is an
initializer with a subgrouping for each command option.

Each subgrouping contains a string constant, that defines the
fixed part of the option name, and the address of a variable.
The variable, type 'char *', is set to the variable part of the
given option if the fixed part matches. The actual option name
is made by appending '-m' to the specified name.

Here is an example which defines '-mshort-data-NUMBER'. If the
given option is '-mshort-data-512', the variable 'm88k_short_data'
will be set to the string '"512"'.

extern char *m88k_short_data;
#define TARGET_OPTIONS { { "short-data-", &m88k_short_data } }

'TARGET_VERSION'
This macro is a C statement to print on 'stderr' a string
describing the particular machine description choice. Every
machine description should define 'TARGET_VERSION'. For example:

#ifdef MOTOROLA
#define TARGET_VERSION fprintf (stderr, " (68k, Motorola syntax)");
#else
#define TARGET_VERSION fprintf (stderr, " (68k, MIT syntax)");
#endif

'OVERRIDE_OPTIONS'
Sometimes certain combinations of command options do not make
sense on a particular target machine. You can define a macro
'OVERRIDE_OPTIONS' to take account of this. This macro, if
defined, is executed once just after all the command options have
been parsed.

Don't use this macro to turn on various extra optimizations for
'-O'. That is what 'OPTIMIZATION_OPTIONS' is for.

'OPTIMIZATION_OPTIONS (LEVEL)'
Some machines may desire to change what optimizations are
performed for various optimization levels. This macro, if
defined, is executed once just after the optimization level is
determined and before the remainder of the command options have
been parsed. Values set in this macro are used as the default
values for the other command line options.

LEVEL is the optimization level specified; 2 if -O2 is specified,
1 if -O is specified, and 0 if neither is specified.

*Do not examine 'write_symbols' in this macro!* The debugging
options are not supposed to alter the generated code.


File: gcc.info, Node: Storage Layout, Next: Type Layout, Prev: Run-time Target, Up: Target Macros

Storage Layout
==============

Note that the definitions of the macros in this table which are
sizes or alignments measured in bits do not need to be constant. They
can be C expressions that refer to static variables, such as the
'target_flags'. *Note Run-time Target::.

'BITS_BIG_ENDIAN'
Define this macro to be the value 1 if the most significant bit
in a byte has the lowest number; otherwise define it to be the
value zero. This means that bit-field instructions count from
the most significant bit. If the machine has no bit-field
instructions, this macro is irrelevant.

This macro does not affect the way structure fields are packed
into bytes or words; that is controlled by 'BYTES_BIG_ENDIAN'.

'BYTES_BIG_ENDIAN'
Define this macro to be 1 if the most significant byte in a word
has the lowest number.

'WORDS_BIG_ENDIAN'
Define this macro to be 1 if, in a multiword object, the most
significant word has the lowest number.

'BITS_PER_UNIT'
Number of bits in an addressable storage unit (byte); normally 8.

'BITS_PER_WORD'
Number of bits in a word; normally 32.

'MAX_BITS_PER_WORD'
Maximum number of bits in a word. If this is undefined, the
default is 'BITS_PER_WORD'. Otherwise, it is the constant value
that is the largest value that 'BITS_PER_WORD' can have at
run-time.

'UNITS_PER_WORD'
Number of storage units in a word; normally 4.

'POINTER_SIZE'
Width of a pointer, in bits.

'PARM_BOUNDARY'
Normal alignment required for function parameters on the stack, in
bits. All stack parameters receive least this much alignment
regardless of data type. On most machines, this is the same as
the size of an integer.

'STACK_BOUNDARY'
Define this macro if you wish to preserve a certain alignment for
the stack pointer. The definition is a C expression for the
desired alignment (measured in bits).

If 'PUSH_ROUNDING' is not defined, the stack will always be
aligned to the specified boundary. If 'PUSH_ROUNDING' is defined
and specifies a less strict alignment than 'STACK_BOUNDARY', the
stack may be momentarily unaligned while pushing arguments.

'FUNCTION_BOUNDARY'
Alignment required for a function entry point, in bits.

'BIGGEST_ALIGNMENT'
Biggest alignment that any data type can require on this machine,
in bits.

'BIGGEST_FIELD_ALIGNMENT'
Biggest alignment that any structure field can require on this
machine, in bits.

'MAX_OFILE_ALIGNMENT'
Biggest alignment supported by the object file format of this
machine. Use this macro to limit the alignment which can be
specified using the '__attribute__ ((aligned (N)))' construct.
If not defined, the default value is 'BIGGEST_ALIGNMENT'.

'DATA_ALIGNMENT (TYPE, BASIC-ALIGN)'
If defined, a C expression to compute the alignment for a static
variable. TYPE is the data type, and BASIC-ALIGN is the
alignment that the object would ordinarily have. The value of
this macro is used instead of that alignment to align the object.

If this macro is not defined, then BASIC-ALIGN is used.

One use of this macro is to increase alignment of medium-size
data to make it all fit in fewer cache lines. Another is to
cause character arrays to be word-aligned so that 'strcpy' calls
that copy constants to character arrays can be done inline.

'CONSTANT_ALIGNMENT (CONSTANT, BASIC-ALIGN)'
If defined, a C expression to compute the alignment given to a
constant that is being placed in memory. CONSTANT is the
constant and BASIC-ALIGN is the alignment that the object would
ordinarily have. The value of this macro is used instead of that
alignment to align the object.

If this macro is not defined, then BASIC-ALIGN is used.

The typical use of this macro is to increase alignment for string
constants to be word aligned so that 'strcpy' calls that copy
constants can be done inline.

'EMPTY_FIELD_BOUNDARY'
Alignment in bits to be given to a structure bit field that
follows an empty field such as 'int : 0;'.

Note that 'PCC_BITFIELD_TYPE_MATTERS' also affects the alignment
that results from an empty field.

'STRUCTURE_SIZE_BOUNDARY'
Number of bits which any structure or union's size must be a
multiple of. Each structure or union's size is rounded up to a
multiple of this.

If you do not define this macro, the default is the same as
'BITS_PER_UNIT'.

'STRICT_ALIGNMENT'
Define this macro to be the value 1 if instructions will fail to
work if given data not on the nominal alignment. If instructions
will merely go slower in that case, define this macro as 0.

'PCC_BITFIELD_TYPE_MATTERS'
Define this if you wish to imitate the way many other C compilers
handle alignment of bitfields and the structures that contain
them.

The behavior is that the type written for a bitfield ('int',
'short', or other integer type) imposes an alignment for the
entire structure, as if the structure really did contain an
ordinary field of that type. In addition, the bitfield is placed
within the structure so that it would fit within such a field,
not crossing a boundary for it.

Thus, on most machines, a bitfield whose type is written as 'int'
would not cross a four-byte boundary, and would force four-byte
alignment for the whole structure. (The alignment used may not
be four bytes; it is controlled by the other alignment
parameters.)

If the macro is defined, its definition should be a C expression;
a nonzero value for the expression enables this behavior.

Note that if this macro is not defined, or its value is zero, some
bitfields may cross more than one alignment boundary. The
compiler can support such references if there are 'insv', 'extv',
and 'extzv' insns that can directly reference memory.

The other known way of making bitfields work is to define
'STRUCTURE_SIZE_BOUNDARY' as large as 'BIGGEST_ALIGNMENT'. Then
every structure can be accessed with fullwords.

Unless the machine has bitfield instructions or you define
'STRUCTURE_SIZE_BOUNDARY' that way, you must define
'PCC_BITFIELD_TYPE_MATTERS' to have a nonzero value.

'BITFIELD_NBYTES_LIMITED'
Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited
to aligning a bitfield within the structure.

'ROUND_TYPE_SIZE (STRUCT, SIZE, ALIGN)'
Define this macro as an expression for the overall size of a
structure (given by STRUCT as a tree node) when the size computed
from the fields is SIZE and the alignment is ALIGN.

The default is to round SIZE up to a multiple of ALIGN.

'ROUND_TYPE_ALIGN (STRUCT, COMPUTED, SPECIFIED)'
Define this macro as an expression for the alignment of a
structure (given by STRUCT as a tree node) if the alignment
computed in the usual way is COMPUTED and the alignment
explicitly specified was SPECIFIED.

The default is to use SPECIFIED if it is larger; otherwise, use
the smaller of COMPUTED and 'BIGGEST_ALIGNMENT'

'MAX_FIXED_MODE_SIZE'
An integer expression for the size in bits of the largest integer
machine mode that should actually be used. All integer machine
modes of this size or smaller can be used for structures and
unions with the appropriate sizes. If this macro is undefined,
'GET_MODE_BITSIZE (DImode)' is assumed.

'CHECK_FLOAT_VALUE (MODE, VALUE)'
A C statement to validate the value VALUE (of type 'double') for
mode MODE. This means that you check whether VALUE fits within
the possible range of values for mode MODE on this target
machine. The mode MODE is always 'SFmode' or 'DFmode'.

If VALUE is not valid, you should call 'error' to print an error
message and then assign some valid value to VALUE. Allowing an
invalid value to go through the compiler can produce incorrect
assembler code which may even cause Unix assemblers to crash.

This macro need not be defined if there is no work for it to do.

'TARGET_FLOAT_FORMAT'
A code distinguishing the floating point format of the target
machine. There are three defined values:

'IEEE_FLOAT_FORMAT'
This code indicates IEEE floating point. It is the default;
there is no need to define this macro when the format is
IEEE.

'VAX_FLOAT_FORMAT'
This code indicates the peculiar format used on the Vax.

'UNKNOWN_FLOAT_FORMAT'
This code indicates any other format.

The value of this macro is compared with 'HOST_FLOAT_FORMAT'
(*note Config::.) to determine whether the target machine has the
same format as the host machine. If any other formats are
actually in use on supported machines, new codes should be
defined for them.


File: gcc.info, Node: Type Layout, Next: Registers, Prev: Storage Layout, Up: Target Macros

Layout of Source Language Data Types
====================================

These macros define the sizes and other characteristics of the
standard basic data types used in programs being compiled. Unlike the
macros in the previous section, these apply to specific features of C
and related languages, rather than to fundamental aspects of storage
layout.

'INT_TYPE_SIZE'
A C expression for the size in bits of the type 'int' on the
target machine. If you don't define this, the default is one
word.

'SHORT_TYPE_SIZE'
A C expression for the size in bits of the type 'short' on the
target machine. If you don't define this, the default is half a
word. (If this would be less than one storage unit, it is
rounded up to one unit.)

'LONG_TYPE_SIZE'
A C expression for the size in bits of the type 'long' on the
target machine. If you don't define this, the default is one
word.

'LONG_LONG_TYPE_SIZE'
A C expression for the size in bits of the type 'long long' on the
target machine. If you don't define this, the default is two
words.

'CHAR_TYPE_SIZE'
A C expression for the size in bits of the type 'char' on the
target machine. If you don't define this, the default is one
quarter of a word. (If this would be less than one storage unit,
it is rounded up to one unit.)

'FLOAT_TYPE_SIZE'
A C expression for the size in bits of the type 'float' on the
target machine. If you don't define this, the default is one
word.

'DOUBLE_TYPE_SIZE'
A C expression for the size in bits of the type 'double' on the
target machine. If you don't define this, the default is two
words.

'LONG_DOUBLE_TYPE_SIZE'
A C expression for the size in bits of the type 'long double' on
the target machine. If you don't define this, the default is two
words.

'DEFAULT_SIGNED_CHAR'
An expression whose value is 1 or 0, according to whether the type
'char' should be signed or unsigned by default. The user can
always override this default with the options '-fsigned-char' and
'-funsigned-char'.

'DEFAULT_SHORT_ENUMS'
A C expression to determine whether to give an 'enum' type only
as many bytes as it takes to represent the range of possible
values of that type. A nonzero value means to do that; a zero
value means all 'enum' types should be allocated like 'int'.

If you don't define the macro, the default is 0.

'SIZE_TYPE'
A C expression for a string describing the name of the data type
to use for size values. The typedef name 'size_t' is defined
using the contents of the string.

The string can contain more than one keyword. If so, separate
them with spaces, and write first any length keyword, then
'unsigned' if appropriate, and finally 'int'. The string must
exactly match one of the data type names defined in the function
'init_decl_processing' in the file 'c-decl.c'. You may not omit
'int' or change the order--that would cause the compiler to crash
on startup.

If you don't define this macro, the default is '"long unsigned
int"'.

'PTRDIFF_TYPE'
A C expression for a string describing the name of the data type
to use for the result of subtracting two pointers. The typedef
name 'ptrdiff_t' is defined using the contents of the string. See
'SIZE_TYPE' above for more information.

If you don't define this macro, the default is '"long int"'.

'WCHAR_TYPE'
A C expression for a string describing the name of the data type
to use for wide characters. The typedef name 'wchar_t' is
defined using the contents of the string. See 'SIZE_TYPE' above
for more information.

If you don't define this macro, the default is '"int"'.

'WCHAR_TYPE_SIZE'
A C expression for the size in bits of the data type for wide
characters. This is used in 'cpp', which cannot make use of
'WCHAR_TYPE'.

'OBJC_INT_SELECTORS'
Define this macro if the type of Objective C selectors should be
'int'.

If this macro is not defined, then selectors should have the type
'struct objc_selector *'.

'OBJC_NONUNIQUE_SELECTORS'
Define this macro if Objective C selector-references will be made
unique by the linker (this is the default). In this case, each
selector-reference will be given a separate assembler label.
Otherwise, the selector-references will be gathered into an array
with a single assembler label.

'MULTIBYTE_CHARS'
Define this macro to enable support for multibyte characters in
the input to GNU CC. This requires that the host system support
the ANSI C library functions for converting multibyte characters
to wide characters.

'TARGET_BELL'
A C constant expression for the integer value for escape sequence
'\a'.

'TARGET_BS'
'TARGET_TAB'
'TARGET_NEWLINE'
C constant expressions for the integer values for escape sequences
'\b', '\t' and '\n'.

'TARGET_VT'
'TARGET_FF'
'TARGET_CR'
C constant expressions for the integer values for escape sequences
'\v', '\f' and '\r'.


File: gcc.info, Node: Registers, Next: Register Classes, Prev: Type Layout, Up: Target Macros

Register Usage
==============

This section explains how to describe what registers the target
machine has, and how (in general) they can be used.

The description of which registers a specific instruction can use is
done with register classes; see *Note Register Classes::. For
information on using registers to access a stack frame, see *Note
Frame Registers::. For passing values in registers, see *Note
Register Arguments::. For returning values in registers, see *Note
Scalar Return::.

* Menu:

* Register Basics:: Number and kinds of registers.
* Allocation Order:: Order in which registers are allocated.
* Values in Registers:: What kinds of values each reg can hold.
* Leaf Functions:: Renumbering registers for leaf functions.
* Stack Registers:: Handling a register stack such as 80387.
* Obsolete Register Macros:: Macros formerly used for the 80387.


File: gcc.info, Node: Register Basics, Next: Allocation Order, Up: Registers

Basic Characteristics of Registers
----------------------------------

'FIRST_PSEUDO_REGISTER'
Number of hardware registers known to the compiler. They receive
numbers 0 through 'FIRST_PSEUDO_REGISTER-1'; thus, the first
pseudo register's number really is assigned the number
'FIRST_PSEUDO_REGISTER'.

'FIXED_REGISTERS'
An initializer that says which registers are used for fixed
purposes all throughout the compiled code and are therefore not
available for general allocation. These would include the stack
pointer, the frame pointer (except on machines where that can be
used as a general register when no frame pointer is needed), the
program counter on machines where that is considered one of the
addressable registers, and any other numbered register with a
standard use.

This information is expressed as a sequence of numbers, separated
by commas and surrounded by braces. The Nth number is 1 if
register N is fixed, 0 otherwise.

The table initialized from this macro, and the table initialized
by the following one, may be overridden at run time either
automatically, by the actions of the macro
'CONDITIONAL_REGISTER_USAGE', or by the user with the command
options '-ffixed-REG', '-fcall-used-REG' and '-fcall-saved-REG'.

'CALL_USED_REGISTERS'
Like 'FIXED_REGISTERS' but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers. This macro therefore identifies the registers that
are not available for general allocation of values that must live
across function calls.

If a register has 0 in 'CALL_USED_REGISTERS', the compiler
automatically saves it on function entry and restores it on
function exit, if the register is used within the function.

'CONDITIONAL_REGISTER_USAGE'
Zero or more C statements that may conditionally modify two
variables 'fixed_regs' and 'call_used_regs' (both of type 'char
[]') after they have been initialized from the two preceding
macros.

This is necessary in case the fixed or call-clobbered registers
depend on target flags.

You need not define this macro if it has no work to do.

If the usage of an entire class of registers depends on the target
flags, you may indicate this to GCC by using this macro to modify
'fixed_regs' and 'call_used_regs' to 1 for each of the registers
in the classes which should not be used by GCC. Also define the
macro 'REG_CLASS_FROM_LETTER' to return 'NO_REGS' if it is called
with a letter for a class that shouldn't be used.

(However, if this class is not included in 'GENERAL_REGS' and all
of the insn patterns whose constraints permit this class are
controlled by target switches, then GCC will automatically avoid
using these registers when the target switches are opposed to
them.)

'NON_SAVING_SETJMP'
If this macro is defined and has a nonzero value, it means that
'setjmp' and related functions fail to save the registers, or that
'longjmp' fails to restore them. To compensate, the compiler
avoids putting variables in registers in functions that use
'setjmp'.


File: gcc.info, Node: Allocation Order, Next: Values in Registers, Prev: Register Basics, Up: Registers

Order of Allocation of Registers
--------------------------------

'REG_ALLOC_ORDER'
If defined, an initializer for a vector of integers, containing
the numbers of hard registers in the order in which GNU CC should
prefer to use them (from most preferred to least).

If this macro is not defined, registers are used lowest numbered
first (all else being equal).

One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers. On
such machines, define 'REG_ALLOC_ORDER' to be an initializer that
lists the highest numbered allocatable register first.

'ORDER_REGS_FOR_LOCAL_ALLOC'
A C statement (sans semicolon) to choose the order in which to
allocate hard registers for pseudo-registers local to a basic
block.

Store the desired order of registers in the array
'reg_alloc_order'. Element 0 should be the register to allocate
first; element 1, the next register; and so on.

The macro body should not assume anything about the contents of
'reg_alloc_order' before execution of the macro.

On most machines, it is not necessary to define this macro.


File: gcc.info, Node: Values in Registers, Next: Leaf Functions, Prev: Allocation Order, Up: Registers

How Values Fit in Registers
---------------------------

This section discusses the macros that describe which kinds of
values (specifically, which machine modes) each register can hold, and
how many consecutive registers are needed for a given mode.

'HARD_REGNO_NREGS (REGNO, MODE)'
A C expression for the number of consecutive hard registers,
starting at register number REGNO, required to hold a value of
mode MODE.

On a machine where all registers are exactly one word, a suitable
definition of this macro is

#define HARD_REGNO_NREGS(REGNO, MODE) \
((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \
/ UNITS_PER_WORD))

'HARD_REGNO_MODE_OK (REGNO, MODE)'
A C expression that is nonzero if it is permissible to store a
value of mode MODE in hard register number REGNO (or in several
registers starting with that one). For a machine where all
registers are equivalent, a suitable definition is

#define HARD_REGNO_MODE_OK(REGNO, MODE) 1

It is not necessary for this macro to check for the numbers of
fixed registers, because the allocation mechanism considers them
to be always occupied.

On some machines, double-precision values must be kept in even/odd
register pairs. The way to implement that is to define this macro
to reject odd register numbers for such modes.

The minimum requirement for a mode to be OK in a register is that
the 'movMODE' instruction pattern support moves between the
register and any other hard register for which the mode is OK;
and that moving a value into the register and back out not alter
it.

Since the same instruction used to move 'SImode' will work for all
narrower integer modes, it is not necessary on any machine for
'HARD_REGNO_MODE_OK' to distinguish between these modes, provided
you define patterns 'movhi', etc., to take advantage of this.
This is useful because of the interaction between
'HARD_REGNO_MODE_OK' and 'MODES_TIEABLE_P'; it is very desirable
for all integer modes to be tieable.

Many machines have special registers for floating point
arithmetic. Often people assume that floating point machine
modes are allowed only in floating point registers. This is not
true. Any registers that can hold integers can safely *hold* a
floating point machine mode, whether or not floating arithmetic
can be done on it in those registers. Integer move instructions
can be used to move the values.

On some machines, though, the converse is true: fixed-point
machine modes may not go in floating registers. This is true if
the floating registers normalize any value stored in them,
because storing a non-floating value there would garble it. In
this case, 'HARD_REGNO_MODE_OK' should reject fixed-point machine
modes in floating registers. But if the floating registers do
not automatically normalize, if you can store any bit pattern in
one and retrieve it unchanged without a trap, then any machine
mode may go in a floating register and this macro should say so.

The primary significance of special floating registers is rather
that they are the registers acceptable in floating point
arithmetic instructions. However, this is of no concern to
'HARD_REGNO_MODE_OK'. You handle it by writing the proper
constraints for those instructions.

On some machines, the floating registers are especially slow to
access, so that it is better to store a value in a stack frame
than in such a register if floating point arithmetic is not being
done. As long as the floating registers are not in class
'GENERAL_REGS', they will not be used unless some pattern's
constraint asks for one.

'MODES_TIEABLE_P (MODE1, MODE2)'
A C expression that is nonzero if it is desirable to choose
register allocation so as to avoid move instructions between a
value of mode MODE1 and a value of mode MODE2.

If 'HARD_REGNO_MODE_OK (R, MODE1)' and 'HARD_REGNO_MODE_OK (R,
MODE2)' are ever different for any R, then 'MODES_TIEABLE_P
(MODE1, MODE2)' must be zero.


File: gcc.info, Node: Leaf Functions, Next: Stack Registers, Prev: Values in Registers, Up: Registers

Handling Leaf Functions
-----------------------

On some machines, a leaf function (i.e., one which make no calls)
can run more efficiently if it does not make its own register window.
Often this means it is required to receive its arguments in the
registers where they are passed by the caller, instead of the
registers where they would normally arrive. Also, the leaf function
may use only those registers for its own variables and temporaries.

GNU CC assigns register numbers before it knows whether the
function is suitable for leaf function treatment. So it needs to
renumber the registers in order to output a leaf function. The
following macros accomplish this.

'LEAF_REGISTERS'
A C initializer for a vector, indexed by hard register number,
which contains 1 for a register that is allowable in a candidate
for leaf function treatment.

If leaf function treatment involves renumbering the registers,
then the registers marked here should be the ones before
renumbering--those that GNU CC would ordinarily allocate. The
registers which will actually be used in the assembler code,
after renumbering, should not be marked with 1 in this vector.

Define this macro only if the target machine offers a way to
optimize the treatment of leaf functions.

'LEAF_REG_REMAP (REGNO)'
A C expression whose value is the register number to which REGNO
should be renumbered, when a function is treated as a leaf
function.

If REGNO is a register number which should not appear in a leaf
function before renumbering, then the expression should yield -1,
which will cause the compiler to abort.

Define this macro only if the target machine offers a way to
optimize the treatment of leaf functions, and registers need to
be renumbered to do this.

'REG_LEAF_ALLOC_ORDER'
If defined, an initializer for a vector of integers, containing
the numbers of hard registers in the order in which the GNU CC
should prefer to use them (from most preferred to least) in a
leaf function. If this macro is not defined, REG_ALLOC_ORDER is
used for both non-leaf and leaf-functions.

Normally, it is necessary for 'FUNCTION_PROLOGUE' and
'FUNCTION_EPILOGUE' to treat leaf functions specially. The C variable
'leaf_function' is nonzero for such a function.


File: gcc.info, Node: Stack Registers, Next: Obsolete Register Macros, Prev: Leaf Functions, Up: Registers

Registers That Form a Stack
---------------------------

There are special features to handle computers where some of the
"registers" form a stack, as in the 80387 coprocessor for the 80386.
Stack registers are normally written by pushing onto the stack, and are
numbered relative to the top of the stack.

Currently, GNU CC can only handle one group of stack-like
registers, and they must be consecutively numbered.

'STACK_REGS'
Define this if the machine has any stack-like registers.

'FIRST_STACK_REG'
The number of the first stack-like register. This one is the top
of the stack.

'LAST_STACK_REG'
The number of the last stack-like register. This one is the
bottom of the stack.


File: gcc.info, Node: Obsolete Register Macros, Prev: Stack Registers, Up: Registers

Obsolete Macros for Controlling Register Usage
----------------------------------------------

These features do not work very well. They exist because they used
to be required to generate correct code for the 80387 coprocessor of
the 80386. They are no longer used by that machine description and
may be removed in a later version of the compiler. Don't use them!

'OVERLAPPING_REGNO_P (REGNO)'
If defined, this is a C expression whose value is nonzero if hard
register number REGNO is an overlapping register. This means a
hard register which overlaps a hard register with a different
number. (Such overlap is undesirable, but occasionally it allows
a machine to be supported which otherwise could not be.) This
macro must return nonzero for *all* the registers which overlap
each other. GNU CC can use an overlapping register only in
certain limited ways. It can be used for allocation within a
basic block, and may be spilled for reloading; that is all.

If this macro is not defined, it means that none of the hard
registers overlap each other. This is the usual situation.

'INSN_CLOBBERS_REGNO_P (INSN, REGNO)'
If defined, this is a C expression whose value should be nonzero
if the insn INSN has the effect of mysteriously clobbering the
contents of hard register number REGNO. By "mysterious" we mean
that the insn's RTL expression doesn't describe such an effect.

If this macro is not defined, it means that no insn clobbers
registers mysteriously. This is the usual situation; all else
being equal, it is best for the RTL expression to show all the
activity.

'PRESERVE_DEATH_INFO_REGNO_P (REGNO)'
If defined, this is a C expression whose value is nonzero if
accurate 'REG_DEAD' notes are needed for hard register number
REGNO at the time of outputting the assembler code. When this is
so, a few optimizations that take place after register allocation
and could invalidate the death notes are not done when this
register is involved.

You would arrange to preserve death info for a register when some
of the code in the machine description which is executed to write
the assembler code looks at the death notes. This is necessary
only when the actual hardware feature which GNU CC thinks of as a
register is not actually a register of the usual sort. (It
might, for example, be a hardware stack.)

If this macro is not defined, it means that no death notes need
to be preserved. This is the usual situation.


File: gcc.info, Node: Register Classes, Next: Stack and Calling, Prev: Registers, Up: Target Macros

Register Classes
================

On many machines, the numbered registers are not all equivalent.
For example, certain registers may not be allowed for indexed
addressing; certain registers may not be allowed in some instructions.
These machine restrictions are described to the compiler using
"register classes".

You define a number of register classes, giving each one a name and
saying which of the registers belong to it. Then you can specify
register classes that are allowed as operands to particular
instruction patterns.

In general, each register will belong to several classes. In fact,
one class must be named 'ALL_REGS' and contain all the registers.
Another class must be named 'NO_REGS' and contain no registers. Often
the union of two classes will be another class; however, this is not
required.

One of the classes must be named 'GENERAL_REGS'. There is nothing
terribly special about the name, but the operand constraint letters
'r' and 'g' specify this class. If 'GENERAL_REGS' is the same as
'ALL_REGS', just define it as a macro which expands to 'ALL_REGS'.

Order the classes so that if class X is contained in class Y then X
has a lower class number than Y.

The way classes other than 'GENERAL_REGS' are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.

You should define a class for the union of two classes whenever some
instruction allows both classes. For example, if an instruction allows
either a floating point (coprocessor) register or a general register
for a certain operand, you should define a class
'FLOAT_OR_GENERAL_REGS' which includes both of them. Otherwise you
will get suboptimal code.

You must also specify certain redundant information about the
register classes: for each class, which classes contain it and which
ones are contained in it; for each pair of classes, the largest class
contained in their union.

When a value occupying several consecutive registers is expected in
a certain class, all the registers used must belong to that class.
Therefore, register classes cannot be used to enforce a requirement for
a register pair to start with an even-numbered register. The way to
specify this requirement is with 'HARD_REGNO_MODE_OK'.

Register classes used for input-operands of bitwise-and or shift
instructions have a special requirement: each such class must have, for
each fixed-point machine mode, a subclass whose registers can transfer
that mode to or from memory. For example, on some machines, the
operations for single-byte values ('QImode') are limited to certain
registers. When this is so, each register class that is used in a
bitwise-and or shift instruction must have a subclass consisting of
registers from which single-byte values can be loaded or stored. This
is so that 'PREFERRED_RELOAD_CLASS' can always have a possible value
to return.

'enum reg_class'
An enumeral type that must be defined with all the register class
names as enumeral values. 'NO_REGS' must be first. 'ALL_REGS'
must be the last register class, followed by one more enumeral
value, 'LIM_REG_CLASSES', which is not a register class but rather
tells how many classes there are.

Each register class has a number, which is the value of casting
the class name to type 'int'. The number serves as an index in
many of the tables described below.

'N_REG_CLASSES'
The number of distinct register classes, defined as follows:

#define N_REG_CLASSES (int) LIM_REG_CLASSES

'REG_CLASS_NAMES'
An initializer containing the names of the register classes as C
string constants. These names are used in writing some of the
debugging dumps.

'REG_CLASS_CONTENTS'
An initializer containing the contents of the register classes,
as integers which are bit masks. The Nth integer specifies the
contents of class N. The way the integer MASK is interpreted is
that register R is in the class if 'MASK & (1 << R)' is 1.

When the machine has more than 32 registers, an integer does not
suffice. Then the integers are replaced by sub-initializers,
braced groupings containing several integers. Each
sub-initializer must be suitable as an initializer for the type
'HARD_REG_SET' which is defined in 'hard-reg-set.h'.

'REGNO_REG_CLASS (REGNO)'
A C expression whose value is a register class containing hard
register REGNO. In general there is more that one such class;
choose a class which is "minimal", meaning that no smaller class
also contains the register.

'BASE_REG_CLASS'
A macro whose definition is the name of the class to which a valid
base register must belong. A base register is one used in an
address which is the register value plus a displacement.

'INDEX_REG_CLASS'
A macro whose definition is the name of the class to which a valid
index register must belong. An index register is one used in an
address where its value is either multiplied by a scale factor or
added to another register (as well as added to a displacement).

'REG_CLASS_FROM_LETTER (CHAR)'
A C expression which defines the machine-dependent operand
constraint letters for register classes. If CHAR is such a
letter, the value should be the register class corresponding to
it. Otherwise, the value should be 'NO_REGS'. The register
letter 'r', corresponding to class 'GENERAL_REGS', will not be
passed to this macro; you do not need to handle it.

'REGNO_OK_FOR_BASE_P (NUM)'
A C expression which is nonzero if register number NUM is
suitable for use as a base register in operand addresses. It may
be either a suitable hard register or a pseudo register that has
been allocated such a hard register.

'REGNO_OK_FOR_INDEX_P (NUM)'
A C expression which is nonzero if register number NUM is
suitable for use as an index register in operand addresses. It
may be either a suitable hard register or a pseudo register that
has been allocated such a hard register.

The difference between an index register and a base register is
that the index register may be scaled. If an address involves
the sum of two registers, neither one of them scaled, then either
one may be labeled the "base" and the other the "index"; but
whichever labeling is used must fit the machine's constraints of
which registers may serve in each capacity. The compiler will
try both labelings, looking for one that is valid, and will
reload one or both registers only if neither labeling works.

'PREFERRED_RELOAD_CLASS (X, CLASS)'
A C expression that places additional restrictions on the
register class to use when it is necessary to copy value X into a
register in class CLASS. The value is a register class; perhaps
CLASS, or perhaps another, smaller class. On many machines, the
definition

#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS

is safe.

Sometimes returning a more restrictive class makes better code.
For example, on the 68000, when X is an integer constant that is
in range for a 'moveq' instruction, the value of this macro is
always 'DATA_REGS' as long as CLASS includes the data registers.
Requiring a data register guarantees that a 'moveq' will be used.

If X is a 'const_double', by returning 'NO_REGS' you can force X
into a memory constant. This is useful on certain machines where
immediate floating values cannot be loaded into certain kinds of
registers.

'LIMIT_RELOAD_CLASS (MODE, CLASS)'
A C expression that places additional restrictions on the
register class to use when it is necessary to be able to hold a
value of mode MODE in a reload register for which class CLASS
would ordinarily be used.

Unlike 'PREFERRED_RELOAD_CLASS', this macro should be used when
there are certain modes that simply can't go in certain reload
classes.

The value is a register class; perhaps CLASS, or perhaps another,
smaller class.

Don't define this macro unless the target machine has limitations
which require the macro to do something nontrivial.

'SECONDARY_RELOAD_CLASS (CLASS, MODE, X)'
'SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X)'
'SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X)'
Many machines have some registers that cannot be copied directly
to or from memory or even from other types of registers. An
example is the 'MQ' register, which on most machines, can only be
copied to or from general registers, but not memory. Some
machines allow copying all registers to and from memory, but
require a scratch register for stores to some memory locations
(e.g., those with symbolic address on the RT, and those with
certain symbolic address on the Sparc when compiling PIC). In
some cases, both an intermediate and a scratch register are
required.

You should define these macros to indicate to the reload phase
that it may need to allocate at least one register for a reload
in addition to the register to contain the data. Specifically,
if copying X to a register CLASS in MODE requires an intermediate
register, you should define 'SECONDARY_INPUT_RELOAD_CLASS' to
return the largest register class all of whose registers can be
used as intermediate registers or scratch registers.

If copying a register CLASS in MODE to X requires an intermediate
or scratch register, you should define
'SECONDARY_OUTPUT_RELOAD_CLASS' to return the largest register
class required. If the requirements for input and output reloads
are the same, the macro 'SECONDARY_RELOAD_CLASS' should be used
instead of defining both macros identically.

The values returned by these macros are often 'GENERAL_REGS'.
Return 'NO_REGS' if no spare register is needed; i.e., if X can
be directly copied to or from a register of CLASS in MODE without
requiring a scratch register. Do not define this macro if it
would always return 'NO_REGS'.

If a scratch register is required (either with or without an
intermediate register), you should define patterns for
'reload_inM' or 'reload_outM', as required (*note Standard
Names::.. These patterns, which will normally be implemented
with a 'define_expand', should be similar to the 'movM' patterns,
except that operand 2 is the scratch register.

Define constraints for the reload register and scratch register
that contain a single register class. If the original reload
register (whose class is CLASS) can meet the constraint given in
the pattern, the value returned by these macros is used for the
class of the scratch register. Otherwise, two additional reload
registers are required. Their classes are obtained from the
constraints in the insn pattern.

X might be a pseudo-register or a 'subreg' of a pseudo-register,
which could either be in a hard register or in memory. Use
'true_regnum' to find out; it will return -1 if the pseudo is in
memory and the hard register number if it is in a register.

These macros should not be used in the case where a particular
class of registers can only be copied to memory and not to
another class of registers. In that case, secondary reload
registers are not needed and would not be helpful. Instead, a
stack location must be used to perform the copy and the 'movM'
pattern should use memory as a intermediate storage. This case
often occurs between floating-point and general registers.

'SMALL_REGISTER_CLASSES'
Normally the compiler will avoid choosing spill registers from
registers that have been explicitly mentioned in the rtl (these
registers are normally those used to pass parameters and return
values). However, some machines have so few registers of certain
classes that there would not be enough registers to use as spill
registers if this were done.

On those machines, you should define 'SMALL_REGISTER_CLASSES'.
When it is defined, the compiler allows registers explicitly used
in the rtl to be used as spill registers but prevents the
compiler from extending the lifetime of these registers.

Defining this macro is always safe, but unnecessarily defining
this macro will reduce the amount of optimizations that can be
performed in some cases. If this macro is not defined but needs
to be, the compiler will run out of reload registers and print a
fatal error message.

For most machines, this macro should not be defined.

'CLASS_MAX_NREGS (CLASS, MODE)'
A C expression for the maximum number of consecutive registers of
class CLASS needed to hold a value of mode MODE.

This is closely related to the macro 'HARD_REGNO_NREGS'. In
fact, the value of the macro 'CLASS_MAX_NREGS (CLASS, MODE)'
should be the maximum value of 'HARD_REGNO_NREGS (REGNO, MODE)'
for all REGNO values in the class CLASS.

This macro helps control the handling of multiple-word values in
the reload pass.

Three other special macros describe which operands fit which
constraint letters.

'CONST_OK_FOR_LETTER_P (VALUE, C)'
A C expression that defines the machine-dependent operand
constraint letters that specify particular ranges of integer
values. If C is one of those letters, the expression should
check that VALUE, an integer, is in the appropriate range and
return 1 if so, 0 otherwise. If C is not one of those letters,
the value should be 0 regardless of VALUE.

'CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C)'
A C expression that defines the machine-dependent operand
constraint letters that specify particular ranges of
'const_double' values.

If C is one of those letters, the expression should check that
VALUE, an RTX of code 'const_double', is in the appropriate range
and return 1 if so, 0 otherwise. If C is not one of those
letters, the value should be 0 regardless of VALUE.

'const_double' is used for all floating-point constants and for
'DImode' fixed-point constants. A given letter can accept either
or both kinds of values. It can use 'GET_MODE' to distinguish
between these kinds.

'EXTRA_CONSTRAINT (VALUE, C)'
A C expression that defines the optional machine-dependent
constraint letters that can be used to segregate specific types
of operands, usually memory references, for the target machine.
Normally this macro will not be defined. If it is required for a
particular target machine, it should return 1 if VALUE
corresponds to the operand type represented by the constraint
letter C. If C is not defined as an extra constraint, the value
returned should be 0 regardless of VALUE.

For example, on the ROMP, load instructions cannot have their
output in r0 if the memory reference contains a symbolic address.
Constraint letter 'Q' is defined as representing a memory
address that does *not* contain a symbolic address. An
alternative is specified with a 'Q' constraint on the input and
'r' on the output. The next alternative specifies 'm' on the
input and a register class that does not include r0 on the output.


File: gcc.info, Node: Stack and Calling, Next: Varargs, Prev: Register Classes, Up: Target Macros

Describing Stack Layout and Calling Conventions
===============================================

* Menu:

* Frame Layout::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::


File: gcc.info, Node: Frame Layout, Next: Frame Registers, Up: Stack and Calling

Basic Stack Layout
------------------

'STACK_GROWS_DOWNWARD'
Define this macro if pushing a word onto the stack moves the stack
pointer to a smaller address.

When we say, "define this macro if ...," it means that the
compiler checks this macro only with '#ifdef' so the precise
definition used does not matter.

'FRAME_GROWS_DOWNWARD'
Define this macro if the addresses of local variable slots are at
negative offsets from the frame pointer.

'ARGS_GROW_DOWNWARD'
Define this macro if successive arguments to a function occupy
decreasing addresses on the stack.

'STARTING_FRAME_OFFSET'
Offset from the frame pointer to the first local variable slot to
be allocated.

If 'FRAME_GROWS_DOWNWARD', the next slot's offset is found by
subtracting the length of the first slot from
'STARTING_FRAME_OFFSET'. Otherwise, it is found by adding the
length of the first slot to the value 'STARTING_FRAME_OFFSET'.

'STACK_POINTER_OFFSET'
Offset from the stack pointer register to the first location at
which outgoing arguments are placed. If not specified, the
default value of zero is used. This is the proper value for most
machines.

If 'ARGS_GROW_DOWNWARD', this is the offset to the location above
the first location at which outgoing arguments are placed.

'FIRST_PARM_OFFSET (FUNDECL)'
Offset from the argument pointer register to the first argument's
address. On some machines it may depend on the data type of the
function.

If 'ARGS_GROW_DOWNWARD', this is the offset to the location above
the first argument's address.

'STACK_DYNAMIC_OFFSET (FUNDECL)'
Offset from the stack pointer register to an item dynamically
allocated on the stack, e.g., by 'alloca'.

The default value for this macro is 'STACK_POINTER_OFFSET' plus
the length of the outgoing arguments. The default is correct for
most machines. See 'function.c' for details.

'DYNAMIC_CHAIN_ADDRESS (FRAMEADDR)'
A C expression whose value is RTL representing the address in a
stack frame where the pointer to the caller's frame is stored.
Assume that FRAMEADDR is an RTL expression for the address of the
stack frame itself.

If you don't define this macro, the default is to return the value
of FRAMEADDR--that is, the stack frame address is also the
address of the stack word that points to the previous frame.


File: gcc.info, Node: Frame Registers, Next: Elimination, Prev: Frame Layout, Up: Stack and Calling

Registers That Address the Stack Frame
--------------------------------------

'STACK_POINTER_REGNUM'
The register number of the stack pointer register, which must
also be a fixed register according to 'FIXED_REGISTERS'. On most
machines, the hardware determines which register this is.

'FRAME_POINTER_REGNUM'
The register number of the frame pointer register, which is used
to access automatic variables in the stack frame. On some
machines, the hardware determines which register this is. On
other machines, you can choose any register you wish for this
purpose.

'ARG_POINTER_REGNUM'
The register number of the arg pointer register, which is used to
access the function's argument list. On some machines, this is
the same as the frame pointer register. On some machines, the
hardware determines which register this is. On other machines,
you can choose any register you wish for this purpose. If this
is not the same register as the frame pointer register, then you
must mark it as a fixed register according to 'FIXED_REGISTERS',
or arrange to be able to eliminate it (*note Elimination::.).

'STATIC_CHAIN_REGNUM'
'STATIC_CHAIN_INCOMING_REGNUM'
Register numbers used for passing a function's static chain
pointer. If register windows are used,
'STATIC_CHAIN_INCOMING_REGNUM' is the register number as seen by
the called function, while 'STATIC_CHAIN_REGNUM' is the register
number as seen by the calling function. If these registers are
the same, 'STATIC_CHAIN_INCOMING_REGNUM' need not be defined.

The static chain register need not be a fixed register.

If the static chain is passed in memory, these macros should not
be defined; instead, the next two macros should be defined.

'STATIC_CHAIN'
'STATIC_CHAIN_INCOMING'
If the static chain is passed in memory, these macros provide rtx
giving 'mem' expressions that denote where they are stored.
'STATIC_CHAIN' and 'STATIC_CHAIN_INCOMING' give the locations as
seen by the calling and called functions, respectively. Often
the former will be at an offset from the stack pointer and the
latter at an offset from the frame pointer.

The variables 'stack_pointer_rtx', 'frame_pointer_rtx', and
'arg_pointer_rtx' will have been initialized prior to the use of
these macros and should be used to refer to those items.

If the static chain is passed in a register, the two previous
macros should be defined instead.


File: gcc.info, Node: Elimination, Next: Stack Arguments, Prev: Frame Registers, Up: Stack and Calling

Eliminating Frame Pointer and Arg Pointer
-----------------------------------------

'FRAME_POINTER_REQUIRED'
A C expression which is nonzero if a function must have and use a
frame pointer. This expression is evaluated in the reload pass.
If its value is nonzero the function will have a frame pointer.

The expression can in principle examine the current function and
decide according to the facts, but on most machines the constant
0 or the constant 1 suffices. Use 0 when the machine allows code
to be generated with no frame pointer, and doing so saves some
time or space. Use 1 when there is no possible advantage to
avoiding a frame pointer.

In certain cases, the compiler does not know how to produce valid
code without a frame pointer. The compiler recognizes those
cases and automatically gives the function a frame pointer
regardless of what 'FRAME_POINTER_REQUIRED' says. You don't need
to worry about them.

In a function that does not require a frame pointer, the frame
pointer register can be allocated for ordinary usage, unless you
mark it as a fixed register. See 'FIXED_REGISTERS' for more
information.

This macro is ignored and need not be defined if 'ELIMINABLE_REGS'
is defined.

'INITIAL_FRAME_POINTER_OFFSET (DEPTH-VAR)'
A C statement to store in the variable DEPTH-VAR the difference
between the frame pointer and the stack pointer values
immediately after the function prologue. The value would be
computed from information such as the result of 'get_frame_size
()' and the tables of registers 'regs_ever_live' and
'call_used_regs'.

If 'ELIMINABLE_REGS' is defined, this macro will be not be used
and need not be defined. Otherwise, it must be defined even if
'FRAME_POINTER_REQUIRED' is defined to always be true; in that
case, you may set DEPTH-VAR to anything.

'ELIMINABLE_REGS'
If defined, this macro specifies a table of register pairs used to
eliminate unneeded registers that point into the stack frame. If
it is not defined, the only elimination attempted by the compiler
is to replace references to the frame pointer with references to
the stack pointer.

The definition of this macro is a list of structure
initializations, each of which specifies an original and
replacement register.

On some machines, the position of the argument pointer is not
known until the compilation is completed. In such a case, a
separate hard register must be used for the argument pointer.
This register can be eliminated by replacing it with either the
frame pointer or the argument pointer, depending on whether or
not the frame pointer has been eliminated.

In this case, you might specify:
#define ELIMINABLE_REGS \
{{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}

Note that the elimination of the argument pointer with the stack
pointer is specified first since that is the preferred
elimination.

'CAN_ELIMINATE (FROM-REG, TO-REG)'
A C expression that returns non-zero if the compiler is allowed
to try to replace register number FROM-REG with register number
TO-REG. This macro need only be defined if 'ELIMINABLE_REGS' is
defined, and will usually be the constant 1, since most of the
cases preventing register elimination are things that the
compiler already knows about.

'INITIAL_ELIMINATION_OFFSET (FROM-REG, TO-REG, OFFSET-VAR)'
This macro is similar to 'INITIAL_FRAME_POINTER_OFFSET'. It
specifies the initial difference between the specified pair of
registers. This macro must be defined if 'ELIMINABLE_REGS' is
defined.

'LONGJMP_RESTORE_FROM_STACK'
Define this macro if the 'longjmp' function restores registers
from the stack frames, rather than from those saved specifically
by 'setjmp'. Certain quantities must not be kept in registers
across a call to 'setjmp' on such machines.


File: gcc.info, Node: Stack Arguments, Next: Register Arguments, Prev: Elimination, Up: Stack and Calling

Passing Function Arguments on the Stack
---------------------------------------

The macros in this section control how arguments are passed on the
stack. See the following section for other macros that control
passing certain arguments in registers.

'PROMOTE_PROTOTYPES'
Define this macro if an argument declared as 'char' or 'short' in
a prototype should actually be passed as an 'int'. In addition
to avoiding errors in certain cases of mismatch, it also makes
for better code on certain machines.

'PUSH_ROUNDING (NPUSHED)'
A C expression that is the number of bytes actually pushed onto
the stack when an instruction attempts to push NPUSHED bytes.

If the target machine does not have a push instruction, do not
define this macro. That directs GNU CC to use an alternate
strategy: to allocate the entire argument block and then store
the arguments into it.

On some machines, the definition

#define PUSH_ROUNDING(BYTES) (BYTES)

will suffice. But on other machines, instructions that appear to
push one byte actually push two bytes in an attempt to maintain
alignment. Then the definition should be

#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)

'ACCUMULATE_OUTGOING_ARGS'
If defined, the maximum amount of space required for outgoing
arguments will be computed and placed into the variable
'current_function_outgoing_args_size'. No space will be pushed
onto the stack for each call; instead, the function prologue
should increase the stack frame size by this amount.

It is not proper to define both 'PUSH_ROUNDING' and
'ACCUMULATE_OUTGOING_ARGS'.

'REG_PARM_STACK_SPACE'
Define this macro if functions should assume that stack space has
been allocated for arguments even when their values are passed in
registers.

The value of this macro is the size, in bytes, of the area
reserved for arguments passed in registers.

This space can either be allocated by the caller or be a part of
the machine-dependent stack frame: 'OUTGOING_REG_PARM_STACK_SPACE'
says which.

'OUTGOING_REG_PARM_STACK_SPACE'
Define this if it is the responsibility of the caller to allocate
the area reserved for arguments passed in registers.

If 'ACCUMULATE_OUTGOING_ARGS' is defined, this macro controls
whether the space for these arguments counts in the value of
'current_function_outgoing_args_size'.

'STACK_PARMS_IN_REG_PARM_AREA'
Define this macro if 'REG_PARM_STACK_SPACE' is defined but stack
parameters don't skip the area specified by
'REG_PARM_STACK_SPACE'.

Normally, when a parameter is not passed in registers, it is
placed on the stack beyond the 'REG_PARM_STACK_SPACE' area.
Defining this macro suppresses this behavior and causes the
parameter to be passed on the stack in its natural location.

'RETURN_POPS_ARGS (FUNTYPE, STACK-SIZE)'
A C expression that should indicate the number of bytes of its own
arguments that a function pops on returning, or 0 if the function
pops no arguments and the caller must therefore pop them all
after the function returns.

FUNTYPE is a C variable whose value is a tree node that describes
the function in question. Normally it is a node of type
'FUNCTION_TYPE' that describes the data type of the function.
From this it is possible to obtain the data types of the value and
arguments (if known).

When a call to a library function is being considered, FUNTYPE
will contain an identifier node for the library function. Thus,
if you need to distinguish among various library functions, you
can do so by their names. Note that "library function" in this
context means a function used to perform arithmetic, whose name
is known specially in the compiler and was not mentioned in the C
code being compiled.

STACK-SIZE is the number of bytes of arguments passed on the
stack. If a variable number of bytes is passed, it is zero, and
argument popping will always be the responsibility of the calling
function.

On the Vax, all functions always pop their arguments, so the
definition of this macro is STACK-SIZE. On the 68000, using the
standard calling convention, no functions pop their arguments, so
the value of the macro is always 0 in this case. But an
alternative calling convention is available in which functions
that take a fixed number of arguments pop them but other
functions (such as 'printf') pop nothing (the caller pops all).
When this convention is in use, FUNTYPE is examined to determine
whether a function takes a fixed number of arguments.


File: gcc.info, Node: Register Arguments, Next: Scalar Return, Prev: Stack Arguments, Up: Stack and Calling

Passing Arguments in Registers
------------------------------

This section describes the macros which let you control how various
types of arguments are passed in registers or how they are arranged in
the stack.

'FUNCTION_ARG (CUM, MODE, TYPE, NAMED)'
A C expression that controls whether a function argument is passed
in a register, and which register.

The arguments are CUM, which summarizes all the previous
arguments; MODE, the machine mode of the argument; TYPE, the data
type of the argument as a tree node or 0 if that is not known
(which happens for C support library functions); and NAMED, which
is 1 for an ordinary argument and 0 for nameless arguments that
correspond to '...' in the called function's prototype.

The value of the expression should either be a 'reg' RTX for the
hard register in which to pass the argument, or zero to pass the
argument on the stack.

For machines like the Vax and 68000, where normally all arguments
are pushed, zero suffices as a definition.

The usual way to make the ANSI library 'stdarg.h' work on a
machine where some arguments are usually passed in registers, is
to cause nameless arguments to be passed on the stack instead.
This is done by making 'FUNCTION_ARG' return 0 whenever NAMED is
0.

You may use the macro 'MUST_PASS_IN_STACK (MODE, TYPE)' in the
definition of this macro to determine if this argument is of a
type that must be passed in the stack. If 'REG_PARM_STACK_SPACE'
is not defined and 'FUNCTION_ARG' returns non-zero for such an
argument, the compiler will abort. If 'REG_PARM_STACK_SPACE' is
defined, the argument will be computed in the stack and then
loaded into a register.

'FUNCTION_INCOMING_ARG (CUM, MODE, TYPE, NAMED)'
Define this macro if the target machine has "register windows", so
that the register in which a function sees an arguments is not
necessarily the same as the one in which the caller passed the
argument.

For such machines, 'FUNCTION_ARG' computes the register in which
the caller passes the value, and 'FUNCTION_INCOMING_ARG' should
be defined in a similar fashion to tell the function being called
where the arguments will arrive.

If 'FUNCTION_INCOMING_ARG' is not defined, 'FUNCTION_ARG' serves
both purposes.

'FUNCTION_ARG_PARTIAL_NREGS (CUM, MODE, TYPE, NAMED)'
A C expression for the number of words, at the beginning of an
argument, must be put in registers. The value must be zero for
arguments that are passed entirely in registers or that are
entirely pushed on the stack.

On some machines, certain arguments must be passed partially in
registers and partially in memory. On these machines, typically
the first N words of arguments are passed in registers, and the
rest on the stack. If a multi-word argument (a 'double' or a
structure) crosses that boundary, its first few words must be
passed in registers and the rest must be pushed. This macro
tells the compiler when this occurs, and how many of the words
should go in registers.

'FUNCTION_ARG' for these arguments should return the first
register to be used by the caller for this argument; likewise
'FUNCTION_INCOMING_ARG', for the called function.

'FUNCTION_ARG_PASS_BY_REFERENCE (CUM, MODE, TYPE, NAMED)'
A C expression that indicates when an argument must be passed by
reference. If nonzero for an argument, a copy of that argument
is made in memory and a pointer to the argument is passed instead
of the argument itself. The pointer is passed in whatever way is
appropriate for passing a pointer to that type.

On machines where 'REG_PARM_STACK_SPACE' is not defined, a
suitable definition of this macro might be
#define FUNCTION_ARG_PASS_BY_REFERENCE(CUM, MODE, TYPE, NAMED) \
MUST_PASS_IN_STACK (MODE, TYPE)

'CUMULATIVE_ARGS'
A C type for declaring a variable that is used as the first
argument of 'FUNCTION_ARG' and other related values. For some
target machines, the type 'int' suffices and can hold the number
of bytes of argument so far.

There is no need to record in 'CUMULATIVE_ARGS' anything about the
arguments that have been passed on the stack. The compiler has
other variables to keep track of that. For target machines on
which all arguments are passed on the stack, there is no need to
store anything in 'CUMULATIVE_ARGS'; however, the data structure
must exist and should not be empty, so use 'int'.

'INIT_CUMULATIVE_ARGS (CUM, FNTYPE, LIBNAME)'
A C statement (sans semicolon) for initializing the variable CUM
for the state at the beginning of the argument list. The
variable has type 'CUMULATIVE_ARGS'. The value of FNTYPE is the
tree node for the data type of the function which will receive
the args, or 0 if the args are to a compiler support library
function.

When processing a call to a compiler support library function,
LIBNAME identifies which one. It is a 'symbol_ref' rtx which
contains the name of the function, as a string. LIBNAME is 0 when
an ordinary C function call is being processed. Thus, each time
this macro is called, either LIBNAME or FNTYPE is nonzero, but
never both of them at once.

'INIT_CUMULATIVE_INCOMING_ARGS (CUM, FNTYPE, LIBNAME)'
Like 'INIT_CUMULATIVE_ARGS' but overrides it for the purposes of
finding the arguments for the function being compiled. If this
macro is undefined, 'INIT_CUMULATIVE_ARGS' is used instead.

The argument LIBNAME exists for symmetry with
'INIT_CUMULATIVE_ARGS'. The value passed for LIBNAME is always
0, since library routines with special calling conventions are
never compiled with GNU CC.

'FUNCTION_ARG_ADVANCE (CUM, MODE, TYPE, NAMED)'
A C statement (sans semicolon) to update the summarizer variable
CUM to advance past an argument in the argument list. The values
MODE, TYPE and NAMED describe that argument. Once this is done,
the variable CUM is suitable for analyzing the *following*
argument with 'FUNCTION_ARG', etc.

This macro need not do anything if the argument in question was
passed on the stack. The compiler knows how to track the amount
of stack space used for arguments without any special help.

'FUNCTION_ARG_PADDING (MODE, TYPE)'
If defined, a C expression which determines whether, and in which
direction, to pad out an argument with extra space. The value
should be of type 'enum direction': either 'upward' to pad above
the argument, 'downward' to pad below, or 'none' to inhibit
padding.

This macro does not control the *amount* of padding; that is
always just enough to reach the next multiple of
'FUNCTION_ARG_BOUNDARY'.

This macro has a default definition which is right for most
systems. For little-endian machines, the default is to pad
upward. For big-endian machines, the default is to pad downward
for an argument of constant size shorter than an 'int', and
upward otherwise.

'FUNCTION_ARG_BOUNDARY (MODE, TYPE)'
If defined, a C expression that gives the alignment boundary, in
bits, of an argument with the specified mode and type. If it is
not defined, 'PARM_BOUNDARY' is used for all arguments.

'FUNCTION_ARG_REGNO_P (REGNO)'
A C expression that is nonzero if REGNO is the number of a hard
register in which function arguments are sometimes passed. This
does *not* include implicit arguments such as the static chain and
the structure-value address. On many machines, no registers can
be used for this purpose since all function arguments are pushed
on the stack.


File: gcc.info, Node: Scalar Return, Next: Aggregate Return, Prev: Register Arguments, Up: Stack and Calling

How Scalar Function Values Are Returned
---------------------------------------

This section discusses the macros that control returning scalars as
values--values that can fit in registers.

'TRADITIONAL_RETURN_FLOAT'
Define this macro if '-traditional' should not cause functions
declared to return 'float' to convert the value to 'double'.

'FUNCTION_VALUE (VALTYPE, FUNC)'
A C expression to create an RTX representing the place where a
function returns a value of data type VALTYPE. VALTYPE is a tree
node representing a data type. Write 'TYPE_MODE (VALTYPE)' to
get the machine mode used to represent that type. On many
machines, only the mode is relevant. (Actually, on most
machines, scalar values are returned in the same place regardless
of mode).

If the precise function being called is known, FUNC is a tree
node ('FUNCTION_DECL') for it; otherwise, FUNC is a null pointer.
This makes it possible to use a different value-returning
convention for specific functions when all their calls are known.

'FUNCTION_VALUE' is not used for return vales with aggregate data
types, because these are returned in another way. See
'STRUCT_VALUE_REGNUM' and related macros, below.

'FUNCTION_OUTGOING_VALUE (VALTYPE, FUNC)'
Define this macro if the target machine has "register windows" so
that the register in which a function returns its value is not
the same as the one in which the caller sees the value.

For such machines, 'FUNCTION_VALUE' computes the register in
which the caller will see the value, and
'FUNCTION_OUTGOING_VALUE' should be defined in a similar fashion
to tell the function where to put the value.

If 'FUNCTION_OUTGOING_VALUE' is not defined, 'FUNCTION_VALUE'
serves both purposes.

'FUNCTION_OUTGOING_VALUE' is not used for return vales with
aggregate data types, because these are returned in another way.
See 'STRUCT_VALUE_REGNUM' and related macros, below.

'LIBCALL_VALUE (MODE)'
A C expression to create an RTX representing the place where a
library function returns a value of mode MODE. If the precise
function being called is known, FUNC is a tree node
('FUNCTION_DECL') for it; otherwise, FUNC is a null pointer.
This makes it possible to use a different value-returning
convention for specific functions when all their calls are known.

Note that "library function" in this context means a compiler
support routine, used to perform arithmetic, whose name is known
specially by the compiler and was not mentioned in the C code
being compiled.

The definition of 'LIBRARY_VALUE' need not be concerned aggregate
data types, because none of the library functions returns such
types.

'FUNCTION_VALUE_REGNO_P (REGNO)'
A C expression that is nonzero if REGNO is the number of a hard
register in which the values of called function may come back.

A register whose use for returning values is limited to serving
as the second of a pair (for a value of type 'double', say) need
not be recognized by this macro. So for most machines, this
definition suffices:

#define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)

If the machine has register windows, so that the caller and the
called function use different registers for the return value,
this macro should recognize only the caller's register numbers.


File: gcc.info, Node: Aggregate Return, Next: Caller Saves, Prev: Scalar Return, Up: Stack and Calling

How Large Values Are Returned
-----------------------------

When a function value's mode is 'BLKmode' (and in some other
cases), the value is not returned according to 'FUNCTION_VALUE' (*note
Scalar Return::.). Instead, the caller passes the address of a block
of memory in which the value should be stored. This address is called
the "structure value address".

This section describes how to control returning structure values in
memory.

'RETURN_IN_MEMORY (TYPE)'
A C expression which can inhibit the returning of certain function
values in registers, based on the type of value. A nonzero value
says to return the function value in memory, just as large
structures are always returned. Here TYPE will be a C expression
of type 'tree', representing the data type of the value.

Note that values of mode 'BLKmode' are returned in memory
regardless of this macro. Also, the option '-fpcc-struct-return'
takes effect regardless of this macro. On most systems, it is
possible to leave the macro undefined; this causes a default
definition to be used, whose value is the constant 0.

'STRUCT_VALUE_REGNUM'
If the structure value address is passed in a register, then
'STRUCT_VALUE_REGNUM' should be the number of that register.

'STRUCT_VALUE'
If the structure value address is not passed in a register, define
'STRUCT_VALUE' as an expression returning an RTX for the place
where the address is passed. If it returns 0, the address is
passed as an "invisible" first argument.

'STRUCT_VALUE_INCOMING_REGNUM'
On some architectures the place where the structure value address
is found by the called function is not the same place that the
caller put it. This can be due to register windows, or it could
be because the function prologue moves it to a different place.

If the incoming location of the structure value address is in a
register, define this macro as the register number.

'STRUCT_VALUE_INCOMING'
If the incoming location is not a register, define
'STRUCT_VALUE_INCOMING' as an expression for an RTX for where the
called function should find the value. If it should find the
value on the stack, define this to create a 'mem' which refers to
the frame pointer. A definition of 0 means that the address is
passed as an "invisible" first argument.

'PCC_STATIC_STRUCT_RETURN'
Define this macro if the usual system convention on the target
machine for returning structures and unions is for the called
function to return the address of a static variable containing
the value. GNU CC does not normally use this convention, even if
it is the usual one, but does use it if '-fpcc-struct-value' is
specified.

Do not define this if the usual system convention is for the
caller to pass an address to the subroutine.


File: gcc.info, Node: Caller Saves, Next: Function Entry, Prev: Aggregate Return, Up: Stack and Calling

Caller-Saves Register Allocation
--------------------------------

If you enable it, GNU CC can save registers around function calls.
This makes it possible to use call-clobbered registers to hold
variables that must live across calls.

'DEFAULT_CALLER_SAVES'
Define this macro if function calls on the target machine do not
preserve any registers; in other words, if 'CALL_USED_REGISTERS'
has 1 for all registers. This macro enables '-fcaller-saves' by
default. Eventually that option will be enabled by default on
all machines and both the option and this macro will be
eliminated.

'CALLER_SAVE_PROFITABLE (REFS, CALLS)'
A C expression to determine whether it is worthwhile to consider
placing a pseudo-register in a call-clobbered hard register and
saving and restoring it around each function call. The
expression should be 1 when this is worth doing, and 0 otherwise.

If you don't define this macro, a default is used which is good
on most machines: '4 * CALLS < REFS'.


File: gcc.info, Node: Function Entry, Next: Profiling, Prev: Caller Saves, Up: Stack and Calling

Function Entry and Exit
-----------------------

This section describes the macros that output function entry
("prologue") and exit ("epilogue") code.

'FUNCTION_PROLOGUE (FILE, SIZE)'
A C compound statement that outputs the assembler code for entry
to a function. The prologue is responsible for setting up the
stack frame, initializing the frame pointer register, saving
registers that must be saved, and allocating SIZE additional
bytes of storage for the local variables. SIZE is an integer.
FILE is a stdio stream to which the assembler code should be
output.

The label for the beginning of the function need not be output by
this macro. That has already been done when the macro is run.

To determine which registers to save, the macro can refer to the
array 'regs_ever_live': element R is nonzero if hard register R
is used anywhere within the function. This implies the function
prologue should save register R, provided it is not one of the
call-used registers. ('FUNCTION_EPILOGUE' must likewise use
'regs_ever_live'.)

On machines that have "register windows", the function entry code
does not save on the stack the registers that are in the windows,
even if they are supposed to be preserved by function calls;
instead it takes appropriate steps to "push" the register stack,
if any non-call-used registers are used in the function.

On machines where functions may or may not have frame-pointers,
the function entry code must vary accordingly; it must set up the
frame pointer if one is wanted, and not otherwise. To determine
whether a frame pointer is in wanted, the macro can refer to the
variable 'frame_pointer_needed'. The variable's value will be 1
at run time in a function that needs a frame pointer. *Note
Elimination::.

The function entry code is responsible for allocating any stack
space required for the function. This stack space consists of
the regions listed below. In most cases, these regions are
allocated in the order listed, with the last listed region
closest to the top of the stack (the lowest address if
'STACK_GROWS_DOWNWARD' is defined, and the highest address if it
is not defined). You can use a different order for a machine if
doing so is more convenient or required for compatibility
reasons. Except in cases where required by standard or by a
debugger, there is no reason why the stack layout used by GCC
need agree with that used by other compilers for a machine.

* A region of 'current_function_pretend_args_size' bytes of
uninitialized space just underneath the first argument
arriving on the stack. (This may not be at the very start
of the allocated stack region if the calling sequence has
pushed anything else since pushing the stack arguments. But
usually, on such machines, nothing else has been pushed yet,
because the function prologue itself does all the pushing.)
This region is used on machines where an argument may be
passed partly in registers and partly in memory, and, in
some cases to support the features in 'varargs.h' and
'stdargs.h'.

* An area of memory used to save certain registers used by the
function. The size of this area, which may also include
space for such things as the return address and pointers to
previous stack frames, is machine-specific and usually
depends on which registers have been used in the function.
Machines with register windows often do not require a save
area.

* A region of at least SIZE bytes, possibly rounded up to an
allocation boundary, to contain the local variables of the
function. On some machines, this region and the save area
may occur in the opposite order, with the save area closer
to the top of the stack.

* Optionally, in the case that 'ACCUMULATE_OUTGOING_ARGS' is
defined, a region of 'current_function_outgoing_args_size'
bytes to be used for outgoing argument lists of the
function. *Note Stack Arguments::.

Normally, it is necessary for 'FUNCTION_PROLOGUE' and
'FUNCTION_EPILOGUE' to treat leaf functions specially. The C
variable 'leaf_function' is nonzero for such a function.

'EXIT_IGNORE_STACK'
Define this macro as a C expression that is nonzero if the return
instruction or the function epilogue ignores the value of the
stack pointer; in other words, if it is safe to delete an
instruction to adjust the stack pointer before a return from the
function.

Note that this macro's value is relevant only for functions for
which frame pointers are maintained. It is never safe to delete
a final stack adjustment in a function that has no frame pointer,
and the compiler knows this regardless of 'EXIT_IGNORE_STACK'.

'FUNCTION_EPILOGUE (FILE, SIZE)'
A C compound statement that outputs the assembler code for exit
from a function. The epilogue is responsible for restoring the
saved registers and stack pointer to their values when the
function was called, and returning control to the caller. This
macro takes the same arguments as the macro 'FUNCTION_PROLOGUE',
and the registers to restore are determined from 'regs_ever_live'
and 'CALL_USED_REGISTERS' in the same way.

On some machines, there is a single instruction that does all the
work of returning from the function. On these machines, give that
instruction the name 'return' and do not define the macro
'FUNCTION_EPILOGUE' at all.

Do not define a pattern named 'return' if you want the
'FUNCTION_EPILOGUE' to be used. If you want the target switches
to control whether return instructions or epilogues are used,
define a 'return' pattern with a validity condition that tests
the target switches appropriately. If the 'return' pattern's
validity condition is false, epilogues will be used.

On machines where functions may or may not have frame-pointers,
the function exit code must vary accordingly. Sometimes the code
for these two cases is completely different. To determine
whether a frame pointer is in wanted, the macro can refer to the
variable 'frame_pointer_needed'. The variable's value will be 1
at run time in a function that needs a frame pointer.

Normally, it is necessary for 'FUNCTION_PROLOGUE' and
'FUNCTION_EPILOGUE' to treat leaf functions specially. The C
variable 'leaf_function' is nonzero for such a function. *Note
Leaf Functions::.

On some machines, some functions pop their arguments on exit while
others leave that for the caller to do. For example, the 68020
when given '-mrtd' pops arguments in functions that take a fixed
number of arguments.

Your definition of the macro 'RETURN_POPS_ARGS' decides which
functions pop their own arguments. 'FUNCTION_EPILOGUE' needs to
know what was decided. The variable 'current_function_pops_args'
is the number of bytes of its arguments that a function should
pop. *Note Scalar Return::.

'DELAY_SLOTS_FOR_EPILOGUE'
Define this macro if the function epilogue contains delay slots
to which instructions from the rest of the function can be
"moved". The definition should be a C expression whose value is
an integer representing the number of delay slots there.

'ELIGIBLE_FOR_EPILOGUE_DELAY (INSN, N)'
A C expression that returns 1 if INSN can be placed in delay slot
number N of the epilogue.

The argument N is an integer which identifies the delay slot now
being considered (since different slots may have different rules
of eligibility). It is never negative and is always less than
the number of epilogue delay slots (what
'DELAY_SLOTS_FOR_EPILOGUE' returns). If you reject a particular
insn for a given delay slot, in principle, it may be reconsidered
for a subsequent delay slot. Also, other insns may (at least in
principle) be considered for the so far unfilled delay slot.

The insns accepted to fill the epilogue delay slots are put in an
RTL list made with 'insn_list' objects, stored in the variable
'current_function_epilogue_delay_list'. The insn for the first
delay slot comes first in the list. Your definition of the macro
'FUNCTION_EPILOGUE' should fill the delay slots by outputting the
insns in this list, usually by calling 'final_scan_insn'.

You need not define this macro if you did not define
'DELAY_SLOTS_FOR_EPILOGUE'.


File: gcc.info, Node: Profiling, Prev: Function Entry, Up: Stack and Calling

Generating Code for Profiling
-----------------------------

'FUNCTION_PROFILER (FILE, LABELNO)'
A C statement or compound statement to output to FILE some
assembler code to call the profiling subroutine 'mcount'. Before
calling, the assembler code must load the address of a counter
variable into a register where 'mcount' expects to find the
address. The name of this variable is 'LP' followed by the
number LABELNO, so you would generate the name using 'LP%d' in a
'fprintf'.

The details of how the address should be passed to 'mcount' are
determined by your operating system environment, not by GNU CC.
To figure them out, compile a small program for profiling using
the system's installed C compiler and look at the assembler code
that results.

'PROFILE_BEFORE_PROLOGUE'
Define this macro if the code for function profiling should come
before the function prologue. Normally, the profiling code comes
after.

'FUNCTION_BLOCK_PROFILER (FILE, LABELNO)'
A C statement or compound statement to output to FILE some
assembler code to initialize basic-block profiling for the current
object module. This code should call the subroutine
'__bb_init_func' once per object module, passing it as its sole
argument the address of a block allocated in the object module.

The name of the block is a local symbol made with this statement:

ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 0);

Of course, since you are writing the definition of
'ASM_GENERATE_INTERNAL_LABEL' as well as that of this macro, you
can take a short cut in the definition of this macro and use the
name that you know will result.

The first word of this block is a flag which will be nonzero if
the object module has already been initialized. So test this
word first, and do not call '__bb_init_func' if the flag is
nonzero.

'BLOCK_PROFILER (FILE, BLOCKNO)'
A C statement or compound statement to increment the count
associated with the basic block number BLOCKNO. Basic blocks are
numbered separately from zero within each compilation. The count
associated with block number BLOCKNO is at index BLOCKNO in a
vector of words; the name of this array is a local symbol made
with this statement:

ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 2);

Of course, since you are writing the definition of
'ASM_GENERATE_INTERNAL_LABEL' as well as that of this macro, you
can take a short cut in the definition of this macro and use the
name that you know will result.


File: gcc.info, Node: Varargs, Next: Trampolines, Prev: Stack and Calling, Up: Target Macros

Implementing the Varargs Macros
===============================

GNU CC comes with an implementation of 'varargs.h' and 'stdarg.h'
that work without change on machines that pass arguments on the stack.
Other machines require their own implementations of varargs, and the
two machine independent header files must have conditionals to include
it.

ANSI 'stdarg.h' differs from traditional 'varargs.h' mainly in the
calling convention for 'va_start'. The traditional implementation
takes just one argument, which is the variable in which to store the
argument pointer. The ANSI implementation takes an additional first
argument, which is the last named argument of the function. However,
it should not use this argument. The way to find the end of the named
arguments is with the built-in functions described below.

'__builtin_saveregs ()'
Use this built-in function to save the argument registers in
memory so that the varargs mechanism can access them. Both ANSI
and traditional versions of 'va_start' must use
'__builtin_saveregs', unless you use 'SETUP_INCOMING_VARARGS'
(see below) instead.

On some machines, '__builtin_saveregs' is open-coded under the
control of the macro 'EXPAND_BUILTIN_SAVEREGS'. On other
machines, it calls a routine written in assembler language, found
in 'libgcc2.c'.

Regardless of what code is generated for the call to
'__builtin_saveregs', it appears at the beginning of the function,
not where the call to '__builtin_saveregs' is written. This is
because the registers must be saved before the function starts to
use them for its own purposes.

'__builtin_args_info (CATEGORY)'
Use this built-in function to find the first anonymous arguments
in registers.

In general, a machine may have several categories of registers
used for arguments, each for a particular category of data types.
(For example, on some machines, floating-point registers are
used for floating-point arguments while other arguments are
passed in the general registers.) To make non-varargs functions
use the proper calling convention, you have defined the
'CUMULATIVE_ARGS' data type to record how many registers in each
category have been used so far

'__builtin_args_info' accesses the same data structure of type
'CUMULATIVE_ARGS' after the ordinary argument layout is finished
with it, with CATEGORY specifying which word to access. Thus, the
value indicates the first unused register in a given category.

Normally, you would use '__builtin_args_info' in the
implementation of 'va_start', accessing each category just once
and storing the value in the 'va_list' object. This is because
'va_list' will have to update the values, and there is no way to
alter the values accessed by '__builtin_args_info'.

'__builtin_next_arg ()'
This is the equivalent of '__builtin_args_info', for stack
arguments. It returns the address of the first anonymous stack
argument, as type 'void *'. If 'ARGS_GROW_DOWNWARD', it returns
the address of the location above the first anonymous stack
argument. Use it in 'va_start' to initialize the pointer for
fetching arguments from the stack.

'__builtin_classify_type (OBJECT)'
Since each machine has its own conventions for which data types
are passed in which kind of register, your implementation of
'va_arg' has to embody these conventions. The easiest way to
categorize the specified data type is to use
'__builtin_classify_type' together with 'sizeof' and
'__alignof__'.

'__builtin_classify_type' ignores the value of OBJECT,
considering only its data type. It returns an integer describing
what kind of type that is--integer, floating, pointer, structure,
and so on.

The file 'typeclass.h' defines an enumeration that you can use to
interpret the values of '__builtin_classify_type'.

These machine description macros help implement varargs:

'EXPAND_BUILTIN_SAVEREGS (ARGS)'
If defined, is a C expression that produces the machine-specific
code for a call to '__builtin_saveregs'. This code will be moved
to the very beginning of the function, before any parameter
access are made. The return value of this function should be an
RTX that contains the value to use as the return of
'__builtin_saveregs'.

The argument ARGS is a 'tree_list' containing the arguments that
were passed to '__builtin_saveregs'.

If this macro is not defined, the compiler will output an ordinary
call to the library function '__builtin_saveregs'.

'SETUP_INCOMING_VARARGS (ARGS_SO_FAR, MODE, TYPE, PRETEND_ARGS_SIZE, SECOND_TIME)'
This macro offers an alternative to using '__builtin_saveregs' and
defining the macro 'EXPAND_BUILTIN_SAVEREGS'. Use it to store the
anonymous register arguments into the stack so that all the
arguments appear to have been passed consecutively on the stack.
Once this is done, you can use the standard implementation of
varargs that works for machines that pass all their arguments on
the stack.

The argument ARGS_SO_FAR is the 'CUMULATIVE_ARGS' data structure,
containing the values that obtain after processing of the named
arguments. The arguments MODE and TYPE describe the last named
argument--its machine mode and its data type as a tree node.

The macro implementation should do two things: first, push onto
the stack all the argument registers *not* used for the named
arguments, and second, store the size of the data thus pushed
into the 'int'-valued variable whose name is supplied as the
argument PRETEND_ARGS_SIZE. The value that you store here will
serve as additional offset for setting up the stack frame.

Because you must generate code to push the anonymous arguments at
compile time without knowing their data types,
'SETUP_INCOMING_VARARGS' is only useful on machines that have just
a single category of argument register and use it uniformly for
all data types.

If the argument SECOND_TIME is nonzero, it means that the
arguments of the function are being analyzed for the second time.
This happens for an inline function, which is not actually
compiled until the end of the source file. The macro
'SETUP_INCOMING_VARARGS' should not generate any instructions in
this case.


File: gcc.info, Node: Trampolines, Next: Library Calls, Prev: Varargs, Up: Target Macros

Trampolines for Nested Functions
================================

A "trampoline" is a small piece of code that is created at run time
when the address of a nested function is taken. It normally resides on
the stack, in the stack frame of the containing function. These macros
tell GNU CC how to generate code to allocate and initialize a
trampoline.

The instructions in the trampoline must do two things: load a
constant address into the static chain register, and jump to the real
address of the nested function. On CISC machines such as the m68k,
this requires two instructions, a move immediate and a jump. Then the
two addresses exist in the trampoline as word-long immediate operands.
On RISC machines, it is often necessary to load each address into a
register in two parts. Then pieces of each address form separate
immediate operands.

The code generated to initialize the trampoline must store the
variable parts--the static chain value and the function address--into
the immediate operands of the instructions. On a CISC machine, this is
simply a matter of copying each address to a memory reference at the
proper offset from the start of the trampoline. On a RISC machine, it
may be necessary to take out pieces of the address and store them
separately.

'TRAMPOLINE_TEMPLATE (FILE)'
A C statement to output, on the stream FILE, assembler code for a
block of data that contains the constant parts of a trampoline.
This code should not include a label--the label is taken care of
automatically.

'TRAMPOLINE_SIZE'
A C expression for the size in bytes of the trampoline, as an
integer.

'TRAMPOLINE_ALIGNMENT'
Alignment required for trampolines, in bits.

If you don't define this macro, the value of 'BIGGEST_ALIGNMENT'
is used for aligning trampolines.

'INITIALIZE_TRAMPOLINE (ADDR, FNADDR, STATIC_CHAIN)'
A C statement to initialize the variable parts of a trampoline.
ADDR is an RTX for the address of the trampoline; FNADDR is an
RTX for the address of the nested function; STATIC_CHAIN is an
RTX for the static chain value that should be passed to the
function when it is called.

'ALLOCATE_TRAMPOLINE (FP)'
A C expression to allocate run-time space for a trampoline. The
expression value should be an RTX representing a memory reference
to the space for the trampoline.

If this macro is not defined, by default the trampoline is
allocated as a stack slot. This default is right for most
machines. The exceptions are machines where it is impossible to
execute instructions in the stack area. On such machines, you
may have to implement a separate stack, using this macro in
conjunction with 'FUNCTION_PROLOGUE' and 'FUNCTION_EPILOGUE'.

FP points to a data structure, a 'struct function', which
describes the compilation status of the immediate containing
function of the function which the trampoline is for. Normally
(when 'ALLOCATE_TRAMPOLINE' is not defined), the stack slot for
the trampoline is in the stack frame of this containing function.
Other allocation strategies probably must do something analogous
with this information.

Implementing trampolines is difficult on many machines because they
have separate instruction and data caches. Writing into a stack
location fails to clear the memory in the instruction cache, so when
the program jumps to that location, it executes the old contents.

Here are two possible solutions. One is to clear the relevant
parts of the instruction cache whenever a trampoline is set up. The
other is to make all trampolines identical, by having them jump to a
standard subroutine. The former technique makes trampoline execution
faster; the latter makes initialization faster.

To clear the instruction cache when a trampoline is initialized,
define the following macros which describe the shape of the cache.

'INSN_CACHE_SIZE'
The total size in bytes of the cache.

'INSN_CACHE_LINE_WIDTH'
The length in bytes of each cache line. The cache is divided
into cache lines which are disjoint slots, each holding a
contiguous chunk of data fetched from memory. Each time data is
brought into the cache, an entire line is read at once. The data
loaded into a cache line is always aligned on a boundary equal to
the line size.

'INSN_CACHE_DEPTH'
The number of alternative cache lines that can hold any
particular memory location.

To use a standard subroutine, define the following macro. In
addition, you must make sure that the instructions in a trampoline
fill an entire cache line with identical instructions, or else ensure
that the beginning of the trampoline code is always aligned at the
same point in its cache line. Look in 'm68k.h' as a guide.

'TRANSFER_FROM_TRAMPOLINE'
Define this macro if trampolines need a special subroutine to do
their work. The macro should expand to a series of 'asm'
statements which will be compiled with GNU CC. They go in a
library function named '__transfer_from_trampoline'.

If you need to avoid executing the ordinary prologue code of a
compiled C function when you jump to the subroutine, you can do
so by placing a special label of your own in the assembler code.
Use one 'asm' statement to generate an assembler label, and
another to make the label global. Then trampolines can use that
label to jump directly to your special assembler code.


File: gcc.info, Node: Library Calls, Next: Addressing Modes, Prev: Trampolines, Up: Target Macros

Implicit Calls to Library Routines
==================================

'MULSI3_LIBCALL'
A C string constant giving the name of the function to call for
multiplication of one signed full-word by another. If you do not
define this macro, the default name is used, which is '__mulsi3',
a function defined in 'libgcc.a'.

'DIVSI3_LIBCALL'
A C string constant giving the name of the function to call for
division of one signed full-word by another. If you do not define
this macro, the default name is used, which is '__divsi3', a
function defined in 'libgcc.a'.

'UDIVSI3_LIBCALL'
A C string constant giving the name of the function to call for
division of one unsigned full-word by another. If you do not
define this macro, the default name is used, which is
'__udivsi3', a function defined in 'libgcc.a'.

'MODSI3_LIBCALL'
A C string constant giving the name of the function to call for
the remainder in division of one signed full-word by another. If
you do not define this macro, the default name is used, which is
'__modsi3', a function defined in 'libgcc.a'.

'UMODSI3_LIBCALL'
A C string constant giving the name of the function to call for
the remainder in division of one unsigned full-word by another.
If you do not define this macro, the default name is used, which
is '__umodsi3', a function defined in 'libgcc.a'.

'MULDI3_LIBCALL'
A C string constant giving the name of the function to call for
multiplication of one signed double-word by another. If you do
not define this macro, the default name is used, which is
'__muldi3', a function defined in 'libgcc.a'.

'DIVDI3_LIBCALL'
A C string constant giving the name of the function to call for
division of one signed double-word by another. If you do not
define this macro, the default name is used, which is '__divdi3',
a function defined in 'libgcc.a'.

'UDIVDI3_LIBCALL'
A C string constant giving the name of the function to call for
division of one unsigned full-word by another. If you do not
define this macro, the default name is used, which is
'__udivdi3', a function defined in 'libgcc.a'.

'MODDI3_LIBCALL'
A C string constant giving the name of the function to call for
the remainder in division of one signed double-word by another.
If you do not define this macro, the default name is used, which
is '__moddi3', a function defined in 'libgcc.a'.

'UMODDI3_LIBCALL'
A C string constant giving the name of the function to call for
the remainder in division of one unsigned full-word by another.
If you do not define this macro, the default name is used, which
is '__umoddi3', a function defined in 'libgcc.a'.

'TARGET_MEM_FUNCTIONS'
Define this macro if GNU CC should generate calls to the System V
(and ANSI C) library functions 'memcpy' and 'memset' rather than
the BSD functions 'bcopy' and 'bzero'.

'LIBGCC_NEEDS_DOUBLE'
Define this macro if only 'float' arguments cannot be passed to
library routines (so they must be converted to 'double'). This
macro affects both how library calls are generated and how the
library routines in 'libgcc1.c' accept their arguments. It is
useful on machines where floating and fixed point arguments are
passed differently, such as the i860.

'FLOAT_ARG_TYPE'
Define this macro to override the type used by the library
routines to pick up arguments of type 'float'. (By default, they
use a union of 'float' and 'int'.)

The obvious choice would be 'float'--but that won't work with
traditional C compilers that expect all arguments declared as
'float' to arrive as 'double'. To avoid this conversion, the
library routines ask for the value as some other type and then
treat it as a 'float'.

On some systems, no other type will work for this. For these
systems, you must use 'LIBGCC_NEEDS_DOUBLE' instead, to force
conversion of the values 'double' before they are passed.

'FLOATIFY (PASSED-VALUE)'
Define this macro to override the way library routines
redesignate a 'float' argument as a 'float' instead of the type
it was passed as. The default is an expression which takes the
'float' field of the union.

'FLOAT_VALUE_TYPE'
Define this macro to override the type used by the library
routines to return values that ought to have type 'float'. (By
default, they use 'int'.)

The obvious choice would be 'float'--but that won't work with
traditional C compilers gratuitously convert values declared as
'float' into 'double'.

'INTIFY (FLOAT-VALUE)'
Define this macro to override the way the value of a
'float'-returning library routine should be packaged in order to
return it. These functions are actually declared to return type
'FLOAT_VALUE_TYPE' (normally 'int').

These values can't be returned as type 'float' because traditional
C compilers would gratuitously convert the value to a 'double'.

A local variable named 'intify' is always available when the macro
'INTIFY' is used. It is a union of a 'float' field named 'f' and
a field named 'i' whose type is 'FLOAT_VALUE_TYPE' or 'int'.

If you don't define this macro, the default definition works by
copying the value through that union.

'SItype'
Define this macro as the name of the data type corresponding to
'SImode' in the system's own C compiler.

You need not define this macro if that type is 'int', as it
usually is.

'perform_...'
Define these macros to supply explicit C statements to carry out
various arithmetic operations on types 'float' and 'double' in the
library routines in 'libgcc1.c'. See that file for a full list
of these macros and their arguments.

On most machines, you don't need to define any of these macros,
because the C compiler that comes with the system takes care of
doing them.

'NEXT_OBJC_RUNTIME'
Define this macro to generate code for Objective C message
sending using the calling convention of the NeXT system. This
calling convention involves passing the object, the selector and
the method arguments all at once to the method-lookup library
function.

The default calling convention passes just the object and the
selector to the lookup function, which returns a pointer to the
method.


File: gcc.info, Node: Addressing Modes, Next: Condition Code, Prev: Library Calls, Up: Target Macros

Addressing Modes
================

'HAVE_POST_INCREMENT'
Define this macro if the machine supports post-increment
addressing.

'HAVE_PRE_INCREMENT'
'HAVE_POST_DECREMENT'
'HAVE_PRE_DECREMENT'
Similar for other kinds of addressing.

'CONSTANT_ADDRESS_P (X)'
A C expression that is 1 if the RTX X is a constant which is a
valid address. On most machines, this can be defined as
'CONSTANT_P (X)', but a few machines are more restrictive in
which constant addresses are supported.

'CONSTANT_P' accepts integer-values expressions whose values are
not explicitly known, such as 'symbol_ref', 'label_ref', and
'high' expressions and 'const' arithmetic expressions, in
addition to 'const_int' and 'const_double' expressions.

'MAX_REGS_PER_ADDRESS'
A number, the maximum number of registers that can appear in a
valid memory address. Note that it is up to you to specify a
value equal to the maximum number that 'GO_IF_LEGITIMATE_ADDRESS'
would ever accept.

'GO_IF_LEGITIMATE_ADDRESS (MODE, X, LABEL)'
A C compound statement with a conditional 'goto LABEL;' executed
if X (an RTX) is a legitimate memory address on the target
machine for a memory operand of mode MODE.

It usually pays to define several simpler macros to serve as
subroutines for this one. Otherwise it may be too complicated to
understand.

This macro must exist in two variants: a strict variant and a
non-strict one. The strict variant is used in the reload pass.
It must be defined so that any pseudo-register that has not been
allocated a hard register is considered a memory reference. In
contexts where some kind of register is required, a
pseudo-register with no hard register must be rejected.

The non-strict variant is used in other passes. It must be
defined to accept all pseudo-registers in every context where
some kind of register is required.

Compiler source files that want to use the strict variant of this
macro define the macro 'REG_OK_STRICT'. You should use an
'#ifdef REG_OK_STRICT' conditional to define the strict variant
in that case and the non-strict variant otherwise.

Typically among the subroutines used to define
'GO_IF_LEGITIMATE_ADDRESS' are subroutines to check for
acceptable registers for various purposes (one for base
registers, one for index registers, and so on). Then only these
subroutine macros need have two variants; the higher levels of
macros may be the same whether strict or not.

Normally, constant addresses which are the sum of a 'symbol_ref'
and an integer are stored inside a 'const' RTX to mark them as
constant. Therefore, there is no need to recognize such sums
specifically as legitimate addresses. Normally you would simply
recognize any 'const' as legitimate.

Usually 'PRINT_OPERAND_ADDRESS' is not prepared to handle constant
sums that are not marked with 'const'. It assumes that a naked
'plus' indicates indexing. If so, then you *must* reject such
naked constant sums as illegitimate addresses, so that none of
them will be given to 'PRINT_OPERAND_ADDRESS'.

On some machines, whether a symbolic address is legitimate
depends on the section that the address refers to. On these
machines, define the macro 'ENCODE_SECTION_INFO' to store the
information into the 'symbol_ref', and then check for it here.
When you see a 'const', you will have to look inside it to find
the 'symbol_ref' in order to determine the section. *Note
Assembler Format::.

The best way to modify the name string is by adding text to the
beginning, with suitable punctuation to prevent any ambiguity.
Allocate the new name in 'saveable_obstack'. You will have to
modify 'ASM_OUTPUT_LABELREF' to remove and decode the added text
and output the name accordingly.

You can check the information stored here into the 'symbol_ref' in
the definitions of 'GO_IF_LEGITIMATE_ADDRESS' and
'PRINT_OPERAND_ADDRESS'.

'REG_OK_FOR_BASE_P (X)'
A C expression that is nonzero if X (assumed to be a 'reg' RTX)
is valid for use as a base register. For hard registers, it
should always accept those which the hardware permits and reject
the others. Whether the macro accepts or rejects pseudo
registers must be controlled by 'REG_OK_STRICT' as described
above. This usually requires two variant definitions, of which
'REG_OK_STRICT' controls the one actually used.

'REG_OK_FOR_INDEX_P (X)'
A C expression that is nonzero if X (assumed to be a 'reg' RTX)
is valid for use as an index register.

The difference between an index register and a base register is
that the index register may be scaled. If an address involves
the sum of two registers, neither one of them scaled, then either
one may be labeled the "base" and the other the "index"; but
whichever labeling is used must fit the machine's constraints of
which registers may serve in each capacity. The compiler will
try both labelings, looking for one that is valid, and will
reload one or both registers only if neither labeling works.

'LEGITIMIZE_ADDRESS (X, OLDX, MODE, WIN)'
A C compound statement that attempts to replace X with a valid
memory address for an operand of mode MODE. WIN will be a C
statement label elsewhere in the code; the macro definition may
use

GO_IF_LEGITIMATE_ADDRESS (MODE, X, WIN);

to avoid further processing if the address has become legitimate.

X will always be the result of a call to 'break_out_memory_refs',
and OLDX will be the operand that was given to that function to
produce X.

The code generated by this macro should not alter the
substructure of X. If it transforms X into a more legitimate
form, it should assign X (which will always be a C variable) a
new value.

It is not necessary for this macro to come up with a legitimate
address. The compiler has standard ways of doing so in all
cases. In fact, it is safe for this macro to do nothing. But
often a machine-dependent strategy can generate better code.

'GO_IF_MODE_DEPENDENT_ADDRESS (ADDR, LABEL)'
A C statement or compound statement with a conditional 'goto
LABEL;' executed if memory address X (an RTX) can have different
meanings depending on the machine mode of the memory reference it
is used for.

Autoincrement and autodecrement addresses typically have
mode-dependent effects because the amount of the increment or
decrement is the size of the operand being addressed. Some
machines have other mode-dependent addresses. Many RISC machines
have no mode-dependent addresses.

You may assume that ADDR is a valid address for the machine.

'LEGITIMATE_CONSTANT_P (X)'
A C expression that is nonzero if X is a legitimate constant for
an immediate operand on the target machine. You can assume that
X satisfies 'CONSTANT_P', so you need not check this. In fact,
'1' is a suitable definition for this macro on machines where
anything 'CONSTANT_P' is valid.

'LEGITIMATE_PIC_OPERAND_P (X)'
A C expression that is nonzero if X is a legitimate immediate
operand on the target machine when generating position
independent code. You can assume that X satisfies 'CONSTANT_P',
so you need not check this. You can also assume FLAG_PIC is
true, so you need not check it either. You need not define this
macro if all constants (including 'SYMBOL_REF') can be immediate
operands when generating position independent code.


File: gcc.info, Node: Condition Code, Next: Costs, Prev: Addressing Modes, Up: Target Macros
File: gcc.info, Node: Costs, Next: Sections, Prev: Condition Code, Up: Target Macros

Describing Relative Costs of Operations
=======================================

These macros let you describe the relative speed of various
operations on the target machine.

'CONST_COSTS (X, CODE)'
A part of a C 'switch' statement that describes the relative costs
of constant RTL expressions. It must contain 'case' labels for
expression codes 'const_int', 'const', 'symbol_ref', 'label_ref'
and 'const_double'. Each case must ultimately reach a 'return'
statement to return the relative cost of the use of that kind of
constant value in an expression. The cost may depend on the
precise value of the constant, which is available for examination
in X.

CODE is the expression code--redundant, since it can be obtained
with 'GET_CODE (X)'.

'RTX_COSTS (X, CODE)'
Like 'CONST_COSTS' but applies to nonconstant RTL expressions.
This can be used, for example, to indicate how costly a multiply
instruction is. In writing this macro, you can use the construct
'COSTS_N_INSNS (N)' to specify a cost equal to N fast
instructions.

This macro is optional; do not define it if the default cost
assumptions are adequate for the target machine.

'ADDRESS_COST (ADDRESS)'
An expression giving the cost of an addressing mode that contains
ADDRESS. If not defined, the cost is computed from the ADDRESS
expression and the 'CONST_COSTS' values.

For most CISC machines, the default cost is a good approximation
of the true cost of the addressing mode. However, on RISC
machines, all instructions normally have the same length and
execution time. Hence all addresses will have equal costs.

In cases where more than one form of an address is known, the
form with the lowest cost will be used. If multiple forms have
the same, lowest, cost, the one that is the most complex will be
used.

For example, suppose an address that is equal to the sum of a
register and a constant is used twice in the same basic block.
When this macro is not defined, the address will be computed in a
register and memory references will be indirect through that
register. On machines where the cost of the addressing mode
containing the sum is no higher than that of a simple indirect
reference, this will produce an additional instruction and
possibly require an additional register. Proper specification of
this macro eliminates this overhead for such machines.

Similar use of this macro is made in strength reduction of loops.

ADDRESS need not be valid as an address. In such a case, the cost
is not relevant and can be any value; invalid addresses need not
be assigned a different cost.

On machines where an address involving more than one register is
as cheap as an address computation involving only one register,
defining 'ADDRESS_COST' to reflect this can cause two registers
to be live over a region of code where only one would have been if
'ADDRESS_COST' were not defined in that manner. This effect
should be considered in the definition of this macro. Equivalent
costs should probably only be given to addresses with different
numbers of registers on machines with lots of registers.

This macro will normally either not be defined or be defined as a
constant.

'REGISTER_MOVE_COST (FROM, TO)'
A C expression for the cost of moving data from a register in
class FROM to one in class TO. The classes are expressed using
the enumeration values such as 'GENERAL_REGS'. A value of 2 is
the default; other values are interpreted relative to that.

It is not required that the cost always equal 2 when FROM is the
same as TO; on some machines it is expensive to move between
registers if they are not general registers.

If reload sees an insn consisting of a single 'set' between two
hard registers, and if 'REGISTER_MOVE_COST' applied to their
classes returns a value of 2, reload does not check to ensure
that the constraints of the insn are met. Setting a cost of
other than 2 will allow reload to verify that the constraints are
met. You should do this if the 'movM' pattern's constraints do
not allow such copying.

'MEMORY_MOVE_COST (M)'
A C expression for the cost of moving data of mode M between a
register and memory. A value of 2 is the default; this cost is
relative to those in 'REGISTER_MOVE_COST'.

If moving between registers and memory is more expensive than
between two registers, you should define this macro to express
the relative cost.

'BRANCH_COST'
A C expression for the cost of a branch instruction. A value of
1 is the default; other values are interpreted relative to that.

Here are additional macros which do not specify precise relative
costs, but only that certain actions are more expensive than GNU CC
would ordinarily expect.

'SLOW_BYTE_ACCESS'
Define this macro as a C expression which is nonzero if accessing
less than a word of memory (i.e. a 'char' or a 'short') is no
faster than accessing a word of memory, i.e., if such access
require more than one instruction or if there is no difference in
cost between byte and (aligned) word loads.

When this macro is not defined, the compiler will access a field
by finding the smallest containing object; when it is defined, a
fullword load will be used if alignment permits. Unless bytes
accesses are faster than word accesses, using word accesses is
preferable since it may eliminate subsequent memory access if
subsequent accesses occur to other fields in the same word of the
structure, but to different bytes.

'SLOW_ZERO_EXTEND'
Define this macro if zero-extension (of a 'char' or 'short' to an
'int') can be done faster if the destination is a register that
is known to be zero.

If you define this macro, you must have instruction patterns that

recognize RTL structures like this:

(set (strict_low_part (subreg:QI (reg:SI ...) 0)) ...)

and likewise for 'HImode'.

'SLOW_UNALIGNED_ACCESS'
Define this macro to be the value 1 if unaligned accesses have a
cost many times greater than aligned accesses, for example if
they are emulated in a trap handler.

When this macro is non-zero, the compiler will act as if
'STRICT_ALIGNMENT' were non-zero when generating code for block
moves. This can cause significantly more instructions to be
produced. Therefore, do not set this macro non-zero if unaligned
accesses only add a cycle or two to the time for a memory access.

If the value of this macro is always zero, it need not be defined.

'DONT_REDUCE_ADDR'
Define this macro to inhibit strength reduction of memory
addresses. (On some machines, such strength reduction seems to
do harm rather than good.)

'MOVE_RATIO'
The number of scalar move insns which should be generated instead
of a string move insn or a library call. Increasing the value
will always make code faster, but eventually incurs high cost in
increased code size.

If you don't define this, a reasonable default is used.

'NO_FUNCTION_CSE'
Define this macro if it is as good or better to call a constant
function address than to call an address kept in a register.

'NO_RECURSIVE_FUNCTION_CSE'
Define this macro if it is as good or better for a function to
call itself with an explicit address than to call an address kept
in a register.


File: gcc.info, Node: Sections, Next: PIC, Prev: Costs, Up: Target Macros

Dividing the Output into Sections (Texts, Data, ...)
====================================================

An object file is divided into sections containing different types
of data. In the most common case, there are three sections: the "text
section", which holds instructions and read-only data; the "data
section", which holds initialized writable data; and the "bss
section", which holds uninitialized data. Some systems have other
kinds of sections.

The compiler must tell the assembler when to switch sections. These
macros control what commands to output to tell the assembler this. You
can also define additional sections.

'TEXT_SECTION_ASM_OP'
A C string constant for the assembler operation that should
precede instructions and read-only data. Normally '".text"' is
right.

'DATA_SECTION_ASM_OP'
A C string constant for the assembler operation to identify the
following data as writable initialized data. Normally '".data"'
is right.

'SHARED_SECTION_ASM_OP'
If defined, a C string constant for the assembler operation to
identify the following data as shared data. If not defined,
'DATA_SECTION_ASM_OP' will be used.

'INIT_SECTION_ASM_OP'
If defined, a C string constant for the assembler operation to
identify the following data as initialization code. If not
defined, GNU CC will assume such a section does not exist.

'EXTRA_SECTIONS'
A list of names for sections other than the standard two, which
are 'in_text' and 'in_data'. You need not define this macro on a
system with no other sections (that GCC needs to use).

'EXTRA_SECTION_FUNCTIONS'
One or more functions to be defined in 'varasm.c'. These
functions should do jobs analogous to those of 'text_section' and
'data_section', for your additional sections. Do not define this
macro if you do not define 'EXTRA_SECTIONS'.

'READONLY_DATA_SECTION'
On most machines, read-only variables, constants, and jump tables
are placed in the text section. If this is not the case on your
machine, this macro should be defined to be the name of a
function (either 'data_section' or a function defined in
'EXTRA_SECTIONS') that switches to the section to be used for
read-only items.

If these items should be placed in the text section, this macro
should not be defined.

'SELECT_SECTION (EXP, RELOC)'
A C statement or statements to switch to the appropriate section
for output of EXP. You can assume that EXP is either a
'VAR_DECL' node or a constant of some sort. RELOC indicates
whether the initial value of EXP requires link-time relocations.
Select the section by calling 'text_section' or one of the
alternatives for other sections.

Do not define this macro if you put all read-only variables and
constants in the read-only data section (usually the text
section).

'SELECT_RTX_SECTION (MODE, RTX)'
A C statement or statements to switch to the appropriate section
for output of RTX in mode MODE. You can assume that RTX is some
kind of constant in RTL. The argument MODE is redundant except
in the case of a 'const_int' rtx. Select the section by calling
'text_section' or one of the alternatives for other sections.

Do not define this macro if you put all constants in the read-only
data section.

'JUMP_TABLES_IN_TEXT_SECTION'
Define this macro if jump tables (for 'tablejump' insns) should be
output in the text section, along with the assembler instructions.
Otherwise, the readonly data section is used.

This macro is irrelevant if there is no separate readonly data
section.

'ENCODE_SECTION_INFO (DECL)'
Define this macro if references to a symbol must be treated
differently depending on something about the variable or function
named by the symbol (such as what section it is in).

The macro definition, if any, is executed immediately after the
rtl for DECL has been created and stored in 'DECL_RTL (DECL)'.
The value of the rtl will be a 'mem' whose address is a
'symbol_ref'.

The usual thing for this macro to do is to record a flag in the
'symbol_ref' (such as 'SYMBOL_REF_FLAG') or to store a modified
name string in the 'symbol_ref' (if one bit is not enough
information).


File: gcc.info, Node: PIC, Next: Assembler Format, Prev: Sections, Up: Target Macros

Position Independent Code
=========================

This section describes macros that help implement generation of
position independent code. Simply defining these macros is not enough
to generate valid PIC; you must also add support to the macros
'GO_IF_LEGITIMATE_ADDRESS' and 'LEGITIMIZE_ADDRESS', and
'PRINT_OPERAND_ADDRESS' as well. You must modify the definition of
'movsi' to do something appropriate when the source operand contains a
symbolic address. You may also need to alter the handling of switch
statements so that they use relative addresses.

'PIC_OFFSET_TABLE_REGNUM'
The register number of the register used to address a table of
static data addresses in memory. In some cases this register is
defined by a processor's "application binary interface" (ABI).
When this macro is defined, RTL is generated for this register
once, as with the stack pointer and frame pointer registers. If
this macro is not defined, it is up to the machine-dependent
files to allocate such a register (if necessary).

'FINALIZE_PIC'
By generating position-independent code, when two different
programs (A and B) share a common library (libC.a), the text of
the library can be shared whether or not the library is linked at
the same address for both programs. In some of these
environments, position-independent code requires not only the use
of different addressing modes, but also special code to enable
the use of these addressing modes.

The 'FINALIZE_PIC' macro serves as a hook to emit these special
codes once the function is being compiled into assembly code, but
not before. (It is not done before, because in the case of
compiling an inline function, it would lead to multiple PIC
prologues being included in functions which used inline functions
and were compiled to assembly language.)


File: gcc.info, Node: Assembler Format, Next: Debugging Info, Prev: PIC, Up: Target Macros

Defining the Output Assembler Language
======================================

This section describes macros whose principal purpose is to
describe how to write instructions in assembler language--rather than
what the instructions do.

* Menu:

* File Framework:: Structural information for the assembler file.
* Data Output:: Output of constants (numbers, strings, addresses).
* Uninitialized Data:: Output of uninitialized variables.
* Label Output:: Output and generation of labels.
* Constructor Output:: Output of initialization and termination routines.
* Instruction Output:: Output of actual instructions.
* Dispatch Tables:: Output of jump tables.
* Alignment Output:: Pseudo ops for alignment and skipping data.


File: gcc.info, Node: File Framework, Next: Data Output, Up: Assembler Format

The Overall Framework of an Assembler File
------------------------------------------

'ASM_FILE_START (STREAM)'
A C expression which outputs to the stdio stream STREAM some
appropriate text to go at the start of an assembler file.

Normally this macro is defined to output a line containing
'#NO_APP', which is a comment that has no effect on most
assemblers but tells the GNU assembler that it can save time by
not checking for certain assembler constructs.

On systems that use SDB, it is necessary to output certain
commands; see 'attasm.h'.

'ASM_FILE_END (STREAM)'
A C expression which outputs to the stdio stream STREAM some
appropriate text to go at the end of an assembler file.

If this macro is not defined, the default is to output nothing
special at the end of the file. Most systems don't require any
definition.

On systems that use SDB, it is necessary to output certain
commands; see 'attasm.h'.

'ASM_IDENTIFY_GCC (FILE)'
A C statement to output assembler commands which will identify
the object file as having been compiled with GNU CC (or another
GNU compiler).

If you don't define this macro, the string 'gcc_compiled.:' is
output. This string is calculated to define a symbol which, on
BSD systems, will never be defined for any other reason. GDB
checks for the presence of this symbol when reading the symbol
table of an executable.

On non-BSD systems, you must arrange communication with GDB in
some other fashion. If GDB is not used on your system, you can
define this macro with an empty body.

'ASM_COMMENT_START'
A C string constant describing how to begin a comment in the
target assembler language. The compiler assumes that the comment
will end at the end of the line.

'ASM_APP_ON'
A C string constant for text to be output before each 'asm'
statement or group of consecutive ones. Normally this is
'"#APP"', which is a comment that has no effect on most
assemblers but tells the GNU assembler that it must check the
lines that follow for all valid assembler constructs.

'ASM_APP_OFF'
A C string constant for text to be output after each 'asm'
statement or group of consecutive ones. Normally this is
'"#NO_APP"', which tells the GNU assembler to resume making the
time-saving assumptions that are valid for ordinary compiler
output.

'ASM_OUTPUT_SOURCE_FILENAME (STREAM, NAME)'
A C statement to output COFF information or DWARF debugging
information which indicates that filename NAME is the current
source file to the stdio stream STREAM.

This macro need not be defined if the standard form of output for
the file format in use is appropriate.

'ASM_OUTPUT_SOURCE_LINE (STREAM, LINE)'
A C statement to output DBX or SDB debugging information before
code for line number LINE of the current source file to the stdio
stream STREAM.

This macro need not be defined if the standard form of debugging
information for the debugger in use is appropriate.

'ASM_OUTPUT_IDENT (STREAM, STRING)'
A C statement to output something to the assembler file to handle
a '#ident' directive containing the text STRING. If this macro
is not defined, nothing is output for a '#ident' directive.

'OBJC_PROLOGUE'
A C statement to output any assembler statements which are
required to precede any Objective C object definitions or message
sending. The statement is executed only when compiling an
Objective C program.


File: gcc.info, Node: Data Output, Next: Uninitialized Data, Prev: File Framework, Up: Assembler Format

Output of Data
--------------

'ASM_OUTPUT_LONG_DOUBLE (STREAM, VALUE)'
'ASM_OUTPUT_DOUBLE (STREAM, VALUE)'
'ASM_OUTPUT_FLOAT (STREAM, VALUE)'
A C statement to output to the stdio stream STREAM an assembler
instruction to assemble a floating-point constant of 'TFmode',
'DFmode' or 'SFmode', respectively, whose value is VALUE. VALUE
will be a C expression of type 'REAL_VALUE__TYPE', usually
'double'.

'ASM_OUTPUT_QUADRUPLE_INT (STREAM, EXP)'
'ASM_OUTPUT_DOUBLE_INT (STREAM, EXP)'
'ASM_OUTPUT_INT (STREAM, EXP)'
'ASM_OUTPUT_SHORT (STREAM, EXP)'
'ASM_OUTPUT_CHAR (STREAM, EXP)'
A C statement to output to the stdio stream STREAM an assembler
instruction to assemble an integer of 16, 8, 4, 2 or 1 bytes,
respectively, whose value is VALUE. The argument EXP will be an
RTL expression which represents a constant value. Use
'output_addr_const (STREAM, EXP)' to output this value as an
assembler expression.

For sizes larger than 'UNITS_PER_WORD', if the action of a macro
would be identical to repeatedly calling the macro corresponding
to a size of 'UNITS_PER_WORD', once for each word, you need not
define the macro.

'ASM_OUTPUT_BYTE (STREAM, VALUE)'
A C statement to output to the stdio stream STREAM an assembler
instruction to assemble a single byte containing the number VALUE.

'ASM_BYTE_OP'
A C string constant giving the pseudo-op to use for a sequence of
single-byte constants. If this macro is not defined, the default
is '"byte"'.

'ASM_OUTPUT_ASCII (STREAM, PTR, LEN)'
A C statement to output to the stdio stream STREAM an assembler
instruction to assemble a string constant containing the LEN
bytes at PTR. PTR will be a C expression of type 'char *' and
LEN a C expression of type 'int'.

If the assembler has a '.ascii' pseudo-op as found in the
Berkeley Unix assembler, do not define the macro
'ASM_OUTPUT_ASCII'.

'ASM_OUTPUT_POOL_PROLOGUE (FILE FUNNAME FUNDECL SIZE)'
A C statement to output assembler commands to define the start of
the constant pool for a function. FUNNAME is a string giving the
name of the function. Should the return type of the function be
required, it can be obtained via FUNDECL. SIZE is the size, in
bytes, of the constant pool that will be written immediately
after this call.

If no constant-pool prefix is required, the usual case, this
macro need not be defined.

'ASM_OUTPUT_SPECIAL_POOL_ENTRY (FILE, X, MODE, ALIGN, LABELNO, JUMPTO)'
A C statement (with or without semicolon) to output a constant in
the constant pool, if it needs special treatment. (This macro
need not do anything for RTL expressions that can be output
normally.)

The argument FILE is the standard I/O stream to output the
assembler code on. X is the RTL expression for the constant to
output, and MODE is the machine mode (in case X is a
'const_int'). ALIGN is the required alignment for the value X;
you should output an assembler directive to force this much
alignment.

The argument LABELNO is a number to use in an internal label for
the address of this pool entry. The definition of this macro is
responsible for outputting the label definition at the proper
place. Here is how to do this:

ASM_OUTPUT_INTERNAL_LABEL (FILE, "LC", LABELNO);

When you output a pool entry specially, you should end with a
'goto' to the label JUMPTO. This will prevent the same pool
entry from being output a second time in the usual manner.

You need not define this macro if it would do nothing.

'ASM_OPEN_PAREN'
'ASM_CLOSE_PAREN'
These macros are defined as C string constant, describing the
syntax in the assembler for grouping arithmetic expressions. The
following definitions are correct for most assemblers:

#define ASM_OPEN_PAREN "("
#define ASM_CLOSE_PAREN ")"


File: gcc.info, Node: Uninitialized Data, Next: Label Output, Prev: Data Output, Up: Assembler Format

Output of Uninitialized Variables
---------------------------------

Each of the macros in this section is used to do the whole job of
outputting a single uninitialized variable.

'ASM_OUTPUT_COMMON (STREAM, NAME, SIZE, ROUNDED)'
A C statement (sans semicolon) to output to the stdio stream
STREAM the assembler definition of a common-label named NAME
whose size is SIZE bytes. The variable ROUNDED is the size
rounded up to whatever alignment the caller wants.

Use the expression 'assemble_name (STREAM, NAME)' to output the
name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.

This macro controls how the assembler definitions of uninitialized
global variables are output.

'ASM_OUTPUT_ALIGNED_COMMON (STREAM, NAME, SIZE, ALIGNMENT)'
Like 'ASM_OUTPUT_COMMON' except takes the required alignment as a
separate, explicit argument. If you define this macro, it is
used in place of 'ASM_OUTPUT_COMMON', and gives you more
flexibility in handling the required alignment of the variable.

'ASM_OUTPUT_SHARED_COMMON (STREAM, NAME, SIZE, ROUNDED)'
If defined, it is similar to 'ASM_OUTPUT_COMMON', except that it
is used when NAME is shared. If not defined, 'ASM_OUTPUT_COMMON'
will be used.

'ASM_OUTPUT_LOCAL (STREAM, NAME, SIZE, ROUNDED)'
A C statement (sans semicolon) to output to the stdio stream
STREAM the assembler definition of a local-common-label named
NAME whose size is SIZE bytes. The variable ROUNDED is the size
rounded up to whatever alignment the caller wants.

Use the expression 'assemble_name (STREAM, NAME)' to output the
name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.

This macro controls how the assembler definitions of uninitialized
static variables are output.

'ASM_OUTPUT_ALIGNED_LOCAL (STREAM, NAME, SIZE, ALIGNMENT)'
Like 'ASM_OUTPUT_LOCAL' except takes the required alignment as a
separate, explicit argument. If you define this macro, it is
used in place of 'ASM_OUTPUT_LOCAL', and gives you more
flexibility in handling the required alignment of the variable.

'ASM_OUTPUT_SHARED_LOCAL (STREAM, NAME, SIZE, ROUNDED)'
If defined, it is similar to 'ASM_OUTPUT_LOCAL', except that it
is used when NAME is shared. If not defined, 'ASM_OUTPUT_LOCAL'
will be used.


File: gcc.info, Node: Label Output, Next: Constructor Output, Prev: Uninitialized Data, Up: Assembler Format

Output and Generation of Labels
-------------------------------

'ASM_OUTPUT_LABEL (STREAM, NAME)'
A C statement (sans semicolon) to output to the stdio stream
STREAM the assembler definition of a label named NAME. Use the
expression 'assemble_name (STREAM, NAME)' to output the name
itself; before and after that, output the additional assembler
syntax for defining the name, and a newline.

'ASM_DECLARE_FUNCTION_NAME (STREAM, NAME, DECL)'
A C statement (sans semicolon) to output to the stdio stream
STREAM any text necessary for declaring the name NAME of a
function which is being defined. This macro is responsible for
outputting the label definition (perhaps using
'ASM_OUTPUT_LABEL'). The argument DECL is the 'FUNCTION_DECL'
tree node representing the function.

If this macro is not defined, then the function name is defined
in the usual manner as a label (by means of 'ASM_OUTPUT_LABEL').

'ASM_DECLARE_FUNCTION_SIZE (STREAM, NAME, DECL)'
A C statement (sans semicolon) to output to the stdio stream
STREAM any text necessary for declaring the size of a function
which is being defined. The argument NAME is the name of the
function. The argument DECL is the 'FUNCTION_DECL' tree node
representing the function.

If this macro is not defined, then the function size is not
defined.

'ASM_DECLARE_OBJECT_NAME (STREAM, NAME, DECL)'
A C statement (sans semicolon) to output to the stdio stream
STREAM any text necessary for declaring the name NAME of an
initialized variable which is being defined. This macro must
output the label definition (perhaps using 'ASM_OUTPUT_LABEL').
The argument DECL is the 'VAR_DECL' tree node representing the
variable.

If this macro is not defined, then the variable name is defined
in the usual manner as a label (by means of 'ASM_OUTPUT_LABEL').

'ASM_GLOBALIZE_LABEL (STREAM, NAME)'
A C statement (sans semicolon) to output to the stdio stream
STREAM some commands that will make the label NAME global; that
is, available for reference from other files. Use the expression
'assemble_name (STREAM, NAME)' to output the name itself; before
and after that, output the additional assembler syntax for making
that name global, and a newline.

'ASM_OUTPUT_EXTERNAL (STREAM, DECL, NAME)'
A C statement (sans semicolon) to output to the stdio stream
STREAM any text necessary for declaring the name of an external
symbol named NAME which is referenced in this compilation but not
defined. The value of DECL is the tree node for the declaration.

This macro need not be defined if it does not need to output
anything. The GNU assembler and most Unix assemblers don't
require anything.

'ASM_OUTPUT_EXTERNAL_LIBCALL (STREAM, SYMREF)'
A C statement (sans semicolon) to output on STREAM an assembler
pseudo-op to declare a library function name external. The name
of the library function is given by SYMREF, which has type 'rtx'
and is a 'symbol_ref'.

This macro need not be defined if it does not need to output
anything. The GNU assembler and most Unix assemblers don't
require anything.

'ASM_OUTPUT_LABELREF (STREAM, NAME)'
A C statement (sans semicolon) to output to the stdio stream
STREAM a reference in assembler syntax to a label named NAME.
This should add '_' to the front of the name, if that is
customary on your operating system, as it is in most Berkeley Unix
systems. This macro is used in 'assemble_name'.

'ASM_OUTPUT_LABELREF_AS_INT (FILE, LABEL)'
Define this macro for systems that use the program 'collect2'.
The definition should be a C statement to output a word containing
a reference to the label LABEL.

'ASM_OUTPUT_INTERNAL_LABEL (STREAM, PREFIX, NUM)'
A C statement to output to the stdio stream STREAM a label whose
name is made from the string PREFIX and the number NUM.

It is absolutely essential that these labels be distinct from the
labels used for user-level functions and variables. Otherwise,
certain programs will have name conflicts with internal labels.

It is desirable to exclude internal labels from the symbol table
of the object file. Most assemblers have a naming convention for
labels that should be excluded; on many systems, the letter 'L'
at the beginning of a label has this effect. You should find out
what convention your system uses, and follow it.

The usual definition of this macro is as follows:

fprintf (STREAM, "L%s%d:\n", PREFIX, NUM)

'ASM_GENERATE_INTERNAL_LABEL (STRING, PREFIX, NUM)'
A C statement to store into the string STRING a label whose name
is made from the string PREFIX and the number NUM.

This string, when output subsequently by 'assemble_name', should
produce the same output that 'ASM_OUTPUT_INTERNAL_LABEL' would
produce with the same PREFIX and NUM.

If the string begins with '*', then 'assemble_name' will output
the rest of the string unchanged. It is often convenient for
'ASM_GENERATE_INTERNAL_LABEL' to use '*' in this way. If the
string doesn't start with '*', then 'ASM_OUTPUT_LABELREF' gets to
output the string, and may change it. (Of course,
'ASM_OUTPUT_LABELREF' is also part of your machine description, so
you should know what it does on your machine.)

'ASM_FORMAT_PRIVATE_NAME (OUTVAR, NAME, NUMBER)'
A C expression to assign to OUTVAR (which is a variable of type
'char *') a newly allocated string made from the string NAME and
the number NUMBER, with some suitable punctuation added. Use
'alloca' to get space for the string.

This string will be used as the argument to 'ASM_OUTPUT_LABELREF'
to produce an assembler label for an internal static variable
whose name is NAME. Therefore, the string must be such as to
result in valid assembler code. The argument NUMBER is different
each time this macro is executed; it prevents conflicts between
similarly-named internal static variables in different scopes.

Ideally this string should not be a valid C identifier, to
prevent any conflict with the user's own symbols. Most
assemblers allow periods or percent signs in assembler symbols;
putting at least one of these between the name and the number
will suffice.

'OBJC_GEN_METHOD_LABEL (BUF, IS_INST, CLASS_NAME, CAT_NAME, SEL_NAME)'
Define this macro to override the default assembler names used for
Objective C methods.

The default name is a unique method number followed by the name
of the class (e.g. '_1_Foo'). For methods in categories, the
name of the category is also included in the assembler name (e.g.
'_1_Foo_Bar').

These names are safe on most systems, but make debugging
difficult since the method's selector is not present in the name.
Therefore, particular systems define other ways of computing
names.

BUF is an expression of type 'char *' which gives you a buffer in
which to store the name; its length is as long as CLASS_NAME,
CAT_NAME and SEL_NAME put together, plus 50 characters extra.

The argument IS_INST specifies whether the method is an instance
method or a class method; CLASS_NAME is the name of the class;
CAT_NAME is the name of the category (or NULL if the method is not
in a category); and SEL_NAME is the name of the selector.

On systems where the assembler can handle quoted names, you can
use this macro to provide more human-readable names.


File: gcc.info, Node: Constructor Output, Next: Instruction Output, Prev: Label Output, Up: Assembler Format

Output of Initialization Routines
---------------------------------

The compiled code for certain languages includes "constructors"
(also called "initialization routines")--functions to initialize data
in the program when the program is started. These functions need to
be called before the program is "started"--that is to say, before
'main' is called.

Compiling some languages generates "destructors" (also called
"termination routines") that should be called when the program
terminates.

To make the initialization and termination functions work, the
compiler must output something in the assembler code to cause those
functions to be called at the appropriate time. When you port the
compiler to a new system, you need to specify what assembler code is
needed to do this.

Here are the two macros you should define if necessary:

'ASM_OUTPUT_CONSTRUCTOR (STREAM, NAME)'
Define this macro as a C statement to output on the stream STREAM
the assembler code to arrange to call the function named NAME at
initialization time.

Assume that NAME is the name of a C function generated
automatically by the compiler. This function takes no arguments.
Use the function 'assemble_name' to output the name NAME; this
performs any system-specific syntactic transformations such as
adding an underscore.

If you don't define this macro, nothing special is output to
arrange to call the function. This is correct when the function
will be called in some other manner--for example, by means of the
'collect' program, which looks through the symbol table to find
these functions by their names. If you want to use 'collect',
then you need to arrange for it to be built and installed and
used on your system.

'ASM_OUTPUT_DESTRUCTOR (STREAM, NAME)'
This is like 'ASM_OUTPUT_CONSTRUCTOR' but used for termination
functions rather than initialization functions.


File: gcc.info, Node: Instruction Output, Next: Dispatch Tables, Prev: Constructor Output, Up: Assembler Format

Output of Assembler Instructions
--------------------------------

'REGISTER_NAMES'
A C initializer containing the assembler's names for the machine
registers, each one as a C string constant. This is what
translates register numbers in the compiler into assembler
language.

'ADDITIONAL_REGISTER_NAMES'
If defined, a C initializer for an array of structures containing
a name and a register number. This macro defines additional
names for hard registers, thus allowing the 'asm' option in
declarations to refer to registers using alternate names.

'ASM_OUTPUT_OPCODE (STREAM, PTR)'
Define this macro if you are using an unusual assembler that
requires different names for the machine instructions.

The definition is a C statement or statements which output an
assembler instruction opcode to the stdio stream STREAM. The
macro-operand PTR is a variable of type 'char *' which points to
the opcode name in its "internal" form--the form that is written
in the machine description. The definition should output the
opcode name to STREAM, performing any translation you desire, and
increment the variable PTR to point at the end of the opcode so
that it will not be output twice.

In fact, your macro definition may process less than the entire
opcode name, or more than the opcode name; but if you want to
process text that includes '%'-sequences to substitute operands,
you must take care of the substitution yourself. Just be sure to
increment PTR over whatever text should not be output normally.

If you need to look at the operand values, they can be found as
the elements of 'recog_operand'.

If the macro definition does nothing, the instruction is output
in the usual way.

'FINAL_PRESCAN_INSN (INSN, OPVEC, NOPERANDS)'
If defined, a C statement to be executed just prior to the output
of assembler code for INSN, to modify the extracted operands so
they will be output differently.

Here the argument OPVEC is the vector containing the operands
extracted from INSN, and NOPERANDS is the number of elements of
the vector which contain meaningful data for this insn. The
contents of this vector are what will be used to convert the insn
template into assembler code, so you can change the assembler
output by changing the contents of the vector.

This macro is useful when various assembler syntaxes share a
single file of instruction patterns; by defining this macro
differently, you can cause a large class of instructions to be
output differently (such as with rearranged operands).
Naturally, variations in assembler syntax affecting individual
insn patterns ought to be handled by writing conditional output
routines in those patterns.

If this macro is not defined, it is equivalent to a null
statement.

'PRINT_OPERAND (STREAM, X, CODE)'
A C compound statement to output to stdio stream STREAM the
assembler syntax for an instruction operand X. X is an RTL
expression.

CODE is a value that can be used to specify one of several ways
of printing the operand. It is used when identical operands must
be printed differently depending on the context. CODE comes from
the '%' specification that was used to request printing of the
operand. If the specification was just '%DIGIT' then CODE is 0;
if the specification was '%LTR DIGIT' then CODE is the ASCII code
for LTR.

If X is a register, this macro should print the register's name.
The names can be found in an array 'reg_names' whose type is
'char *[]'. 'reg_names' is initialized from 'REGISTER_NAMES'.

When the machine description has a specification '%PUNCT' (a '%'
followed by a punctuation character), this macro is called with a
null pointer for X and the punctuation character for CODE.

'PRINT_OPERAND_PUNCT_VALID_P (CODE)'
A C expression which evaluates to true if CODE is a valid
punctuation character for use in the 'PRINT_OPERAND' macro. If
'PRINT_OPERAND_PUNCT_VALID_P' is not defined, it means that no
punctuation characters (except for the standard one, '%') are used
in this way.

'PRINT_OPERAND_ADDRESS (STREAM, X)'
A C compound statement to output to stdio stream STREAM the
assembler syntax for an instruction operand that is a memory
reference whose address is X. X is an RTL expression.

On some machines, the syntax for a symbolic address depends on the
section that the address refers to. On these machines, define
the macro 'ENCODE_SECTION_INFO' to store the information into the
'symbol_ref', and then check for it here. *Note Assembler
Format::.

'DBR_OUTPUT_SEQEND(FILE)'
A C statement, to be executed after all slot-filler instructions
have been output. If necessary, call 'dbr_sequence_length' to
determine the number of slots filled in a sequence (zero if not
currently outputting a sequence), to decide how many no-ops to
output, or whatever.

Don't define this macro if it has nothing to do, but it is
helpful in reading assembly output if the extent of the delay
sequence is made explicit (e.g. with white space).

Note that output routines for instructions with delay slots must
be prepared to deal with not being output as part of a sequence
(i.e. when the scheduling pass is not run, or when no slot
fillers could be found.) The variable 'final_sequence' is null
when not processing a sequence, otherwise it contains the
'sequence' rtx being output.

'REGISTER_PREFIX'
'LOCAL_LABEL_PREFIX'
'USER_LABEL_PREFIX'
'IMMEDIATE_PREFIX'
If defined, C string expressions to be used for the '%R', '%L',
'%U', and '%I' options of 'asm_fprintf' (see 'final.c'). These
are useful when a single 'md' file must support multiple
assembler formats. In that case, the various 'tm.h' files can
define these macros differently.

'ASM_OUTPUT_REG_PUSH (STREAM, REGNO)'
A C expression to output to STREAM some assembler code which will
push hard register number REGNO onto the stack. The code need
not be optimal, since this macro is used only when profiling.

'ASM_OUTPUT_REG_POP (STREAM, REGNO)'
A C expression to output to STREAM some assembler code which will
pop hard register number REGNO off of the stack. The code need
not be optimal, since this macro is used only when profiling.


File: gcc.info, Node: Dispatch Tables, Next: Alignment Output, Prev: Instruction Output, Up: Assembler Format

Output of Dispatch Tables
-------------------------

'ASM_OUTPUT_ADDR_DIFF_ELT (STREAM, VALUE, REL)'
This macro should be provided on machines where the addresses in
a dispatch table are relative to the table's own address.

The definition should be a C statement to output to the stdio
stream STREAM an assembler pseudo-instruction to generate a
difference between two labels. VALUE and REL are the numbers of
two internal labels. The definitions of these labels are output
using 'ASM_OUTPUT_INTERNAL_LABEL', and they must be printed in
the same way here. For example,

fprintf (STREAM, "\t.word L%d-L%d\n",
VALUE, REL)

'ASM_OUTPUT_ADDR_VEC_ELT (STREAM, VALUE)'
This macro should be provided on machines where the addresses in
a dispatch table are absolute.

The definition should be a C statement to output to the stdio
stream STREAM an assembler pseudo-instruction to generate a
reference to a label. VALUE is the number of an internal label
whose definition is output using 'ASM_OUTPUT_INTERNAL_LABEL'.
For example,

fprintf (STREAM, "\t.word L%d\n", VALUE)

'ASM_OUTPUT_CASE_LABEL (STREAM, PREFIX, NUM, TABLE)'
Define this if the label before a jump-table needs to be output
specially. The first three arguments are the same as for
'ASM_OUTPUT_INTERNAL_LABEL'; the fourth argument is the
jump-table which follows (a 'jump_insn' containing an 'addr_vec'
or 'addr_diff_vec').

This feature is used on system V to output a 'swbeg' statement
for the table.

If this macro is not defined, these labels are output with
'ASM_OUTPUT_INTERNAL_LABEL'.

'ASM_OUTPUT_CASE_END (STREAM, NUM, TABLE)'
Define this if something special must be output at the end of a
jump-table. The definition should be a C statement to be executed
after the assembler code for the table is written. It should
write the appropriate code to stdio stream STREAM. The argument
TABLE is the jump-table insn, and NUM is the label-number of the
preceding label.

If this macro is not defined, nothing special is output at the
end of the jump-table.


File: gcc.info, Node: Alignment Output, Prev: Dispatch Tables, Up: Assembler Format

Assembler Commands for Alignment
--------------------------------

'ASM_OUTPUT_ALIGN_CODE (FILE)'
A C expression to output text to align the location counter in
the way that is desirable at a point in the code that is reached
only by jumping.

This macro need not be defined if you don't want any special
alignment to be done at such a time. Most machine descriptions
do not currently define the macro.

'ASM_OUTPUT_LOOP_ALIGN (FILE)'
A C expression to output text to align the location counter in
the way that is desirable at the beginning of a loop.

This macro need not be defined if you don't want any special
alignment to be done at such a time. Most machine descriptions
do not currently define the macro.

'ASM_OUTPUT_SKIP (STREAM, NBYTES)'
A C statement to output to the stdio stream STREAM an assembler
instruction to advance the location counter by NBYTES bytes.
Those bytes should be zero when loaded. NBYTES will be a C
expression of type 'int'.

'ASM_NO_SKIP_IN_TEXT'
Define this macro if 'ASM_OUTPUT_SKIP' should not be used in the
text section because it fails put zeros in the bytes that are
skipped. This is true on many Unix systems, where the pseudo--op
to skip bytes produces no-op instructions rather than zeros when
used in the text section.

'ASM_OUTPUT_ALIGN (STREAM, POWER)'
A C statement to output to the stdio stream STREAM an assembler
command to advance the location counter to a multiple of 2 to the
POWER bytes. POWER will be a C expression of type 'int'.



Condition Code Status
=====================

The file 'conditions.h' defines a variable 'cc_status' to describe
how the condition code was computed (in case the interpretation of the
condition code depends on the instruction that it was set by). This
variable contains the RTL expressions on which the condition code is
currently based, and several standard flags.

Sometimes additional machine-specific flags must be defined in the
machine description header file. It can also add additional
machine-specific information by defining 'CC_STATUS_MDEP'.

'CC_STATUS_MDEP'
C code for a data type which is used for declaring the 'mdep'
component of 'cc_status'. It defaults to 'int'.

This macro is not used on machines that do not use 'cc0'.

'CC_STATUS_MDEP_INIT'
A C expression to initialize the 'mdep' field to "empty". The
default definition does nothing, since most machines don't use
the field anyway. If you want to use the field, you should
probably define this macro to initialize it.

This macro is not used on machines that do not use 'cc0'.

'NOTICE_UPDATE_CC (EXP, INSN)'
A C compound statement to set the components of 'cc_status'
appropriately for an insn INSN whose body is EXP. It is this
macro's responsibility to recognize insns that set the condition
code as a byproduct of other activity as well as those that
explicitly set '(cc0)'.

This macro is not used on machines that do not use 'cc0'.

If there are insns that do not set the condition code but do alter
other machine registers, this macro must check to see whether they
invalidate the expressions that the condition code is recorded as
reflecting. For example, on the 68000, insns that store in
address registers do not set the condition code, which means that
usually 'NOTICE_UPDATE_CC' can leave 'cc_status' unaltered for
such insns. But suppose that the previous insn set the condition
code based on location '[email protected](102)' and the current insn stores a
new value in 'a4'. Although the condition code is not changed by
this, it will no longer be true that it reflects the contents of
'[email protected](102)'. Therefore, 'NOTICE_UPDATE_CC' must alter 'cc_status'
in this case to say that nothing is known about the condition
code value.

The definition of 'NOTICE_UPDATE_CC' must be prepared to deal
with the results of peephole optimization: insns whose patterns
are 'parallel' RTXs containing various 'reg', 'mem' or constants
which are just the operands. The RTL structure of these insns is
not sufficient to indicate what the insns actually do. What
'NOTICE_UPDATE_CC' should do when it sees one is just to run
'CC_STATUS_INIT'.

A possible definition of 'NOTICE_UPDATE_CC' is to call a function
that looks at an attribute (*note Insn Attributes::.) named, for
example, 'cc'. This avoids having detailed information about
patterns in two places, the 'md' file and in 'NOTICE_UPDATE_CC'.

'EXTRA_CC_MODES'
A list of names to be used for additional modes for condition code
values in registers (*note Jump Patterns::.). These names are
added to 'enum machine_mode' and all have class 'MODE_CC'. By
convention, they should start with 'CC' and end with 'mode'.

You should only define this macro if your machine does not use
'cc0' and only if additional modes are required.

'EXTRA_CC_NAMES'
A list of C strings giving the names for the modes listed in
'EXTRA_CC_MODES'. For example, the Sparc defines this macro and
'EXTRA_CC_MODES' as

#define EXTRA_CC_MODES CC_NOOVmode, CCFPmode
#define EXTRA_CC_NAMES "CC_NOOV", "CCFP"

This macro is not required if 'EXTRA_CC_MODES' is not defined.

'SELECT_CC_MODE (OP, X)'
Returns a mode from class 'MODE_CC' to be used when comparison
operation code OP is applied to rtx X. For example, on the Sparc,
'SELECT_CC_MODE' is defined as (see *note Jump Patterns::. for a
description of the reason for this definition)

#define SELECT_CC_MODE(OP,X) \
(GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT ? CCFPmode \
: (GET_CODE (X) == PLUS || GET_CODE (X) == MINUS \
|| GET_CODE (X) == NEG) \
? CC_NOOVmode : CCmode)

This macro is not required if 'EXTRA_CC_MODES' is not defined.


File: gcc.info, Node: Debugging Info, Next: Cross-compilation, Prev: Assembler Format, Up: Target Macros

Controlling Debugging Information Format
========================================

'DBX_REGISTER_NUMBER (REGNO)'
A C expression that returns the DBX register number for the
compiler register number REGNO. In simple cases, the value of
this expression may be REGNO itself. But sometimes there are some
registers that the compiler knows about and DBX does not, or vice
versa. In such cases, some register may need to have one number
in the compiler and another for DBX.

If two registers have consecutive numbers inside GNU CC, and they
can be used as a pair to hold a multiword value, then they *must*
have consecutive numbers after renumbering with
'DBX_REGISTER_NUMBER'. Otherwise, debuggers will be unable to
access such a pair, because they expect register pairs to be
consecutive in their own numbering scheme.

If you find yourself defining 'DBX_REGISTER_NUMBER' in way that
does not preserve register pairs, then what you must do instead is
redefine the actual register numbering scheme.

'DBX_DEBUGGING_INFO'
Define this macro if GNU CC should produce debugging output for
DBX in response to the '-g' option.

'SDB_DEBUGGING_INFO'
Define this macro if GNU CC should produce COFF-style debugging
output for SDB in response to the '-g' option.

'DWARF_DEBUGGING_INFO'
Define this macro if GNU CC should produce dwarf format debugging
output in response to the '-g' option.

'XCOFF_DEBUGGING_INFO'
Define this macro if GNU CC should produce XCOFF format debugging
output in response to the '-g' option.

'DEFAULT_GDB_EXTENSIONS'
Define this macro to control whether GNU CC should by default
generate GDB's extended version of DBX debugging information
(assuming DBX-format debugging information is enabled at all).
If you don't define the macro, the default is 1: always generate
the extended information.

'DEBUG_SYMS_TEXT'
Define this macro if all '.stabs' commands should be output while
in the text section.

'DEBUGGER_AUTO_OFFSET (X)'
A C expression that returns the integer offset value for an
automatic variable having address X (an RTL expression). The
default computation assumes that X is based on the frame-pointer
and gives the offset from the frame-pointer. This is required
for targets that produce debugging output for DBX or COFF-style
debugging output for SDB and allow the frame-pointer to be
eliminated when the '-g' options is used.

'DEBUGGER_ARG_OFFSET (OFFSET, X)'
A C expression that returns the integer offset value for an
argument having address X (an RTL expression). The nominal
offset is OFFSET.

'ASM_STABS_OP'
A C string constant naming the assembler pseudo op to use instead
of '.stabs' to define an ordinary debugging symbol. If you don't
define this macro, '.stabs' is used. This macro applies only to
DBX debugging information format.

'ASM_STABD_OP'
A C string constant naming the assembler pseudo op to use instead
of '.stabd' to define a debugging symbol whose value is the
current location. If you don't define this macro, '.stabd' is
used. This macro applies only to DBX debugging information
format.

'ASM_STABN_OP'
A C string constant naming the assembler pseudo op to use instead
of '.stabn' to define a debugging symbol with no name. If you
don't define this macro, '.stabn' is used. This macro applies
only to DBX debugging information format.

'PUT_SDB_...'
Define these macros to override the assembler syntax for the
special SDB assembler directives. See 'sdbout.c' for a list of
these macros and their arguments. If the standard syntax is
used, you need not define them yourself.

'SDB_DELIM'
Some assemblers do not support a semicolon as a delimiter, even
between SDB assembler directives. In that case, define this
macro to be the delimiter to use (usually '\n'). It is not
necessary to define a new set of 'PUT_SDB_OP' macros if this is
the only change required.

'SDB_GENERATE_FAKE'
Define this macro to override the usual method of constructing a
dummy name for anonymous structure and union types. See
'sdbout.c' for more information.

'SDB_ALLOW_UNKNOWN_REFERENCES'
Define this macro to allow references to unknown structure,
union, or enumeration tags to be emitted. Standard COFF does not
allow handling of unknown references, MIPS ECOFF has support for
it.

'SDB_ALLOW_FORWARD_REFERENCES'
Define this macro to allow references to structure, union, or
enumeration tags that have not yet been seen to be handled. Some
assemblers choke if forward tags are used, while some require it.

'DBX_NO_XREFS'
Define this macro if DBX on your system does not support the
construct 'xsTAGNAME'. On some systems, this construct is used to
describe a forward reference to a structure named TAGNAME. On
other systems, this construct is not supported at all.

'DBX_CONTIN_LENGTH'
A symbol name in DBX-format debugging information is normally
continued (split into two separate '.stabs' directives) when it
exceeds a certain length (by default, 80 characters). On some
operating systems, DBX requires this splitting; on others,
splitting must not be done. You can inhibit splitting by
defining this macro with the value zero. You can override the
default splitting-length by defining this macro as an expression
for the length you desire.

'DBX_CONTIN_CHAR'
Normally continuation is indicated by adding a '\' character to
the end of a '.stabs' string when a continuation follows. To use
a different character instead, define this macro as a character
constant for the character you want to use. Do not define this
macro if backslash is correct for your system.

'DBX_WORKING_DIRECTORY'
Define this if DBX wants to have the current directory recorded
in each object file.

Note that the working directory is always recorded if GDB
extensions are enabled.

'DBX_STATIC_STAB_DATA_SECTION'
Define this macro if it is necessary to go to the data section
before outputting the '.stabs' pseudo-op for a non-global static
variable.

'DBX_LBRAC_FIRST'
Define this macro if the 'N_LBRAC' symbol for a block should
precede the debugging information for variables and functions
defined in that block. Normally, in DBX format, the 'N_LBRAC'
symbol comes first.

'DBX_FUNCTION_FIRST'
Define this macro if the DBX information for a function and its
arguments should precede the assembler code for the function.
Normally, in DBX format, the debugging information entirely
follows the assembler code.

'DBX_OUTPUT_FUNCTION_END (STREAM, FUNCTION)'
Define this macro if the target machine requires special output
at the end of the debugging information for a function. The
definition should be a C statement (sans semicolon) to output the
appropriate information to STREAM. FUNCTION is the
'FUNCTION_DECL' node for the function.

'DBX_OUTPUT_STANDARD_TYPES (SYMS)'
Define this macro if you need to control the order of output of
the standard data types at the beginning of compilation. The
argument SYMS is a 'tree' which is a chain of all the predefined
global symbols, including names of data types.

Normally, DBX output starts with definitions of the types for
integers and characters, followed by all the other predefined
types of the particular language in no particular order.

On some machines, it is necessary to output different particular
types first. To do this, define 'DBX_OUTPUT_STANDARD_TYPES' to
output those symbols in the necessary order. Any predefined
types that you don't explicitly output will be output afterward
in no particular order.

Be careful not to define this macro so that it works only for C.
There are no global variables to access most of the built-in
types, because another language may have another set of types.
The way to output a particular type is to look through SYMS to
see if you can find it. Here is an example:

{
tree decl;
for (decl = syms; decl; decl = TREE_CHAIN (decl))
if (!strcmp (IDENTIFIER_POINTER (DECL_NAME (decl)), "long int"))
dbxout_symbol (decl);
...
}

This does nothing if the expected type does not exist.

See the function 'init_decl_processing' in source file 'c-decl.c'
to find the names to use for all the built-in C types.

'DBX_OUTPUT_MAIN_SOURCE_FILENAME (STREAM, NAME)'
A C statement to output DBX debugging information to the stdio
stream STREAM which indicates that file NAME is the main source
file--the file specified as the input file for compilation. This
macro is called only once, at the beginning of compilation.

This macro need not be defined if the standard form of output for
DBX debugging information is appropriate.

'DBX_OUTPUT_MAIN_SOURCE_DIRECTORY (STREAM, NAME)'
A C statement to output DBX debugging information to the stdio
stream STREAM which indicates that the current directory during
compilation is named NAME.

This macro need not be defined if the standard form of output for
DBX debugging information is appropriate.

'DBX_OUTPUT_MAIN_SOURCE_FILE_END (STREAM, NAME)'
A C statement to output DBX debugging information at the end of
compilation of the main source file NAME.

If you don't define this macro, nothing special is output at the
end of compilation, which is correct for most machines.

'DBX_OUTPUT_SOURCE_FILENAME (STREAM, NAME)'
A C statement to output DBX debugging information to the stdio
stream STREAM which indicates that file NAME is the current source
file. This output is generated each time input shifts to a
different source file as a result of '#include', the end of an
included file, or a '#line' command.

This macro need not be defined if the standard form of output for
DBX debugging information is appropriate.


File: gcc.info, Node: Cross-compilation, Next: Misc, Prev: Debugging Info, Up: Target Macros

Cross Compilation and Floating Point Format
===========================================

While all modern machines use 2's complement representation for
integers, there are a variety of representations for floating point
numbers. This means that in a cross-compiler the representation of
floating point numbers in the compiled program may be different from
that used in the machine doing the compilation.

Because different representation systems may offer different
amounts of range and precision, the cross compiler cannot safely use
the host machine's floating point arithmetic. Therefore, floating
point constants must be represented in the target machine's format.
This means that the cross compiler cannot use 'atof' to parse a
floating point constant; it must have its own special routine to use
instead. Also, constant folding must emulate the target machine's
arithmetic (or must not be done at all).

The macros in the following table should be defined only if you are
cross compiling between different floating point formats.

Otherwise, don't define them. Then default definitions will be set
up which use 'double' as the data type, '==' to test for equality, etc.

You don't need to worry about how many times you use an operand of
any of these macros. The compiler never uses operands which have side
effects.

'REAL_VALUE_TYPE'
A macro for the C data type to be used to hold a floating point
value in the target machine's format. Typically this would be a
'struct' containing an array of 'int'.

'REAL_VALUES_EQUAL (X, Y)'
A macro for a C expression which compares for equality the two
values, X and Y, both of type 'REAL_VALUE_TYPE'.

'REAL_VALUES_LESS (X, Y)'
A macro for a C expression which tests whether X is less than Y,
both values being of type 'REAL_VALUE_TYPE' and interpreted as
floating point numbers in the target machine's representation.

'REAL_VALUE_LDEXP (X, SCALE)'
A macro for a C expression which performs the standard library
function 'ldexp', but using the target machine's floating point
representation. Both X and the value of the expression have type
'REAL_VALUE_TYPE'. The second argument, SCALE, is an integer.

'REAL_VALUE_FIX (X)'
A macro whose definition is a C expression to convert the
target-machine floating point value X to a signed integer. X has
type 'REAL_VALUE_TYPE'.

'REAL_VALUE_UNSIGNED_FIX (X)'
A macro whose definition is a C expression to convert the
target-machine floating point value X to an unsigned integer. X
has type 'REAL_VALUE_TYPE'.

'REAL_VALUE_FIX_TRUNCATE (X)'
A macro whose definition is a C expression to convert the
target-machine floating point value X to a signed integer,
rounding toward 0. X has type 'REAL_VALUE_TYPE'.

'REAL_VALUE_UNSIGNED_FIX_TRUNCATE (X)'
A macro whose definition is a C expression to convert the
target-machine floating point value X to an unsigned integer,
rounding toward 0. X has type 'REAL_VALUE_TYPE'.

'REAL_VALUE_ATOF (STRING)'
A macro for a C expression which converts STRING, an expression
of type 'char *', into a floating point number in the target
machine's representation. The value has type 'REAL_VALUE_TYPE'.

'REAL_INFINITY'
Define this macro if infinity is a possible floating point value,
and therefore division by 0 is legitimate.

'REAL_VALUE_ISINF (X)'
A macro for a C expression which determines whether X, a floating
point value, is infinity. The value has type 'int'. By default,
this is defined to call 'isinf'.

'REAL_VALUE_ISNAN (X)'
A macro for a C expression which determines whether X, a floating
point value, is a "nan" (not-a-number). The value has type
'int'. By default, this is defined to call 'isnan'.

Define the following additional macros if you want to make floating
point constant folding work while cross compiling. If you don't
define them, cross compilation is still possible, but constant folding
will not happen for floating point values.

'REAL_ARITHMETIC (OUTPUT, CODE, X, Y)'
A macro for a C statement which calculates an arithmetic
operation of the two floating point values X and Y, both of type
'REAL_VALUE_TYPE' in the target machine's representation, to
produce a result of the same type and representation which is
stored in OUTPUT (which will be a variable).

The operation to be performed is specified by CODE, a tree code
which will always be one of the following: 'PLUS_EXPR',
'MINUS_EXPR', 'MULT_EXPR', 'RDIV_EXPR', 'MAX_EXPR', 'MIN_EXPR'.

The expansion of this macro is responsible for checking for
overflow. If overflow happens, the macro expansion should
execute the statement 'return 0;', which indicates the inability
to perform the arithmetic operation requested.

'REAL_VALUE_NEGATE (X)'
A macro for a C expression which returns the negative of the
floating point value X. Both X and the value of the expression
have type 'REAL_VALUE_TYPE' and are in the target machine's
floating point representation.

There is no way for this macro to report overflow, since overflow
can't happen in the negation operation.

'REAL_VALUE_TRUNCATE (X)'
A macro for a C expression which converts the double-precision
floating point value X to single-precision.

Both X and the value of the expression have type
'REAL_VALUE_TYPE' and are in the target machine's floating point
representation. However, the value should have an appropriate bit
pattern to be output properly as a single-precision floating
constant.

There is no way for this macro to report overflow.

'REAL_VALUE_TO_INT (LOW, HIGH, X)'
A macro for a C expression which converts a floating point value
X into a double-precision integer which is then stored into LOW
and HIGH, two variables of type INT.

'REAL_VALUE_FROM_INT (X, LOW, HIGH)'
A macro for a C expression which converts a double-precision
integer found in LOW and HIGH, two variables of type INT, into a
floating point value which is then stored into X.


File: gcc.info, Node: Misc, Prev: Cross-compilation, Up: Target Macros

Miscellaneous Parameters
========================

'PREDICATE_CODES'
Optionally define this if you have added predicates to
'MACHINE.c'. This macro is called within an initializer of an
array of structures. The first field in the structure is the
name of a predicate and the second field is an array of rtl
codes. For each predicate, list all rtl codes that can be in
expressions matched by the predicate. The list should have a
trailing comma. Here is an example of two entries in the list
for a typical RISC machine:

#define PREDICATE_CODES \
{"gen_reg_rtx_operand", {SUBREG, REG}}, \
{"reg_or_short_cint_operand", {SUBREG, REG, CONST_INT}},

Defining this macro does not affect the generated code (however,
incorrect definitions that omit an rtl code that may be matched
by the predicate can cause the compiler to malfunction).
Instead, it allows the table built by 'genrecog' to be more
compact and efficient, thus speeding up the compiler. The most
important predicates to include in the list specified by this
macro are thoses used in the most insn patterns.

'CASE_VECTOR_MODE'
An alias for a machine mode name. This is the machine mode that
elements of a jump-table should have.

'CASE_VECTOR_PC_RELATIVE'
Define this macro if jump-tables should contain relative
addresses.

'CASE_DROPS_THROUGH'
Define this if control falls through a 'case' insn when the index
value is out of range. This means the specified default-label is
actually ignored by the 'case' insn proper.

'BYTE_LOADS_ZERO_EXTEND'
Define this macro if an instruction to load a value narrower than
a word from memory into a register also zero-extends the value to
the whole register.

'IMPLICIT_FIX_EXPR'
An alias for a tree code that should be used by default for
conversion of floating point values to fixed point. Normally,
'FIX_ROUND_EXPR' is used.

'FIXUNS_TRUNC_LIKE_FIX_TRUNC'
Define this macro if the same instructions that convert a floating
point number to a signed fixed point number also convert validly
to an unsigned one.

'EASY_DIV_EXPR'
An alias for a tree code that is the easiest kind of division to
compile code for in the general case. It may be
'TRUNC_DIV_EXPR', 'FLOOR_DIV_EXPR', 'CEIL_DIV_EXPR' or
'ROUND_DIV_EXPR'. These four division operators differ in how
they round the result to an integer. 'EASY_DIV_EXPR' is used
when it is permissible to use any of those kinds of division and
the choice should be made on the basis of efficiency.

'MOVE_MAX'
The maximum number of bytes that a single instruction can move
quickly from memory to memory.

'SHIFT_COUNT_TRUNCATED'
Defining this macro causes the compiler to omit a sign-extend,
zero-extend, or bitwise 'and' instruction that truncates the
count of a shift operation to a width equal to the number of bits
needed to represent the size of the object being shifted. On
machines that have instructions that act on bitfields at variable
positions, including 'bit test' instructions, defining
'SHIFT_COUNT_TRUNCATED' also causes truncation not to be applied
to these instructions.

If both types of instructions truncate the count (for shifts) and
position (for bitfield operations), or if no variable-position
bitfield instructions exist, you should define this macro.

However, on some machines, such as the 80386 and the 680x0,
truncation only applies to shift operations and not the (real or
pretended) bitfield operations. Do not define
'SHIFT_COUNT_TRUNCATED' on such machines. Instead, add patterns
to the 'md' file that include the implied truncation of the shift
instructions.

'TRULY_NOOP_TRUNCATION (OUTPREC, INPREC)'
A C expression which is nonzero if on this machine it is safe to
"convert" an integer of INPREC bits to one of OUTPREC bits (where
OUTPREC is smaller than INPREC) by merely operating on it as if
it had only OUTPREC bits.

On many machines, this expression can be 1.

It is reported that suboptimal code can result when
'TRULY_NOOP_TRUNCATION' returns 1 for a pair of sizes for modes
for which 'MODES_TIEABLE_P' is 0. If this is the case, making
'TRULY_NOOP_TRUNCATION' return 0 in such cases may improve things.

'STORE_FLAG_VALUE'
A C expression describing the value returned by a comparison
operator and stored by a store-flag instruction ('sCOND') when the
condition is true. This description must apply to *all* the
'sCOND' patterns and all the comparison operators.

A value of 1 or -1 means that the instruction implementing the
comparison operator returns exactly 1 or -1 when the comparison
is true and 0 when the comparison is false. Otherwise, the value
indicates which bits of the result are guaranteed to be 1 when
the comparison is true. This value is interpreted in the mode of
the comparison operation, which is given by the mode of the first
operand in the 'sCOND' pattern. Either the low bit or the sign
bit of 'STORE_FLAG_VALUE' be on. Presently, only those bits are
used by the compiler.

If 'STORE_FLAG_VALUE' is neither 1 or -1, the compiler will
generate code that depends only on the specified bits. It can
also replace comparison operators with equivalent operations if
they cause the required bits to be set, even if the remaining
bits are undefined. For example, on a machine whose comparison
operators return an 'SImode' value and where 'STORE_FLAG_VALUE'
is defined as '0x80000000', saying that just the sign bit is
relevant, the expression

(ne:SI (and:SI X (const_int POWER-OF-2)) (const_int 0))

can be converted to

(ashift:SI X (const_int N))

where N is the appropriate shift count to move the bit being
tested into the sign bit.

There is no way to describe a machine that always sets the
low-order bit for a true value, but does not guarantee the value
of any other bits, but we do not know of any machine that has
such an instruction. If you are trying to port GNU CC to such a
machine, include an instruction to perform a logical-and of the
result with 1 in the pattern for the comparison operators and let
us know (*note Bug Reporting::.).

Often, a machine will have multiple instructions that obtain a
value from a comparison (or the condition codes). Here are rules
to guide the choice of value for 'STORE_FLAG_VALUE', and hence
the instructions to be used:

* Use the shortest sequence that yields a valid definition for
'STORE_FLAG_VALUE'. It is more efficent for the compiler to
"normalize" the value (convert it to, e.g., 1 or 0) than for
the comparison operators to do so because there may be
opportunities to combine the normalization with other
operations.

* For equal-length sequences, use a value of 1 or -1, with -1
being slightly preferred on machines with expensive jumps
and 1 preferred on other machines.

* As a second choice, choose a value of '0x80000001' if
instructions exist that set both the sign and low-order bits
but do not define the others.

* Otherwise, use a value of '0x80000000'.

You need not define 'STORE_FLAG_VALUE' if the machine has no
store-flag instructions.

'Pmode'
An alias for the machine mode for pointers. Normally the
definition can be

#define Pmode SImode

'FUNCTION_MODE'
An alias for the machine mode used for memory references to
functions being called, in 'call' RTL expressions. On most
machines this should be 'QImode'.

'INTEGRATE_THRESHOLD (DECL)'
A C expression for the maximum number of instructions above which
the function DECL should not be inlined. DECL is a
'FUNCTION_DECL' node.

The default definition of this macro is 64 plus 8 times the
number of arguments that the function accepts. Some people think
a larger threshold should be used on RISC machines.

'SCCS_DIRECTIVE'
Define this if the preprocessor should ignore '#sccs' directives
and print no error message.

'HANDLE_PRAGMA (STREAM)'
Define this macro if you want to implement any pragmas. If
defined, it should be a C statement to be executed when '#pragma'
is seen. The argument STREAM is the stdio input stream from
which the source text can be read.

It is generally a bad idea to implement new uses of '#pragma'.
The only reason to define this macro is for compatibility with
other compilers that do support '#pragma' for the sake of any user
programs which already use it.

'DOLLARS_IN_IDENTIFIERS'
Define this macro to control use of the character '$' in
identifier names. The value should be 0, 1, or 2. 0 means '$'
is not allowed by default; 1 means it is allowed by default if
'-traditional' is used; 2 means it is allowed by default provided
'-ansi' is not used. 1 is the default; there is no need to
define this macro in that case.

'NO_DOLLAR_IN_LABEL'
Define this macro if the assembler does not accept the character
'$' in label names. By default constructors and destructors in
G++ have '$' in the identifiers. If this macro is defined, '.'
is used instead.

'DEFAULT_MAIN_RETURN'
Define this macro if the target system expects every program's
'main' function to return a standard "success" value by default
(if no other value is explicitly returned).

The definition should be a C statement (sans semicolon) to
generate the appropriate rtl instructions. It is used only when
compiling the end of 'main'.

'HAVE_ATEXIT'
Define this if the target system supports the function 'atexit'
from the ANSI C standard. If this is not defined, and
'INIT_SECTION_ASM_OP' is not defined, a default 'exit' function
will be provided to support C++.

'EXIT_BODY'
Define this if your 'exit' function needs to do something besides
calling an external function '_cleanup' before terminating with
'_exit'. The 'EXIT_BODY' macro is only needed if netiher
'HAVE_ATEXIT' nor 'INIT_SECTION_ASM_OP' are defined.


File: gcc.info, Node: Config, Next: Index, Prev: Target Macros, Up: Top

The Configuration File
**********************

The configuration file 'xm-MACHINE.h' contains macro definitions
that describe the machine and system on which the compiler is running,
unlike the definitions in 'MACHINE.h', which describe the machine for
which the compiler is producing output. Most of the values in
'xm-MACHINE.h' are actually the same on all machines that GNU CC runs
on, so large parts of all configuration files are identical. But
there are some macros that vary:

'USG'
Define this macro if the host system is System V.

'VMS'
Define this macro if the host system is VMS.

'FAILURE_EXIT_CODE'
A C expression for the status code to be returned when the
compiler exits after serious errors.

'SUCCESS_EXIT_CODE'
A C expression for the status code to be returned when the
compiler exits without serious errors.

'HOST_WORDS_BIG_ENDIAN'
Defined if the host machine stores words of multi-word values in
big-endian order. (GNU CC does not depend on the host byte
ordering within a word.)

'HOST_FLOAT_FORMAT'
A numeric code distinguishing the floating point format for the
host machine. See 'TARGET_FLOAT_FORMAT' in *Note Storage
Layout:: for the alternatives and default.

'HOST_BITS_PER_CHAR'
A C expression for the number of bits in 'char' on the host
machine.

'HOST_BITS_PER_SHORT'
A C expression for the number of bits in 'short' on the host
machine.

'HOST_BITS_PER_INT'
A C expression for the number of bits in 'int' on the host
machine.

'HOST_BITS_PER_LONG'
A C expression for the number of bits in 'long' on the host
machine.

'ONLY_INT_FIELDS'
Define this macro to indicate that the host compiler only supports
'int' bit fields, rather than other integral types, including
'enum', as do most C compilers.

'EXECUTABLE_SUFFIX'
Define this macro if the host system uses a naming convention for
executable files that involves a common suffix (such as, in some
systems, '.exe') that must be mentioned explicitly when you run
the program.

'OBSTACK_CHUNK_SIZE'
A C expression for the size of ordinary obstack chunks. If you
don't define this, a usually-reasonable default is used.

'OBSTACK_CHUNK_ALLOC'
The function used to allocate obstack chunks. If you don't
define this, 'xmalloc' is used.

'OBSTACK_CHUNK_FREE'
The function used to free obstack chunks. If you don't define
this, 'free' is used.

'USE_C_ALLOCA'
Define this macro to indicate that the compiler is running with
the 'alloca' implemented in C. This version of 'alloca' can be
found in the file 'alloca.c'; to use it, you must also alter the
'Makefile' variable 'ALLOCA'. (This is done automatically for
the systems on which we know it is needed.)

If you do define this macro, you should probably do it as follows:

#ifndef __GNUC__
#define USE_C_ALLOCA
#else
#define alloca __builtin_alloca
#endif

so that when the compiler is compiled with GNU CC it uses the more
efficient built-in 'alloca' function.

'FUNCTION_CONVERSION_BUG'
Define this macro to indicate that the host compiler does not
properly handle converting a function value to a
pointer-to-function when it is used in an expression.

'HAVE_VPRINTF'
Define this if the library function 'vprintf' is available on your
system.

'HAVE_PUTENV'
Define this if the library function 'putenv' is available on your
system.

'NO_SYS_SIGLIST'
Define this if your system *does not* provide the variable
'sys_siglist'.

Some systems do provide this variable, but with a different name
such as '_sys_siglist'. On these systems, you can define
'sys_siglist' as a macro which expands into the name actually
provided.

'NO_STAB_H'
Define this if your system does not have the include file
'stab.h'. If 'USG' is defined, 'NO_STAB_H' is assumed.

In addition, configuration files for system V define 'bcopy',
'bzero' and 'bcmp' as aliases. Some files define 'alloca' as a macro
when compiled with GNU CC, in order to take advantage of the benefit
of GNU CC's built-in 'alloca'.


File: gcc.info, Node: Index, Prev: Config, Up: Top

Index
*****

* Menu:

* #pragma: Misc.
* $: Dollar Signs.
* ': Incompatibilities.
* (nil): RTL Objects.
* 3b1 installation: 3b1 Install.
* ?: side effect: Conditionals.
* #pragma, reason for not using: Function Attributes.
* * in template: Output Statement.
* ?: extensions: Conditionals.
* ?: extensions: Lvalues.
* ACCUMULATE_OUTGOING_ARGS and stack frames: Function Entry.
* ARG_POINTER_REGNUM and virtual registers: Regs and Memory.
* BITS_BIG_ENDIAN, effect on sign_extract: Bit Fields.
* BLKmode, and function return values: Calls.
* ENCODE_SECTION_INFO and address validation: Addressing Modes.
* ENCODE_SECTION_INFO usage: Instruction Output.
* FIRST_PARM_OFFSET and virtual registers: Regs and Memory.
* FRAME_GROWS_DOWNWARD and virtual registers: Regs and Memory.
* FRAME_POINTER_REGNUM and virtual registers: Regs and Memory.
* FUNCTION_EPILOGUE and trampolines: Trampolines.
* FUNCTION_PROLOGUE and trampolines: Trampolines.
* HImode, in insn: Insns.
* MUST_PASS_IN_STACK, and FUNCTION_ARG: Register Arguments.
* PUSH_ROUNDING, interaction with STACK_BOUNDARY: Storage Layout.
* QImode, in insn: Insns.
* REG_PARM_STACK_SPACE, and FUNCTION_ARG: Register Arguments.
* STACK_DYNAMIC_OFFSET and virtual registers: Regs and Memory.
* STACK_POINTER_OFFSET and virtual registers: Regs and Memory.
* STACK_POINTER_REGNUM and virtual registers: Regs and Memory.
* STARTING_FRAME_OFFSET and virtual registers: Regs and Memory.
* SYMBOL_REF_FLAG, in ENCODE_SECTION_INFO: Sections.
* WORDS_BIG_ENDIAN, effect on subreg: Regs and Memory.
* absM2 instruction pattern: Standard Names.
* abs and attributes: Expressions.
* addM3 instruction pattern: Standard Names.
* addr_diff_vec, length of: Insn Lengths.
* addr_vec, length of: Insn Lengths.
* alloca and SunOs: Installation.
* alloca vs variable-length arrays: Variable Length.
* alloca, for SunOs: Sun Install.
* alloca, for Unos: Unos Install.
* andM3 instruction pattern: Standard Names.
* and and attributes: Expressions.
* and, canonicalization of: Insn Canonicalizations.
* ashiftrt and attributes: Expressions.
* ashift and attributes: Expressions.
* ashlM3 instruction pattern: Standard Names.
* ashrM3 instruction pattern: Standard Names.
* asm_operands, RTL sharing: Sharing.
* asm_operands, usage: Assembler.
* asm expressions: Extended Asm.
* bCOND instruction pattern: Standard Names.
* bcopy, implicit usage: Library Calls.
* bzero, implicit usage: Library Calls.
* call_insn and /u: Flags.
* call_pop instruction pattern: Standard Names.
* call_value_pop instruction pattern: Standard Names.
* call_value instruction pattern: Standard Names.
* call instruction pattern: Standard Names.
* call usage: Calls.
* casesi instruction pattern: Standard Names.
* cc0, RTL sharing: Sharing.
* cmpM instruction pattern: Standard Names.
* cmpstrM instruction pattern: Standard Names.
* code_label and /i: Flags.
* compare, canonicalization of: Insn Canonicalizations.
* cond and attributes: Expressions.
* const_double, RTL sharing: Sharing.
* const_int and attribute tests: Expressions.
* const_int and attributes: Expressions.
* const_int, RTL sharing: Sharing.
* const_string and attributes: Expressions.
* const applied to function: Function Attributes.
* define_insn example: Example.
* divM3 instruction pattern: Standard Names.
* divmodM4 instruction pattern: Standard Names.
* div and attributes: Expressions.
* eq and attributes: Expressions.
* extendMN instruction pattern: Standard Names.
* extv instruction pattern: Standard Names.
* extzv instruction pattern: Standard Names.
* ffsM2 instruction pattern: Standard Names.
* fixMN2 instruction pattern: Standard Names.
* fix_truncMN2 instruction pattern: Standard Names.
* fixunsMN2 instruction pattern: Standard Names.
* fixuns_truncMN2 instruction pattern: Standard Names.
* floatMN2 instruction pattern: Standard Names.
* floatunsMN2 instruction pattern: Standard Names.
* float as function value type: Incompatibilities.
* fscanf, and constant strings: Incompatibilities.
* ftruncM2 instruction pattern: Standard Names.
* genflags, crash on Sun 4: Trouble.
* geu and attributes: Expressions.
* ge and attributes: Expressions.
* gprof: Debugging Options.
* gtu and attributes: Expressions.
* gt and attributes: Expressions.
* if_then_else and attributes: Expressions.
* if_then_else usage: Side Effects.
* in_struct, in code_label: Flags.
* in_struct, in insn: Flags.
* in_struct, in insn: Flags.
* in_struct, in label_ref: Flags.
* in_struct, in mem: Flags.
* in_struct, in reg: Flags.
* indirect_jump instruction pattern: Standard Names.
* insn and /i: Flags.
* insn and /s: Flags.
* insn and /u: Flags.
* insv instruction pattern: Standard Names.
* integrated, in insn: Flags.
* integrated, in reg: Flags.
* iorM3 instruction pattern: Standard Names.
* ior and attributes: Expressions.
* ior, canonicalization of: Insn Canonicalizations.
* label_ref and /s: Flags.
* label_ref, RTL sharing: Sharing.
* leu and attributes: Expressions.
* le and attributes: Expressions.
* long long data types: Long Long.
* longjmp and automatic variables: Dialect Options.
* longjmp and automatic variables: Interface.
* longjmp incompatibilities: Incompatibilities.
* longjmp warnings: Warning Options.
* lshiftrt and attributes: Expressions.
* lshift and attributes: Expressions.
* lshlM3 instruction pattern: Standard Names.
* lshrM3 instruction pattern: Standard Names.
* lt and attributes: Expressions.
* main and the exit status: VMS Misc.
* match_dup and attributes: Insn Lengths.
* match_operand and attributes: Expressions.
* maxM3 instruction pattern: Standard Names.
* memcpy, implicit usage: Library Calls.
* memset, implicit usage: Library Calls.
* mem and /s: Flags.
* mem and /u: Flags.
* mem and /v: Flags.
* mem, RTL sharing: Sharing.
* minM3 instruction pattern: Standard Names.
* minus and attributes: Expressions.
* minus, canonicalization of: Insn Canonicalizations.
* mktemp, and constant strings: Incompatibilities.
* modM3 instruction pattern: Standard Names.
* mod and attributes: Expressions.
* movM instruction pattern: Standard Names.
* movstrM instruction pattern: Standard Names.
* movstrictM instruction pattern: Standard Names.
* mulM3 instruction pattern: Standard Names.
* mulhisi3 instruction pattern: Standard Names.
* mulqihi3 instruction pattern: Standard Names.
* mulsidi3 instruction pattern: Standard Names.
* mult and attributes: Expressions.
* mult, canonicalization of: Insn Canonicalizations.
* negM2 instruction pattern: Standard Names.
* neg and attributes: Expressions.
* neg, canonicalization of: Insn Canonicalizations.
* ne and attributes: Expressions.
* nop instruction pattern: Standard Names.
* not and attributes: Expressions.
* not, canonicalization of: Insn Canonicalizations.
* one_cmplM2 instruction pattern: Standard Names.
* pc and attributes: Insn Lengths.
* pc, RTL sharing: Sharing.
* plus and attributes: Expressions.
* plus, canonicalization of: Insn Canonicalizations.
* prof: Debugging Options.
* qsort, and global register variables: Global Reg Vars.
* reg and /i: Flags.
* reg and /s: Flags.
* reg and /u: Flags.
* reg and /v: Flags.
* reg, RTL sharing: Sharing.
* return instruction pattern: Standard Names.
* rotlM3 instruction pattern: Standard Names.
* rotrM3 instruction pattern: Standard Names.
* sCOND instruction pattern: Standard Names.
* scanf, and constant strings: Incompatibilities.
* scratch, RTL sharing: Sharing.
* setjmp incompatibilities: Incompatibilities.
* sign_extract, canonicalization of: Insn Canonicalizations.
* sqrtM2 instruction pattern: Standard Names.
* sscanf, and constant strings: Incompatibilities.
* subM3 instruction pattern: Standard Names.
* subreg, in strict_low_part: RTL Declarations.
* subreg, special reload handling: Regs and Memory.
* symbol_ref and /u: Flags.
* symbol_ref and /v: Flags.
* symbol_ref, RTL sharing: Sharing.
* tablejump instruction pattern: Standard Names.
* tcov: Debugging Options.
* truncMN instruction pattern: Standard Names.
* tstM instruction pattern: Standard Names.
* udivM3 instruction pattern: Standard Names.
* udivmodM4 instruction pattern: Standard Names.
* umaxM3 instruction pattern: Standard Names.
* uminM3 instruction pattern: Standard Names.
* umodM3 instruction pattern: Standard Names.
* umulhisi3 instruction pattern: Standard Names.
* umulqihi3 instruction pattern: Standard Names.
* umulsidi3 instruction pattern: Standard Names.
* unchanging, in call_insn: Flags.
* unchanging, in insn: Flags.
* unchanging, in reg and mem: Flags.
* unchanging, in symbol_ref: Flags.
* used, in symbol_ref: Flags.
* volatile applied to function: Function Attributes.
* volatil, in insn: Flags.
* volatil, in mem: Flags.
* volatil, in reg: Flags.
* volatil, in symbol_ref: Flags.
* xorM3 instruction pattern: Standard Names.
* xor, canonicalization of: Insn Canonicalizations.
* zero_extendMN instruction pattern: Standard Names.
* zero_extract, canonicalization of: Insn Canonicalizations.
* VAXCRTL: VMS Misc.
* libgcc.a: Library Calls.
* stdarg.h and RT PC: RT Options.
* stdarg.h and register arguments: Register Arguments.
* tm.h macros: Target Macros.
* varargs.h and RT PC: RT Options.
* xm-MACHINE.h: Config.
* ! in constraint: Multi-Alternative.
* # in constraint: Modifiers.
* % in constraint: Modifiers.
* % in template: Output Template.
* & in constraint: Modifiers.
* * in constraint: Modifiers.
* + in constraint: Modifiers.
* /i in RTL dump: Flags.
* /s in RTL dump: Flags.
* /s in RTL dump: Flags.
* /u in RTL dump: Flags.
* /v in RTL dump: Flags.
* 0 in constraint: Simple Constraints.
* < in constraint: Simple Constraints.
* = in constraint: Modifiers.
* > in constraint: Simple Constraints.
* ? in constraint: Multi-Alternative.
* E in constraint: Simple Constraints.
* F in constraint: Simple Constraints.
* G in constraint: Simple Constraints.
* H in constraint: Simple Constraints.
* I in constraint: Simple Constraints.
* Q, in constraint: Simple Constraints.
* V in constraint: Simple Constraints.
* X in constraint: Simple Constraints.
* _ in variables in macros: Naming Types.
* d in constraint: Simple Constraints.
* g in constraint: Simple Constraints.
* i in constraint: Simple Constraints.
* m in constraint: Simple Constraints.
* n in constraint: Simple Constraints.
* o in constraint: Simple Constraints.
* p in constraint: Simple Constraints.
* r in constraint: Simple Constraints.
* s in constraint: Simple Constraints.
* ACCUMULATE_OUTGOING_ARGS: Stack Arguments.
* ADDITIONAL_REGISTER_NAMES: Instruction Output.
* ADDRESS_COST: Costs.
* ADJUST_INSN_LENGTH: Insn Lengths.
* ALLOCATE_TRAMPOLINE: Trampolines.
* ALL_REGS: Register Classes.
* AMD29K options: AMD29K Options.
* ANSI support: Dialect Options.
* ARGS_GROW_DOWNWARD: Frame Layout.
* ARG_POINTER_REGNUM: Frame Registers.
* ASM_APP_OFF: File Framework.
* ASM_APP_ON: File Framework.
* ASM_BYTE_OP: Data Output.
* ASM_CLOSE_PAREN: Data Output.
* ASM_COMMENT_START: File Framework.
* ASM_DECLARE_FUNCTION_NAME: Label Output.
* ASM_DECLARE_FUNCTION_SIZE: Label Output.
* ASM_DECLARE_OBJECT_NAME: Label Output.
* ASM_FILE_END: File Framework.
* ASM_FILE_START: File Framework.
* ASM_FINAL_SPEC: Driver.
* ASM_FORMAT_PRIVATE_NAME: Label Output.
* ASM_GENERATE_INTERNAL_LABEL: Label Output.
* ASM_GLOBALIZE_LABEL: Label Output.
* ASM_IDENTIFY_GCC: File Framework.
* ASM_NO_SKIP_IN_TEXT: Alignment Output.
* ASM_OPEN_PAREN: Data Output.
* ASM_OUTPUT_ADDR_DIFF_ELT: Dispatch Tables.
* ASM_OUTPUT_ADDR_VEC_ELT: Dispatch Tables.
* ASM_OUTPUT_ALIGN: Alignment Output.
* ASM_OUTPUT_ALIGNED_COMMON: Uninitialized Data.
* ASM_OUTPUT_ALIGNED_LOCAL: Uninitialized Data.
* ASM_OUTPUT_ALIGN_CODE: Alignment Output.
* ASM_OUTPUT_ASCII: Data Output.
* ASM_OUTPUT_BYTE: Data Output.
* ASM_OUTPUT_CASE_END: Dispatch Tables.
* ASM_OUTPUT_CASE_LABEL: Dispatch Tables.
* ASM_OUTPUT_CHAR: Data Output.
* ASM_OUTPUT_COMMON: Uninitialized Data.
* ASM_OUTPUT_CONSTRUCTOR: Constructor Output.
* ASM_OUTPUT_DESTRUCTOR: Constructor Output.
* ASM_OUTPUT_DOUBLE: Data Output.
* ASM_OUTPUT_DOUBLE_INT: Data Output.
* ASM_OUTPUT_EXTERNAL: Label Output.
* ASM_OUTPUT_EXTERNAL_LIBCALL: Label Output.
* ASM_OUTPUT_FLOAT: Data Output.
* ASM_OUTPUT_IDENT: File Framework.
* ASM_OUTPUT_INT: Data Output.
* ASM_OUTPUT_INTERNAL_LABEL: Label Output.
* ASM_OUTPUT_LABEL: Label Output.
* ASM_OUTPUT_LABELREF: Label Output.
* ASM_OUTPUT_LABELREF_AS_INT: Label Output.
* ASM_OUTPUT_LOCAL: Uninitialized Data.
* ASM_OUTPUT_LONG_DOUBLE: Data Output.
* ASM_OUTPUT_LOOP_ALIGN: Alignment Output.
* ASM_OUTPUT_OPCODE: Instruction Output.
* ASM_OUTPUT_POOL_PROLOGUE: Data Output.
* ASM_OUTPUT_QUADRUPLE_INT: Data Output.
* ASM_OUTPUT_REG_POP: Instruction Output.
* ASM_OUTPUT_REG_PUSH: Instruction Output.
* ASM_OUTPUT_SHARED_COMMON: Uninitialized Data.
* ASM_OUTPUT_SHARED_LOCAL: Uninitialized Data.
* ASM_OUTPUT_SHORT: Data Output.
* ASM_OUTPUT_SKIP: Alignment Output.
* ASM_OUTPUT_SOURCE_FILENAME: File Framework.
* ASM_OUTPUT_SOURCE_LINE: File Framework.
* ASM_OUTPUT_SPECIAL_POOL_ENTRY: Data Output.
* ASM_SPEC: Driver.
* ASM_STABD_OP: Debugging Info.
* ASM_STABN_OP: Debugging Info.
* ASM_STABS_OP: Debugging Info.
* Alliant: Incompatibilities.
* BASE_REG_CLASS: Register Classes.
* BIGGEST_ALIGNMENT: Storage Layout.
* BIGGEST_FIELD_ALIGNMENT: Storage Layout.
* BITFIELD_NBYTES_LIMITED: Storage Layout.
* BITS_BIG_ENDIAN: Storage Layout.
* BITS_PER_UNIT: Storage Layout.
* BITS_PER_WORD: Storage Layout.
* BLKmode: Machine Modes.
* BLOCK_PROFILER: Profiling.
* BRANCH_COST: Costs.
* BYTES_BIG_ENDIAN: Storage Layout.
* BYTE_LOADS_ZERO_EXTEND: Misc.
* Bison parser generator: Installation.
* C language extensions: Extensions.
* C language, traditional: Dialect Options.
* C statements for assembler output: Output Statement.
* CALLER_SAVE_PROFITABLE: Caller Saves.
* CALL_USED_REGISTERS: Register Basics.
* CAN_ELIMINATE: Elimination.
* CASE_DROPS_THROUGH: Misc.
* CASE_VECTOR_MODE: Misc.
* CASE_VECTOR_PC_RELATIVE: Misc.
* CC1PLUS_SPEC: Driver.
* CC1_SPEC: Driver.
* CC_STATUS_MDEP: Condition Code.
* CC_STATUS_MDEP_INIT: Condition Code.
* CCmode: Machine Modes.
* CHAR_TYPE_SIZE: Type Layout.
* CHECK_FLOAT_VALUE: Storage Layout.
* CLASS_MAX_NREGS: Register Classes.
* CODE_LABEL_NUMBER: Insns.
* COMPILER_PATH: Environment Variables.
* CONDITIONAL_REGISTER_USAGE: Register Basics.
* CONST0_RTX: Constants.
* CONST1_RTX: Constants.
* CONST2_RTX: Constants.
* CONSTANT_ADDRESS_P: Addressing Modes.
* CONSTANT_ALIGNMENT: Storage Layout.
* CONSTANT_P: Addressing Modes.
* CONSTANT_POOL_ADDRESS_P: Flags.
* CONST_CALL_P: Flags.
* CONST_COSTS: Costs.
* CONST_DOUBLE_CHAIN: Constants.
* CONST_DOUBLE_LOW: Constants.
* CONST_DOUBLE_MEM: Constants.
* CONST_DOUBLE_OK_FOR_LETTER_P: Register Classes.
* CONST_OK_FOR_LETTER_P: Register Classes.
* COSTS_N_INSNS: Costs.
* CPLUS_INCLUDE_PATH: Environment Variables.
* CPP_PREDEFINES: Run-time Target.
* CPP_SPEC: Driver.
* CUMULATIVE_ARGS: Register Arguments.
* C_INCLUDE_PATH: Environment Variables.
* Convex options: Convex Options.
* DATA_ALIGNMENT: Storage Layout.
* DATA_SECTION_ASM_OP: Sections.
* DBR_OUTPUT_SEQEND: Instruction Output.
* DBX: Incompatibilities.
* DBX_CONTIN_CHAR: Debugging Info.
* DBX_CONTIN_LENGTH: Debugging Info.
* DBX_DEBUGGING_INFO: Debugging Info.
* DBX_FUNCTION_FIRST: Debugging Info.
* DBX_LBRAC_FIRST: Debugging Info.
* DBX_NO_XREFS: Debugging Info.
* DBX_OUTPUT_FUNCTION_END: Debugging Info.
* DBX_OUTPUT_MAIN_SOURCE_DIRECTORY: Debugging Info.
* DBX_OUTPUT_MAIN_SOURCE_FILENAME: Debugging Info.
* DBX_OUTPUT_MAIN_SOURCE_FILE_END: Debugging Info.
* DBX_OUTPUT_SOURCE_FILENAME: Debugging Info.
* DBX_OUTPUT_STANDARD_TYPES: Debugging Info.
* DBX_REGISTER_NUMBER: Debugging Info.
* DBX_STATIC_STAB_DATA_SECTION: Debugging Info.
* DBX_WORKING_DIRECTORY: Debugging Info.
* DCmode: Machine Modes.
* DEBUGGER_ARG_OFFSET: Debugging Info.
* DEBUGGER_AUTO_OFFSET: Debugging Info.
* DEBUG_SYMS_TEXT: Debugging Info.
* DEFAULT_CALLER_SAVES: Caller Saves.
* DEFAULT_GDB_EXTENSIONS: Debugging Info.
* DEFAULT_MAIN_RETURN: Misc.
* DEFAULT_SHORT_ENUMS: Type Layout.
* DEFAULT_SIGNED_CHAR: Type Layout.
* DELAY_SLOTS_FOR_EPILOGUE: Function Entry.
* DEPENDENCIES_OUTPUT: Environment Variables.
* DFmode: Machine Modes.
* DIVDI3_LIBCALL: Library Calls.
* DIVSI3_LIBCALL: Library Calls.
* DImode: Machine Modes.
* DOLLARS_IN_IDENTIFIERS: Misc.
* DONE: Expander Definitions.
* DONT_REDUCE_ADDR: Costs.
* DOUBLE_TYPE_SIZE: Type Layout.
* DWARF_DEBUGGING_INFO: Debugging Info.
* DYNAMIC_CHAIN_ADDRESS: Frame Layout.
* De Morgan's law: Insn Canonicalizations.
* Dependent Patterns: Dependent Patterns.
* EASY_DIV_EXPR: Misc.
* ELIGIBLE_FOR_EPILOGUE_DELAY: Function Entry.
* ELIMINABLE_REGS: Elimination.
* EMPTY_FIELD_BOUNDARY: Storage Layout.
* ENCODE_SECTION_INFO: Sections.
* ENDFILE_SPEC: Driver.
* EXECUTABLE_SUFFIX: Config.
* EXIT_BODY: Misc.
* EXIT_IGNORE_STACK: Function Entry.
* EXPAND_BUILTIN_SAVEREGS: Varargs.
* EXTRA_CC_MODES: Condition Code.
* EXTRA_CC_NAMES: Condition Code.
* EXTRA_CONSTRAINT: Register Classes.
* EXTRA_SECTIONS: Sections.
* EXTRA_SECTION_FUNCTIONS: Sections.
* FAIL: Expander Definitions.
* FAILURE_EXIT_CODE: Config.
* FINALIZE_PIC: PIC.
* FINAL_PRESCAN_INSN: Instruction Output.
* FIRST_INSN_ADDRESS: Insn Lengths.
* FIRST_PARM_OFFSET: Frame Layout.
* FIRST_PSEUDO_REGISTER: Register Basics.
* FIRST_STACK_REG: Stack Registers.
* FIRST_VIRTUAL_REGISTER: Regs and Memory.
* FIXED_REGISTERS: Register Basics.
* FIXUNS_TRUNC_LIKE_FIX_TRUNC: Misc.
* FLOATIFY: Library Calls.
* FLOAT_ARG_TYPE: Library Calls.
* FLOAT_TYPE_SIZE: Type Layout.
* FLOAT_VALUE_TYPE: Library Calls.
* FRAME_GROWS_DOWNWARD: Frame Layout.
* FRAME_POINTER_REGNUM: Frame Registers.
* FRAME_POINTER_REQUIRED: Elimination.
* FUNCTION_ARG: Register Arguments.
* FUNCTION_ARG_ADVANCE: Register Arguments.
* FUNCTION_ARG_BOUNDARY: Register Arguments.
* FUNCTION_ARG_PADDING: Register Arguments.
* FUNCTION_ARG_PARTIAL_NREGS: Register Arguments.
* FUNCTION_ARG_PASS_BY_REFERENCE: Register Arguments.
* FUNCTION_ARG_REGNO_P: Register Arguments.
* FUNCTION_BLOCK_PROFILER: Profiling.
* FUNCTION_BOUNDARY: Storage Layout.
* FUNCTION_CONVERSION_BUG: Config.
* FUNCTION_EPILOGUE: Function Entry.
* FUNCTION_INCOMING_ARG: Register Arguments.
* FUNCTION_MODE: Misc.
* FUNCTION_OUTGOING_VALUE: Scalar Return.
* FUNCTION_PROFILER: Profiling.
* FUNCTION_PROLOGUE: Function Entry.
* FUNCTION_VALUE: Scalar Return.
* FUNCTION_VALUE_REGNO_P: Scalar Return.
* GCC_EXEC_PREFIX: Environment Variables.
* GENERAL_REGS: Register Classes.
* GET_CLASS_NARROWEST_MODE: Machine Modes.
* GET_CODE: RTL Objects.
* GET_MODE: Machine Modes.
* GET_MODE_ALIGNMENT: Machine Modes.
* GET_MODE_BITSIZE: Machine Modes.
* GET_MODE_CLASS: Machine Modes.
* GET_MODE_MASK: Machine Modes.
* GET_MODE_NAME: Machine Modes.
* GET_MODE_NUNITS: Machine Modes.
* GET_MODE_SIZE: Machine Modes.
* GET_MODE_UNIT_SIZE: Machine Modes.
* GET_MODE_WIDER_MODE: Machine Modes.
* GET_RTX_CLASS: Accessors.
* GET_RTX_FORMAT: Accessors.
* GET_RTX_LENGTH: Accessors.
* GLOBALDEF: Global Declarations.
* GLOBALREF: Global Declarations.
* GLOBALVALUEDEF: Global Declarations.
* GLOBALVALUEREF: Global Declarations.
* GNU CC and portability: Portability.
* GNU CC command options: Invoking GCC.
* GNU extensions to the C language: Extensions.
* GO_IF_LEGITIMATE_ADDRESS: Addressing Modes.
* GO_IF_MODE_DEPENDENT_ADDRESS: Addressing Modes.
* HANDLE_PRAGMA: Misc.
* HARD_REGNO_MODE_OK: Values in Registers.
* HARD_REGNO_NREGS: Values in Registers.
* HAVE_ATEXIT: Misc.
* HAVE_POST_DECREMENT: Addressing Modes.
* HAVE_POST_INCREMENT: Addressing Modes.
* HAVE_PRE_DECREMENT: Addressing Modes.
* HAVE_PRE_INCREMENT: Addressing Modes.
* HAVE_PUTENV: Config.
* HAVE_VPRINTF: Config.
* HImode: Machine Modes.
* HOST_BITS_PER_CHAR: Config.
* HOST_BITS_PER_INT: Config.
* HOST_BITS_PER_LONG: Config.
* HOST_BITS_PER_SHORT: Config.
* HOST_FLOAT_FORMAT: Config.
* HOST_WORDS_BIG_ENDIAN: Config.
* IBM RS/6000 Options: RS/6000 Options.
* IBM RT PC: Incompatibilities.
* IBM RT options: RT Options.
* IEEE_FLOAT_FORMAT: Storage Layout.
* IMMEDIATE_PREFIX: Instruction Output.
* IMPLICIT_FIX_EXPR: Misc.
* INCLUDE_DEFAULTS: Driver.
* INDEX_REG_CLASS: Register Classes.
* INITIALIZE_TRAMPOLINE: Trampolines.
* INITIAL_ELIMINATION_OFFSET: Elimination.
* INITIAL_FRAME_POINTER_OFFSET: Elimination.
* INIT_CUMULATIVE_ARGS: Register Arguments.
* INIT_CUMULATIVE_INCOMING_ARGS: Register Arguments.
* INIT_SECTION_ASM_OP: Sections.
* INSN_ANNULLED_BRANCH_P: Flags.
* INSN_CACHE_DEPTH: Trampolines.
* INSN_CACHE_LINE_WIDTH: Trampolines.
* INSN_CACHE_SIZE: Trampolines.
* INSN_CLOBBERS_REGNO_P: Obsolete Register Macros.
* INSN_CODE: Insns.
* INSN_DELETED_P: Flags.
* INSN_FROM_TARGET_P: Flags.
* INSN_UID: Insns.
* INTEGRATE_THRESHOLD: Misc.
* INTIFY: Library Calls.
* INT_TYPE_SIZE: Type Layout.
* Intel 386 Options: i386 Options.
* Interdependence of Patterns: Dependent Patterns.
* JUMP_LABEL: Insns.
* JUMP_TABLES_IN_TEXT_SECTION: Sections.
* LABEL_NUSES: Insns.
* LABEL_OUTSIDE_LOOP_P: Flags.
* LABEL_PRESERVE_P: Flags.
* LAST_STACK_REG: Stack Registers.
* LAST_VIRTUAL_REGISTER: Regs and Memory.
* LEAF_REGISTERS: Leaf Functions.
* LEAF_REG_REMAP: Leaf Functions.
* LEGITIMATE_CONSTANT_P: Addressing Modes.
* LEGITIMATE_PIC_OPERAND_P: Addressing Modes.
* LEGITIMIZE_ADDRESS: Addressing Modes.
* LIBCALL_VALUE: Scalar Return.
* LIBGCC_NEEDS_DOUBLE: Library Calls.
* LIBRARY_PATH: Environment Variables.
* LIB_SPEC: Driver.
* LIMIT_RELOAD_CLASS: Register Classes.
* LINK_LIBGCC_SPECIAL: Driver.
* LINK_SPEC: Driver.
* LOCAL_INCLUDE_DIR: Driver.
* LOCAL_LABEL_PREFIX: Instruction Output.
* LOG_LINKS: Insns.
* LONGJMP_RESTORE_FROM_STACK: Elimination.
* LONG_DOUBLE_TYPE_SIZE: Type Layout.
* LONG_LONG_TYPE_SIZE: Type Layout.
* LONG_TYPE_SIZE: Type Layout.
* Libraries: Link Options.
* M680x0 options: M680x0 Options.
* M88k options: M88K Options.
* MAX_BITS_PER_WORD: Storage Layout.
* MAX_FIXED_MODE_SIZE: Storage Layout.
* MAX_OFILE_ALIGNMENT: Storage Layout.
* MAX_REGS_PER_ADDRESS: Addressing Modes.
* MD_EXEC_PREFIX: Driver.
* MD_STARTFILE_PREFIX: Driver.
* MEMORY_MOVE_COST: Costs.
* MEM_IN_STRUCT_P: Flags.
* MEM_VOLATILE_P: Flags.
* MIPS options: MIPS Options.
* MODDI3_LIBCALL: Library Calls.
* MODES_TIEABLE_P: Values in Registers.
* MODE_CC: Machine Modes.
* MODE_COMPLEX_FLOAT: Machine Modes.
* MODE_COMPLEX_INT: Machine Modes.
* MODE_FLOAT: Machine Modes.
* MODE_FUNCTION: Machine Modes.
* MODE_INT: Machine Modes.
* MODE_PARTIAL_INT: Machine Modes.
* MODE_RANDOM: Machine Modes.
* MODSI3_LIBCALL: Library Calls.
* MOVE_MAX: Misc.
* MOVE_RATIO: Costs.
* MULDI3_LIBCALL: Library Calls.
* MULSI3_LIBCALL: Library Calls.
* MULTIBYTE_CHARS: Type Layout.
* NEXT_INSN: Insns.
* NEXT_OBJC_RUNTIME: Library Calls.
* NON_SAVING_SETJMP: Register Basics.
* NOTE_INSN_BLOCK_BEG: Insns.
* NOTE_INSN_BLOCK_END: Insns.
* NOTE_INSN_DELETED: Insns.
* NOTE_INSN_FUNCTION_END: Insns.
* NOTE_INSN_LOOP_BEG: Insns.
* NOTE_INSN_LOOP_CONT: Insns.
* NOTE_INSN_LOOP_END: Insns.
* NOTE_INSN_LOOP_VTOP: Insns.
* NOTE_INSN_SETJMP: Insns.
* NOTE_LINE_NUMBER: Insns.
* NOTE_SOURCE_FILE: Insns.
* NOTICE_UPDATE_CC: Condition Code.
* NO_DOLLAR_IN_LABEL: Misc.
* NO_FUNCTION_CSE: Costs.
* NO_RECURSIVE_FUNCTION_CSE: Costs.
* NO_REGS: Register Classes.
* NO_STAB_H: Config.
* NO_SYS_SIGLIST: Config.
* NUM_MACHINE_MODES: Machine Modes.
* N_REG_CLASSES: Register Classes.
* OBJC_GEN_METHOD_LABEL: Label Output.
* OBJC_INCLUDE_PATH: Environment Variables.
* OBJC_INT_SELECTORS: Type Layout.
* OBJC_NONUNIQUE_SELECTORS: Type Layout.
* OBJC_PROLOGUE: File Framework.
* OBSTACK_CHUNK_ALLOC: Config.
* OBSTACK_CHUNK_FREE: Config.
* OBSTACK_CHUNK_SIZE: Config.
* OCS (88k): M88K Options.
* ONLY_INT_FIELDS: Config.
* OPTIMIZATION_OPTIONS: Run-time Target.
* ORDER_REGS_FOR_LOCAL_ALLOC: Allocation Order.
* OUTGOING_REG_PARM_STACK_SPACE: Stack Arguments.
* OVERLAPPING_REGNO_P: Obsolete Register Macros.
* OVERRIDE_OPTIONS: Run-time Target.
* Ordering of Patterns: Pattern Ordering.
* PARM_BOUNDARY: Storage Layout.
* PATTERN: Insns.
* PCC_BITFIELD_TYPE_MATTERS: Storage Layout.
* PCC_STATIC_STRUCT_RETURN: Aggregate Return.
* PDImode: Machine Modes.
* PIC: PIC.
* PIC_OFFSET_TABLE_REGNUM: PIC.
* POINTER_SIZE: Storage Layout.
* PREDICATE_CODES: Misc.
* PREFERRED_RELOAD_CLASS: Register Classes.
* PRESERVE_DEATH_INFO_REGNO_P: Obsolete Register Macros.
* PREV_INSN: Insns.
* PRINT_OPERAND: Instruction Output.
* PRINT_OPERAND_ADDRESS: Instruction Output.
* PRINT_OPERAND_PUNCT_VALID_P: Instruction Output.
* PROFILE_BEFORE_PROLOGUE: Profiling.
* PROMOTE_PROTOTYPES: Stack Arguments.
* PSImode: Machine Modes.
* PTRDIFF_TYPE: Type Layout.
* PUSH_ROUNDING: Stack Arguments.
* PUT_CODE: RTL Objects.
* PUT_MODE: Machine Modes.
* PUT_REG_NOTE_KIND: Insns.
* PUT_SDB_...: Debugging Info.
* Pattern Ordering: Pattern Ordering.
* Pmode: Misc.
* QImode: Machine Modes.
* READONLY_DATA_SECTION: Sections.
* REAL_ARITHMETIC: Cross-compilation.
* REAL_INFINITY: Cross-compilation.
* REAL_VALUES_EQUAL: Cross-compilation.
* REAL_VALUES_LESS: Cross-compilation.
* REAL_VALUE_ATOF: Cross-compilation.
* REAL_VALUE_FIX: Cross-compilation.
* REAL_VALUE_FIX_TRUNCATE: Cross-compilation.
* REAL_VALUE_FROM_INT: Cross-compilation.
* REAL_VALUE_ISINF: Cross-compilation.
* REAL_VALUE_ISNAN: Cross-compilation.
* REAL_VALUE_LDEXP: Cross-compilation.
* REAL_VALUE_NEGATE: Cross-compilation.
* REAL_VALUE_TO_INT: Cross-compilation.
* REAL_VALUE_TRUNCATE: Cross-compilation.
* REAL_VALUE_TYPE: Cross-compilation.
* REAL_VALUE_UNSIGNED_FIX: Cross-compilation.
* REAL_VALUE_UNSIGNED_FIX_TRUNCATE: Cross-compilation.
* REGISTER_MOVE_COST: Costs.
* REGISTER_NAMES: Instruction Output.
* REGISTER_PREFIX: Instruction Output.
* REGNO_OK_FOR_BASE_P: Register Classes.
* REGNO_OK_FOR_INDEX_P: Register Classes.
* REGNO_REG_CLASS: Register Classes.
* REG_ALLOC_ORDER: Allocation Order.
* REG_CC_SETTER: Insns.
* REG_CC_USER: Insns.
* REG_CLASS_CONTENTS: Register Classes.
* REG_CLASS_FROM_LETTER: Register Classes.
* REG_CLASS_NAMES: Register Classes.
* REG_DEAD: Insns.
* REG_DEP_ANTI: Insns.
* REG_DEP_OUTPUT: Insns.
* REG_EQUAL: Insns.
* REG_EQUIV: Insns.
* REG_FUNCTION_VALUE_P: Flags.
* REG_INC: Insns.
* REG_LABEL: Insns.
* REG_LEAF_ALLOC_ORDER: Leaf Functions.
* REG_LIBCALL: Insns.
* REG_LOOP_TEST_P: Flags.
* REG_NONNEG: Insns.
* REG_NOTES: Insns.
* REG_NOTE_KIND: Insns.
* REG_NO_CONFLICT: Insns.
* REG_OK_FOR_BASE_P: Addressing Modes.
* REG_OK_FOR_INDEX_P: Addressing Modes.
* REG_OK_STRICT: Addressing Modes.
* REG_PARM_STACK_SPACE: Stack Arguments.
* REG_RETVAL: Insns.
* REG_UNUSED: Insns.
* REG_USERVAR_P: Flags.
* REG_WAS_0: Insns.
* RELATIVE_PREFIX_NOT_LINKDIR: Driver.
* RETURN_IN_MEMORY: Aggregate Return.
* RETURN_POPS_ARGS: Stack Arguments.
* ROUND_TYPE_ALIGN: Storage Layout.
* ROUND_TYPE_SIZE: Storage Layout.
* RS/6000 Options: RS/6000 Options.
* RT PC: Incompatibilities.
* RT options: RT Options.
* RTL addition: Arithmetic.
* RTL comparison: Arithmetic.
* RTL comparison operations: Comparisons.
* RTL constant expression types: Constants.
* RTL constants: Constants.
* RTL declarations: RTL Declarations.
* RTL difference: Arithmetic.
* RTL expression: RTL Objects.
* RTL expressions for arithmetic: Arithmetic.
* RTL format: Accessors.
* RTL format characters: Accessors.
* RTL function-call insns: Calls.
* RTL generation: Passes.
* RTL insn template: RTL Template.
* RTL integers: RTL Objects.
* RTL memory expressions: Regs and Memory.
* RTL object types: RTL Objects.
* RTL postdecrement: Incdec.
* RTL postincrement: Incdec.
* RTL predecrement: Incdec.
* RTL preincrement: Incdec.
* RTL register expressions: Regs and Memory.
* RTL representation: RTL.
* RTL side effect expressions: Side Effects.
* RTL strings: RTL Objects.
* RTL structure sharing assumptions: Sharing.
* RTL subtraction: Arithmetic.
* RTL sum: Arithmetic.
* RTL vectors: RTL Objects.
* RTX (See RTL): RTL Objects.
* RTX_COSTS: Costs.
* RTX_INTEGRATED_P: Flags.
* RTX_UNCHANGING_P: Flags.
* Register Transfer Language (RTL): RTL.
* SCCS_DIRECTIVE: Misc.
* SCHED_GROUP_P: Flags.
* SCmode: Machine Modes.
* SDB_ALLOW_FORWARD_REFERENCES: Debugging Info.
* SDB_ALLOW_UNKNOWN_REFERENCES: Debugging Info.
* SDB_DEBUGGING_INFO: Debugging Info.
* SDB_DELIM: Debugging Info.
* SDB_GENERATE_FAKE: Debugging Info.
* SECONDARY_INPUT_RELOAD_CLASS: Register Classes.
* SECONDARY_OUTPUT_RELOAD_CLASS: Register Classes.
* SECONDARY_RELOAD_CLASS: Register Classes.
* SELECT_CC_MODE: Condition Code.
* SELECT_RTX_SECTION: Sections.
* SELECT_SECTION: Sections.
* SETUP_INCOMING_VARARGS: Varargs.
* SET_DEST: Side Effects.
* SET_SRC: Side Effects.
* SFmode: Machine Modes.
* SHARED_SECTION_ASM_OP: Sections.
* SHIFT_COUNT_TRUNCATED: Misc.
* SHORT_TYPE_SIZE: Type Layout.
* SIGNED_CHAR_SPEC: Driver.
* SIZE_TYPE: Type Layout.
* SImode: Machine Modes.
* SItype: Library Calls.
* SLOW_BYTE_ACCESS: Costs.

* SLOW_UNALIGNED_ACCESS: Costs.
* SLOW_ZERO_EXTEND: Costs.
* SMALL_REGISTER_CLASSES: Register Classes.
* SPARC options: Sparc Options.
* STACK_BOUNDARY: Storage Layout.
* STACK_DYNAMIC_OFFSET: Frame Layout.
* STACK_GROWS_DOWNWARD: Frame Layout.
* STACK_PARMS_IN_REG_PARM_AREA: Stack Arguments.
* STACK_POINTER_OFFSET: Frame Layout.
* STACK_POINTER_REGNUM: Frame Registers.
* STACK_REGS: Stack Registers.
* STANDARD_EXEC_PREFIX: Driver.
* STANDARD_INCLUDE_DIR: Driver.
* STANDARD_STARTFILE_PREFIX: Driver.
* STARTFILE_SPEC: Driver.
* STARTING_FRAME_OFFSET: Frame Layout.
* STATIC_CHAIN: Frame Registers.
* STATIC_CHAIN_INCOMING: Frame Registers.
* STATIC_CHAIN_INCOMING_REGNUM: Frame Registers.
* STATIC_CHAIN_REGNUM: Frame Registers.
* STDC_VALUE: Run-time Target.
* STORE_FLAG_VALUE: Misc.
* STRICT_ALIGNMENT: Storage Layout.
* STRUCTURE_SIZE_BOUNDARY: Storage Layout.
* STRUCT_VALUE: Aggregate Return.
* STRUCT_VALUE_INCOMING: Aggregate Return.
* STRUCT_VALUE_INCOMING_REGNUM: Aggregate Return.
* STRUCT_VALUE_REGNUM: Aggregate Return.
* SUBREG_REG: Regs and Memory.
* SUBREG_WORD: Regs and Memory.
* SUCCESS_EXIT_CODE: Config.
* SVr4: M88K Options.
* SWITCHES_NEED_SPACES: Driver.
* SWITCH_TAKES_ARG: Driver.
* SYMBOL_REF_FLAG: Flags.
* SYMBOL_REF_USED: Flags.
* SYSTEM_INCLUDE_DIR: Driver.
* Sun installation: Sun Install.
* TARGET_BELL: Type Layout.
* TARGET_BS: Type Layout.
* TARGET_CR: Type Layout.
* TARGET_FF: Type Layout.
* TARGET_FLOAT_FORMAT: Storage Layout.
* TARGET_MEM_FUNCTIONS: Library Calls.
* TARGET_NEWLINE: Type Layout.
* TARGET_OPTIONS: Run-time Target.
* TARGET_SWITCHES: Run-time Target.
* TARGET_TAB: Type Layout.
* TARGET_VERSION: Run-time Target.
* TARGET_VT: Type Layout.
* TCmode: Machine Modes.
* TEXT_SECTION_ASM_OP: Sections.
* TFmode: Machine Modes.
* TImode: Machine Modes.
* TMPDIR: Environment Variables.
* TRADITIONAL_RETURN_FLOAT: Scalar Return.
* TRAMPOLINE_ALIGNMENT: Trampolines.
* TRAMPOLINE_SIZE: Trampolines.
* TRAMPOLINE_TEMPLATE: Trampolines.
* TRANSFER_FROM_TRAMPOLINE: Trampolines.
* TRULY_NOOP_TRUNCATION: Misc.
* UDIVDI3_LIBCALL: Library Calls.
* UDIVSI3_LIBCALL: Library Calls.
* UMODDI3_LIBCALL: Library Calls.
* UMODSI3_LIBCALL: Library Calls.
* UNITS_PER_WORD: Storage Layout.
* UNKNOWN_FLOAT_FORMAT: Storage Layout.
* USER_LABEL_PREFIX: Instruction Output.
* USE_C_ALLOCA: Config.
* USG: Config.
* Ultrix calling convention: Incompatibilities.
* Unos installation: Unos Install.
* VAX options: VAX Options.
* VAX_FLOAT_FORMAT: Storage Layout.
* VIRTUAL_INCOMING_ARGS_REGNUM: Regs and Memory.
* VIRTUAL_OUTGOING_ARGS_REGNUM: Regs and Memory.
* VIRTUAL_STACK_DYNAMIC_REGNUM: Regs and Memory.
* VIRTUAL_STACK_VARS_REGNUM: Regs and Memory.
* VMS: Config.
* VMS and case sensitivity: VMS Misc.
* VMS and include files: Include Files and VMS.
* VMS installation: VMS Install.
* VOIDmode: Machine Modes.
* Vax calling convention: Incompatibilities.
* WCHAR_TYPE: Type Layout.
* WCHAR_TYPE_SIZE: Type Layout.
* WORDS_BIG_ENDIAN: Storage Layout.
* WORD_SWITCH_TAKES_ARG: Driver.
* XCOFF_DEBUGGING_INFO: Debugging Info.
* XCmode: Machine Modes.
* XEXP: Accessors.
* XFmode: Machine Modes.
* XINT: Accessors.
* XSTR: Accessors.
* XVEC: Accessors.
* XVECEXP: Accessors.
* XVECLEN: Accessors.
* \: Output Template.
* __bb_init_func: Profiling.
* __builtin_args_info: Varargs.
* __builtin_classify_type: Varargs.
* __builtin_next_arg: Varargs.
* __builtin_saveregs: Varargs.
* abort: Portability.
* abs: Arithmetic.
* absolute value: Arithmetic.
* access to operands: Accessors.
* accessors: Accessors.
* addr_diff_vec: Side Effects.
* addr_vec: Side Effects.
* address: RTL Template.
* address constraints: Simple Constraints.
* address of a label: Labels as Values.
* address_operand: Simple Constraints.
* addressing modes: Addressing Modes.
* aggregates as return values: Aggregate Return.
* alignment: Alignment.
* alternate keywords: Alternate Keywords.
* analysis, data flow: Passes.
* and: Arithmetic.
* apostrophes: Incompatibilities.
* arg_pointer_rtx: Frame Registers.
* argument passing: Interface.
* arguments in frame (88k): M88K Options.
* arguments in registers: Register Arguments.
* arguments on stack: Stack Arguments.
* arithmetic libraries: Interface.
* arithmetic shift: Arithmetic.
* arithmetic simplifications: Passes.
* arithmetic, in RTL: Arithmetic.
* arrays of length zero: Zero Length.
* arrays of variable length: Variable Length.
* arrays, non-lvalue: Subscripting.
* ashift: Arithmetic.
* ashiftrt: Arithmetic.
* asm_fprintf: Instruction Output.
* asm_input: Side Effects.
* asm_noperands: Insns.
* assemble_name: Label Output.
* assembler format: File Framework.
* assembler instructions: Extended Asm.
* assembler instructions in RTL: Assembler.
* assembler names for identifiers: Asm Labels.
* assembler syntax, 88k: M88K Options.
* assembly code, invalid: Bug Criteria.
* assigning attribute values to insns: Tagging Insns.
* asterisk in template: Output Statement.
* atof: Cross-compilation.
* attr: Tagging Insns.
* attribute expressions: Expressions.
* attribute of variables: Variable Attributes.
* attribute specifications: Attr Example.
* attribute specifications example: Attr Example.
* attributes, defining: Defining Attributes.
* autoincrement addressing, availability: Portability.
* autoincrement/decrement addressing: Simple Constraints.
* autoincrement/decrement analysis: Passes.
* backslash: Output Template.
* backtrace for bug reports: Bug Reporting.
* barrier: Insns.
* basic blocks: Passes.
* bcmp: Config.
* bit fields: Bit Fields.
* bit shift overflow (88k): M88K Options.
* bitwise complement: Arithmetic.
* bitwise exclusive-or: Arithmetic.
* bitwise inclusive-or: Arithmetic.
* bitwise logical-and: Arithmetic.
* break_out_memory_refs: Addressing Modes.
* bug criteria: Bug Criteria.
* bug reports: Bug Reporting.
* bugs: Bugs.
* bugs, known: Trouble.
* byte_mode: Machine Modes.
* bzero: Config.
* call: Side Effects.
* call-clobbered register: Register Basics.
* call-saved register: Register Basics.
* call-used register: Register Basics.
* call_insn: Insns.
* call_used_regs: Register Basics.
* calling conventions: Stack and Calling.
* calling functions in RTL: Calls.
* canonicalization of instructions: Insn Canonicalizations.
* case labels in initializers: Labeled Elements.
* case ranges: Case Ranges.
* case sensitivity and VMS: VMS Misc.
* cast to a union: Cast to Union.
* casts as lvalues: Lvalues.
* cc0: Regs and Memory.
* cc0_rtx: Regs and Memory.
* cc_status: Condition Code.
* change_address: Standard Names.
* class definitions, register: Register Classes.
* class preference constraints: Class Preferences.
* classes of RTX codes: Accessors.
* clobber: Side Effects.
* code generation RTL sequences: Expander Definitions.
* code generation conventions: Code Gen Options.
* code motion: Passes.
* code_label: Insns.
* codes, RTL expression: RTL Objects.
* combiner pass: Regs and Memory.
* command options: Invoking GCC.
* common subexpression elimination: Passes.
* compare: Arithmetic.
* compilation in a separate directory: Other Dir.
* compiler bugs, reporting: Bug Reporting.
* compiler passes and files: Passes.
* compiler version, specifying: Target Options.
* complement, bitwise: Arithmetic.
* compound expressions as lvalues: Lvalues.
* computed gotos: Labels as Values.
* computing the length of an insn: Insn Lengths.
* cond: Comparisons.
* condition code register: Regs and Memory.
* condition code status: Condition Code.
* condition codes: Comparisons.
* conditional expressions as lvalues: Lvalues.
* conditional expressions, extensions: Conditionals.
* conditions, in patterns: Patterns.
* configuration file: Config.
* conflicting types: Trouble.
* const0_rtx: Constants.
* const1_rtx: Constants.
* const2_rtx: Constants.
* const_double: Constants.
* const_int: Constants.
* const_string: Constants.
* const_true_rtx: Constants.
* constant attributes: Constant Attributes.
* constant folding: Passes.
* constant folding and floating point: Cross-compilation.
* constant propagation: Passes.
* constants in constraints: Simple Constraints.
* constm1_rtx: Constants.
* constraint modifier characters: Modifiers.
* constraint, matching: Simple Constraints.
* constraints: Constraints.
* constructor expressions: Constructors.
* constructors, output of: Constructor Output.
* contributors: Contributors.
* controlling register usage: Register Basics.
* controlling the compilation driver: Driver.
* conventions, run-time: Interface.
* conversions: Conversions.
* copy_rtx_if_shared: Sharing.
* core dump: Bug Criteria.
* costs of instructions: Costs.
* cross compilation and floating point: Cross-compilation.
* cross compiling: Target Options.
* cross-jumping: Passes.
* current_function_epilogue_delay_list: Function Entry.
* current_function_outgoing_args_size: Stack Arguments.
* current_function_pops_args: Function Entry.
* current_function_pretend_args_size: Function Entry.
* data flow analysis: Passes.
* data_section: Sections.
* dbr_sequence_length: Instruction Output.
* dead code: Passes.
* dead_or_set_p: Peephole Definitions.
* deallocating variable length arrays: Variable Length.
* death notes: Obsolete Register Macros.
* debug_rtx: Bug Reporting.
* debugging information generation: Passes.
* debugging information options: Debugging Options.
* debugging, 88k OCS: M88K Options.
* declaration scope: Incompatibilities.
* declarations inside expressions: Statement Exprs.
* declarations, RTL: RTL Declarations.
* declaring attributes of functions: Function Attributes.
* define_asm_attributes: Tagging Insns.
* define_attr: Defining Attributes.
* define_delay: Delay Slots.
* define_expand: Expander Definitions.
* define_function_unit: Function Units.
* define_insn: Patterns.
* define_peephole: Expander Definitions.
* defining RTL sequences for code generation: Expander Definitions.
* defining attributes and their values: Defining Attributes.
* defining jump instruction patterns: Jump Patterns.
* defining peephole optimizers: Peephole Definitions.
* delay slots, defining: Delay Slots.
* delayed branch scheduling: Passes.
* dependencies for make as output: Environment Variables.
* dependencies, make: Preprocessor Options.
* destructors, output of: Constructor Output.
* dialect options: Dialect Options.
* digits in constraint: Simple Constraints.
* directory options: Directory Options.
* disabling certain registers: Register Basics.
* dispatch table: Dispatch Tables.
* div: Arithmetic.
* divide instruction, 88k: M88K Options.
* division: Arithmetic.
* division: Arithmetic.
* division: Arithmetic.
* dollar signs in identifier names: Dollar Signs.
* double-word arithmetic: Long Long.
* downward funargs: Nested Functions.
* driver: Driver.
* empty constraints: No Constraints.
* endianness: Portability.
* enum machine_mode: Machine Modes.
* enum reg_class: Register Classes.
* environment variables: Environment Variables.
* epilogue: Function Entry.
* eq: Comparisons.
* eq_attr: Expressions.
* equal: Comparisons.
* error: Storage Layout.
* exclamation point: Multi-Alternative.
* exclusive-or, bitwise: Arithmetic.
* exit status and VMS: VMS Misc.
* expander definitions: Expander Definitions.
* explicit register variables: Explicit Reg Vars.
* expr_list: Insns.
* expression codes: RTL Objects.
* expressions containing statements: Statement Exprs.
* expressions, compound, as lvalues: Lvalues.
* expressions, conditional, as lvalues: Lvalues.
* expressions, constructor: Constructors.
* extended asm: Extended Asm.
* extensible constraints: Simple Constraints.
* extensions, ?:: Conditionals.
* extensions, ?:: Lvalues.
* extensions, C language: Extensions.
* extern int target_flags: Run-time Target.
* external declaration scope: Incompatibilities.
* fatal signal: Bug Criteria.
* features, optional, in system conventions: Run-time Target.
* ffs: Arithmetic.
* file name suffix: Overall Options.
* file names: Link Options.
* files and passes of the compiler: Passes.
* final pass: Passes.
* final_scan_insn: Function Entry.
* final_sequence: Instruction Output.
* fix: Conversions.
* fix: Conversions.
* fixed register: Register Basics.
* fixed_regs: Register Basics.
* flags in RTL expression: Flags.
* float: Conversions.
* float_extend: Conversions.
* float_truncate: Conversions.
* floating point format and cross compilation: Cross-compilation.
* force_reg: Standard Names.
* frame layout: Frame Layout.
* frame_pointer_needed: Function Entry.
* frame_pointer_rtx: Frame Registers.
* function attributes: Function Attributes.
* function call conventions: Interface.
* function entry and exit: Function Entry.
* function pointers, arithmetic: Pointer Arith.
* function units, for scheduling: Function Units.
* function, size of pointer to: Pointer Arith.
* function-call insns: Calls.
* functions that have no side effects: Function Attributes.
* functions that never return: Function Attributes.
* functions, leaf: Leaf Functions.
* ge: Comparisons.
* gencodes: Passes.
* genconfig: Passes.
* general_operand: RTL Template.
* generalized lvalues: Lvalues.
* generating assembler output: Output Statement.
* generating insns: RTL Template.
* genflags: Passes.
* get_attr: Expressions.
* get_attr_length: Insn Lengths.
* get_frame_size: Elimination.
* get_insns: Insns.
* get_last_insn: Insns.
* geu: Comparisons.
* global offset table: Code Gen Options.
* global register after longjmp: Global Reg Vars.
* global register allocation: Passes.
* global register variables: Global Reg Vars.
* goto with computed label: Labels as Values.
* gp-relative references (MIPS): MIPS Options.
* greater than: Comparisons.
* greater than: Comparisons.
* greater than: Comparisons.
* grouping options: Invoking GCC.
* gt: Comparisons.
* gtu: Comparisons.
* hard registers: Regs and Memory.
* hardware models and configurations, specifying: Submodel Options.
* header files and VMS: Include Files and VMS.
* high: Constants.
* i386 Options: i386 Options.
* identifier names, dollar signs in: Dollar Signs.
* identifiers, names in assembler code: Asm Labels.
* identifying source, compiler (88k): M88K Options.
* if_then_else: Comparisons.
* immediate_operand: RTL Template.
* in_data: Sections.
* in_struct: Flags.
* in_text: Sections.
* include files and VMS: Include Files and VMS.
* inclusive-or, bitwise: Arithmetic.
* incompatibilities of GNU CC: Incompatibilities.
* increment operators: Bug Criteria.
* initialization routines: Constructor Output.
* initializations in expressions: Constructors.
* initializers with labeled elements: Labeled Elements.
* initializers, non-constant: Initializers.
* inline functions: Inline.
* inline functions, omission of: Inline.
* inline, automatic: Passes.
* insn: Insns.
* insn attributes: Insn Attributes.
* insn canonicalization: Insn Canonicalizations.
* insn lengths, computing: Insn Lengths.
* insn splitting: Insn Splitting.
* insn-attr.h: Defining Attributes.
* insn_list: Insns.
* insns: Insns.
* insns, generating: RTL Template.
* insns, recognizing: RTL Template.
* installation trouble: Trouble.
* installing GNU CC: Installation.
* installing GNU CC on Unos: Unos Install.
* installing GNU CC on VMS: VMS Install.
* installing GNU CC on the 3b1: 3b1 Install.
* installing GNU CC on the Sun: Sun Install.
* instruction attributes: Insn Attributes.
* instruction combination: Passes.
* instruction patterns: Patterns.
* instruction recognizer: Passes.
* instruction scheduling: Passes.
* instruction scheduling: Passes.
* instruction splitting: Insn Splitting.
* integrated: Flags.
* integrating function code: Inline.
* interfacing to GNU CC output: Interface.
* introduction: Top.
* invalid assembly code: Bug Criteria.
* invalid input: Bug Criteria.
* ior: Arithmetic.
* isinf: Cross-compilation.
* isnan: Cross-compilation.
* jump instruction patterns: Jump Patterns.
* jump instructions and set: Side Effects.
* jump optimization: Passes.
* jump threading: Passes.
* jump_insn: Insns.
* keywords, alternate: Alternate Keywords.
* known causes of trouble: Trouble.
* label_ref: Constants.
* labeled elements in initializers: Labeled Elements.
* labels as values: Labels as Values.
* language dialect options: Dialect Options.
* large bit shifts (88k): M88K Options.
* large return values: Aggregate Return.
* ldexp: Cross-compilation.
* le: Comparisons.
* leaf functions: Leaf Functions.
* leaf_function: Leaf Functions.
* leaf_function_p: Standard Names.
* left rotate: Arithmetic.
* left shift: Arithmetic.
* left shift: Arithmetic.
* length-zero arrays: Zero Length.
* less than: Comparisons.
* less than or equal: Comparisons.
* leu: Comparisons.
* library subroutine names: Library Calls.
* link options: Link Options.
* lo_sum: Arithmetic.
* load address instruction: Simple Constraints.
* local labels: Local Labels.
* local register allocation: Passes.
* local variables in macros: Naming Types.
* local variables, specifying registers: Local Reg Vars.
* logical shift: Arithmetic.
* logical-and, bitwise: Arithmetic.
* longjmp: Global Reg Vars.
* loop optimization: Passes.
* lshift: Arithmetic.
* lshiftrt: Arithmetic.
* lt: Comparisons.
* ltu: Comparisons.
* lvalues, generalized: Lvalues.
* machine dependent options: Submodel Options.
* machine description macros: Target Macros.
* machine descriptions: Machine Desc.
* machine mode conversions: Conversions.
* machine modes: Machine Modes.
* macros containing asm: Extended Asm.
* macros, inline alternative: Inline.
* macros, local labels: Local Labels.
* macros, local variables in: Naming Types.
* macros, statements in expressions: Statement Exprs.
* macros, target description: Target Macros.
* macros, types of arguments: Typeof.
* make: Preprocessor Options.
* make_safe_from: Expander Definitions.
* match_dup: RTL Template.
* match_operand: RTL Template.
* match_operator: RTL Template.
* match_scratch: RTL Template.
* matching constraint: Simple Constraints.
* matching operands: Output Template.
* math libraries: Interface.
* math, in RTL: Arithmetic.
* mcount: Profiling.
* mem: Regs and Memory.
* memory reference, nonoffsettable: Simple Constraints.
* memory references in constraints: Simple Constraints.
* messages, warning: Warning Options.
* middle-operands, omitted: Conditionals.
* minus: Arithmetic.
* mod: Arithmetic.
* mode classes: Machine Modes.
* modifiers in constraints: Modifiers.
* mult: Arithmetic.
* multiple alternative constraints: Multi-Alternative.
* multiplication: Arithmetic.
* multiprecision arithmetic: Long Long.
* name augmentation: VMS Misc.
* named patterns and conditions: Patterns.
* names used in assembler code: Asm Labels.
* names, pattern: Standard Names.
* naming types: Naming Types.
* ne: Comparisons.
* neg: Arithmetic.
* nested functions: Nested Functions.
* nested functions, trampolines for: Trampolines.
* next_cc0_user: Jump Patterns.
* nil: RTL Objects.
* no constraints: No Constraints.
* no-op move instructions: Passes.
* non-constant initializers: Initializers.
* non-static inline function: Inline.
* nonoffsettable memory reference: Simple Constraints.
* not: Arithmetic.
* not equal: Comparisons.
* not using constraints: No Constraints.
* note: Insns.
* obstack_free: 3b1 Install.
* offsettable address: Simple Constraints.
* omitted middle-operands: Conditionals.
* open coding: Inline.
* operand access: Accessors.
* operand constraints: Constraints.
* operand substitution: Output Template.
* operands: Patterns.
* optimize options: Optimize Options.
* optional hardware or system features: Run-time Target.
* options to control warnings: Warning Options.
* options, GNU CC command: Invoking GCC.
* options, code generation: Code Gen Options.
* options, debugging: Debugging Options.
* options, dialect: Dialect Options.
* options, directory search: Directory Options.
* options, grouping: Invoking GCC.
* options, linking: Link Options.
* options, optimization: Optimize Options.
* options, order: Invoking GCC.
* options, preprocessor: Preprocessor Options.
* order of options: Invoking GCC.
* order of register allocation: Allocation Order.
* other directory, compilation in: Other Dir.
* output file option: Overall Options.
* output of assembler code: File Framework.
* output statements: Output Statement.
* output templates: Output Template.
* output_addr_const: Data Output.
* output_asm_insn: Output Statement.
* overflow while constant folding: Cross-compilation.
* parallel: Side Effects.
* parameter forward declaration: Variable Length.
* parameters, miscellaneous: Misc.
* parser generator, Bison: Installation.
* parsing pass: Passes.
* passes and files of the compiler: Passes.
* passing arguments: Interface.
* pattern conditions: Patterns.
* pattern names: Standard Names.
* patterns: Patterns.
* pc: Regs and Memory.
* pc_rtx: Regs and Memory.
* peephole optimization: Passes.
* peephole optimization, RTL representation: Side Effects.
* peephole optimizer definitions: Peephole Definitions.
* percent sign: Output Template.
* perform_...: Library Calls.
* plus: Arithmetic.
* pointer arguments: Function Attributes.
* portability: Portability.
* position independent code: PIC.
* post_dec: Incdec.
* post_inc: Incdec.
* pragma: Misc.
* pragma, reason for not using: Function Attributes.
* pre_dec: Incdec.
* pre_inc: Incdec.
* predefined macros: Run-time Target.
* preprocessor options: Preprocessor Options.
* prev_cc0_setter: Jump Patterns.
* prev_nonnote_insn: Peephole Definitions.
* product: Arithmetic.
* profiling, code generation: Profiling.
* program counter: Regs and Memory.
* prologue: Function Entry.
* promotion of formal parameters: Trouble.
* pseudo registers: Regs and Memory.
* push address instruction: Simple Constraints.
* putenv: Config.
* question mark: Multi-Alternative.
* quotient: Arithmetic.
* r0-relative references (88k): M88K Options.
* ranges in case statements: Case Ranges.
* read-only strings: Incompatibilities.
* recog_operand: Instruction Output.
* recognizing insns: RTL Template.
* reg: Regs and Memory.
* reg_names: Instruction Output.
* register allocation: Passes.
* register allocation order: Allocation Order.
* register allocation, stupid: Passes.
* register class definitions: Register Classes.
* register class preference constraints: Class Preferences.
* register class preference pass: Passes.
* register pairs: Values in Registers.
* register positions in frame (88k): M88K Options.
* register positions in frame (88k): M88K Options.
* register usage: Registers.
* register use analysis: Passes.
* register variable after longjmp: Global Reg Vars.
* register-to-stack conversion: Passes.
* register_operand: RTL Template.
* registers: Extended Asm.
* registers arguments: Register Arguments.
* registers for local variables: Local Reg Vars.
* registers in constraints: Simple Constraints.
* registers, global allocation: Explicit Reg Vars.
* registers, global variables in: Global Reg Vars.
* regs_ever_live: Function Entry.
* relative costs: Costs.
* reload pass: Regs and Memory.
* reload_completed: Standard Names.
* reload_in: Standard Names.
* reload_in_progress: Standard Names.
* reload_out: Standard Names.
* reloading: Passes.
* remainder: Arithmetic.
* reporting bugs: Bugs.
* representation of RTL: RTL.
* rest_of_compilation: Passes.
* rest_of_decl_compilation: Passes.
* return: Side Effects.
* return value of main: VMS Misc.
* return values in registers: Scalar Return.
* returning aggregate values: Aggregate Return.
* returning structures and unions: Interface.
* right rotate: Arithmetic.
* right shift: Arithmetic.
* rotate: Arithmetic.
* rotate: Arithmetic.
* rotatert: Arithmetic.
* run-time conventions: Interface.
* run-time options: Code Gen Options.
* run-time target specification: Run-time Target.
* saveable_obstack: Addressing Modes.
* scalars, returned as values: Scalar Return.
* scheduling, delayed branch: Passes.
* scheduling, instruction: Passes.
* scheduling, instruction: Passes.
* scope of a variable length array: Variable Length.
* scope of declaration: Trouble.
* scope of external declarations: Incompatibilities.
* scratch: Regs and Memory.
* scratch operands: Regs and Memory.
* search path: Directory Options.
* separate directory, compilation in: Other Dir.
* sequence: Side Effects.
* set: Side Effects.
* set_attr: Tagging Insns.
* set_attr_alternative: Tagging Insns.
* setjmp: Global Reg Vars.
* shared VMS run time system: VMS Misc.
* shared strings: Incompatibilities.
* sharing of RTL components: Sharing.
* shift: Arithmetic.
* side effect in ?:: Conditionals.
* side effects, macro argument: Statement Exprs.
* sign_extend: Conversions.
* sign_extract: Bit Fields.
* signed division: Arithmetic.
* signed maximum: Arithmetic.
* signed minimum: Arithmetic.
* simple constraints: Simple Constraints.
* simplifications, arithmetic: Passes.
* sizeof: Typeof.
* smaller data references (88k): M88K Options.
* smaller data references (MIPS): MIPS Options.
* smax: Arithmetic.
* smin: Arithmetic.
* specified registers: Explicit Reg Vars.
* specifying compiler version and target machine: Target Options.
* specifying hardware config: Submodel Options.
* specifying machine version: Target Options.
* specifying registers for local variables: Local Reg Vars.
* speed of instructions: Costs.
* splitting instructions: Insn Splitting.
* sqrt: Arithmetic.
* square root: Arithmetic.
* stack arguments: Stack Arguments.
* stack frame layout: Frame Layout.
* stack_pointer_rtx: Frame Registers.
* stage1: Installation.
* standard pattern names: Standard Names.
* statements inside expressions: Statement Exprs.
* storage layout: Storage Layout.
* strcpy: Storage Layout.
* strength-reduction: Passes.
* strict_low_part: RTL Declarations.
* string constants: Incompatibilities.
* structure passing (88k): M88K Options.
* structure value address: Aggregate Return.
* structures: Incompatibilities.
* structures, constructor expression: Constructors.
* structures, returning: Interface.
* stupid register allocation: Passes.
* submodel options: Submodel Options.
* subreg: Regs and Memory.
* subscripting: Subscripting.
* subscripting and function values: Subscripting.
* suppressing warnings: Warning Options.
* symbol_ref: Constants.
* symbolic label: Sharing.
* syntax checking: Warning Options.
* sys_siglist: Config.
* tagging insns: Tagging Insns.
* tail recursion optimization: Passes.
* target description macros: Target Macros.
* target machine, specifying: Target Options.
* target options: Target Options.
* target specifications: Run-time Target.
* target-parameter-dependent code: Passes.
* termination routines: Constructor Output.
* text_section: Sections.
* thunks: Nested Functions.
* top level of compiler: Passes.
* traditional C language: Dialect Options.
* trampolines for nested functions: Trampolines.
* truncate: Conversions.
* type alignment: Alignment.
* typedef names as function parameters: Incompatibilities.
* typeof: Typeof.
* udiv: Arithmetic.
* umax: Arithmetic.
* umin: Arithmetic.
* umod: Arithmetic.
* unchanging: Flags.
* undefined behavior: Bug Criteria.
* undefined function value: Bug Criteria.
* underscores in variables in macros: Naming Types.
* underscores, avoiding (88k): M88K Options.
* union, casting to a: Cast to Union.
* unions: Incompatibilities.
* unions, returning: Interface.
* unreachable code: Passes.
* unshare_all_rtl: Sharing.
* unsigned division: Arithmetic.
* unsigned greater than: Comparisons.
* unsigned greater than: Comparisons.
* unsigned less than: Comparisons.
* unsigned less than: Comparisons.
* unsigned minimum and maximum: Arithmetic.
* unsigned_fix: Conversions.
* unsigned_float: Conversions.
* unspec: Side Effects.
* unspec_volatile: Side Effects.
* use: Side Effects.
* used: Flags.
* value after longjmp: Global Reg Vars.
* values, returned by functions: Scalar Return.
* varargs implementation: Varargs.
* variable alignment: Alignment.
* variable attributes: Variable Attributes.
* variable-length array scope: Variable Length.
* variable-length arrays: Variable Length.
* variables in specified registers: Explicit Reg Vars.
* variables, local, in macros: Naming Types.
* void pointers, arithmetic: Pointer Arith.
* void, size of pointer to: Pointer Arith.
* volatil: Flags.
* volatile memory references: Flags.
* voting between constraint alternatives: Class Preferences.
* vprintf: Config.
* warning messages: Warning Options.
* which_alternative: Output Statement.
* whitespace: Incompatibilities.
* word_mode: Machine Modes.
* xor: Arithmetic.
* zero division on 88k: M88K Options.
* zero-length arrays: Zero Length.
* zero_extend: Conversions.
* zero_extract: Bit Fields.