GNU SUPEROPTIMIZER The superoptimizer is a
function sequence generator that uses a ex-
haustive generate-and-test approach to find
the shortest instruction sequence for a
given function.You have to tell the super
optimizer which function and which CPU you
want to get code for, and how many instruc-
tions you can accept.
The superoptimizer is a function sequence generator that uses a exhaustive
generate-and-test approach to find the shortest instruction sequence for a
given function. You have to tell the superoptimizer which function and
which CPU you want to get code for, and how many instructions you can
The superoptimizer can't generate very long sequences, unless you have a
very fast computer or very much spare time. The time complexity of the
used algorithm is approximately
O(m n )
where m is the number of available instructions on the architecture and n
is the shortest sequence for the goal function.
The superoptimizer can't guarantee that it finds the best possible
instruction sequences for all possible functions. For example, it doesn't
even try to include immediate constants (other that -1, 0, +1, and the
smallest negative and biggest positive numbers) in the sequences. It often
makes a good job for functions that depend on registers only.
WARNING! The generated sequences might be incorrect with a very small
probability. Always make sure a sequence is correct before using it. So
far, I have never discovered any incorrect sequences. If you find one,
please let me know about it!
The superoptimizer supports 7 CPUs, SPARC, Motorola 68000 and 88000, IBM
RS/6000, AMD 29000, Intel 80x86, and Pyramid.
You need an ANSI C compiler, for example GCC, to compile the
make CPU=-D gso
gcc -O -g -D superopt.c -o gso
where is one of SPARC, M68000, M88000, RS6000, AM29K, I386, or
PYR. To run the superoptimizer, type
gso -f [-assembler] [-max-cost n] [-no-carry-insns]
and wait until the found instructions sequences are printed.
The `-f' option has always to be defined to tell the superoptimizer for
which function it should try to to find an instruction sequence. See below
for possible function names.
Option names may be abbreviated.
Output assembler suitable to feed /bin/as instead of pseudo-code
suitable for humans.
Limit the `cost' of the instruction sequence to n. May be used to
stop the search if no instruction sequence of that length or
shorter is found. By default this is 5.
Search for sequences n more expensive than the cheapest found
sequence. Default is 0 meaning that only the cheapest sequence(s)
Don't use instructions that use the carry flag. This might be
desirable on RISCs to simplify instruction scheduling.
where is one of eq, ne, les, ges, lts, gts,
leu, geu, ltu, gtu, eq0, ne0, les0, ges0, lts0, gts0, neq, nne,
nles, nges, nlts, ngts, nleu, ngeu, nltu, ngtu, neq0, nne0, nles0,
nges0, nlts0, ngts0, maxs, mins, maxu, minu, sgn, abs, nabs, gray,
or gray2, etc, etc.
eq, ne, les, etc, computes the C expression "a == b", "a != b", "a
<= b", etc, where the operation codes ending in `s' indicates
signed comparison; `u` indicates unsigned comparison.
eq0,... computes "a == 0", ...
The `n' before the names means that the corresponding function
value is negated, e.g. nlt is the C expression "-(a < b)".
maxs, mins, maxu, minu are binary (i.e. two argument) signed
respectively unsigned max and min.
sgn is the unary sign function; -1 for negative, 0 for zero, and +1
for positive arguments.
abs and nabs are absolute value and negative absolute value,
For a complete list of goal function and their definitions, look in
the file goal.def. You can easily add your own goal function to
READING SUPEROPTIMIZER OUTPUT
The superoptimizer by default outputs sequences in high-level language like
syntax. For example, this is the output for M88000/abs:
r1:=arith_shift_right(r0,0x1f) means "shift r0 right 31 steps
arithmetically and put the result in r1". add_co is "add and set carry".
adc_co is the subtraction instruction found on most RISCs, i.e. "add with
complement and set carry". This may seem dumb, but there is an important
difference in the way carry is set after an addition-with-complement and a
subtraction. The suffixes "_ci" and "_cio" means respectively that carry
is input but not affected, and that carry is both input and generated.
The interesting value is always the value computed by the last instruction.
Please send comments, improvements and new ports to [email protected]
This superoptimizer was written by Torbjorn Granlund of SICS. Tom Wood of
DG made several improvements, like the clean way to describe goal functions
and internal instructions. The original superoptimizer idea is due to