Sun, 31 Oct 1993 13:30:57 GMT

Related articles |
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Re: Folk Theorem: Assemblers are superior to Compilers napi@cs.indiana.edu (Mohd Hanafiah Abdullah) (1993-10-26) |

Re: Folk Theorem: Assemblers are superior to Compilers synaptx!thymus!daveg@uunet.UU.NET (Dave Gillespie) (1993-10-27) |

Superoptimizer (was Re: Folk Theorem ...) mike@vlsivie.tuwien.ac.at (1993-10-31) |

Re: Superoptimizer (was Re: Folk Theorem ...) korz@cs.columbia.edu (1993-11-01) |

Re: Superoptimizer (was Re: Folk Theorem ...) synaptx!thymus!daveg@uunet.UU.NET (Dave Gillespie) (1993-11-02) |

Newsgroups: | comp.compilers |

From: | mike@vlsivie.tuwien.ac.at (Michael Gschwind) |

Keywords: | optimize, assembler |

Organization: | TU Wien |

References: | 93-10-114 93-10-127 |

Date: | Sun, 31 Oct 1993 13:30:57 GMT |

Dave Gillespie <synaptx!thymus!daveg@uunet.UU.NET> writes:

This thread seems short on concrete examples, so here are a few from my

personal experience:

* There will always be things no compiler writer will think of.

Are you so sure? There is a program called superoptimizer, which does the

trick (for short sequences!) I cannot imagine that any human would come up

with the same tricks that this program came up by searching all possible

code combinations by executing them on the spot and comparing results. It

would construct all one instruction sequences, then all two instruction

sequences, etc. The original idea comes from Henry Massalin of Columbia

University, and there is now a GNU version available also.

References:

Henry Massalin, "Superoptimizer -- A Look at the Smallest Program", ACM,

1/1987 (my copies don't say the publication, but I would suspect SIGPLAN)

a paper in the 1992 ACM SIGPLAN conference on Programming Language Design

and Implementation, about "the GNU superoptimizer and its application in

GCC"

The README in the GNU superopt package, available from isy.liu.se via

anonymous ftp in /pub/gnu/superopt-2.3.tar.gz

*>From the README file:*

<quote>

GNU SUPEROPTIMIZER

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

accept.

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

2 n

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!

OPTIONS

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.

-assembler

Output assembler suitable to feed /bin/as instead of pseudo-code

suitable for humans.

-max-cost n

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.

-extra-cost n

Search for sequences n more expensive than the cheapest found

sequence. Default is 0 meaning that only the cheapest sequence(s)

are printed.

-no-carry-insns

Don't use instructions that use the carry flag. This might be

desirable on RISCs to simplify instruction scheduling.

-f<goal-function-name>

where <goal-function-name> 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,

respectively.

For a complete list of goal function and their definitions, look in

the file goal.def. You can easily add your own goal functions to

that file.

READING SUPEROPTIMIZER OUTPUT

The superoptimizer by default outputs sequences in high-level language like

syntax. For example, this is the output for M88000/abs:

1: r1:=arith_shift_right(r0,0x1f)

r2:=add_co(r1,r0)

r3:=xor(r2,r1)

2: r1:=arith_shift_right(r0,0x1f)

r2:=add(r1,r0)

r3:=xor(r2,r1)

3: r1:=arith_shift_right(r0,0x1f)

r2:=xor(r1,r0)

r3:=adc_co(r2,r1)

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.

...

The GNU superoptimizer and its application in GCC are described in the

proceedings of the ACM SIGPLAN conference on Programming Language Design

an Implementation, 1992.

<end of quote>

mike

--

Michael Gschwind, Institut fuer Technische Informatik, TU Wien

snail: Treitlstrasse 3-182-2 || A-1040 Wien || Austria

email: mike@vlsivie.tuwien.ac.at

phone: +(43)(1)58801 8156 fax: +(43)(1)569 697

--

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