FACTS instead of FUD about gcc3

Matthias Benkmann matthias at winterdrache.de
Wed May 8 07:41:59 PDT 2002


After reading a 2nd-hand account about GCC3's 20% performance regression I
thought you might be interested in the actual facts. I quote below
excerpts from the 1st and last message of the reporter of the issue. You
can find them archived in full at

http://gcc.gnu.org/ml/gcc/2002-04/msg01013.html

and

http://gcc.gnu.org/ml/gcc/2002-04/msg01194.html

Note: I added ***...*** for emphasis around important (IMHO) phrases.

     From: Michel LESPINASSE <walken at zoy dot org>
     Date: Sat, 20 Apr 2002 17:57:18 -0700
     Subject: GCC performance regression - up to 20% ?

I have downloaded the latest 3.1 snapshot (20020415) and ran some
performance tests. So far I've been ***impressed by the FP performance***,
but kinda ***disappointed by the integer performance***.

The benchmarks I've run are two libraries I maintain, libmpeg2 and
liba52:

First the ***good news***: liba52 (mostly FP intensive workload)
on athlon tbird 950, using -mcpu=pentiumpro:
gcc-3.1 snapshot is between ***8% and 9.5% faster*** than 2.95.4
from these measurements 3.1 has a very nice performance, very close to
intel's icc. Great work ! Also using -march=athlon-tbird and
generating sse code, I can get yet a few extra % of performance.

Now the ***bad news***: for libmepg2, which is an integer-only workload, I
get a ***10% to 20% performance regression*** between 2.95.4 and 3.1...

libmpeg2, on celeron 366, with mmx optimizations:
gcc-3.0 is about 4% slower than 2.95.4
gcc-3.1 snapshot is about ***20.5% slower*** than 2.95.4 (!!!!)


    From: Michel LESPINASSE <walken at zoy dot org>
    Date: Tue, 23 Apr 2002 13:35:26 -0700
    Subject: Re: GCC performance regression - its memset !

On Tue, Apr 23, 2002 at 11:25:40AM +0200, Jan Hubicka wrote:

> Concerning the inlining, gcc inlines all memcpys with size smaller
> than 64 bytes. Perhaps this should be extended to 128 bytes in case
> we are still about 2 times as bad. This is 
> ***partly due to lame implementation of memset in glibc too*** :(


Also as I've been only giving bad news up to now, I wanted to say that
now that I've worked around the two issues I had with inlining and
with memset, the 3.1 snapshot does provide ***superior performance on my
libmpeg2*** codebase, about 5% faster than 2.95.4, and that gets up to 8%
when using -fbranch-probabilities and 9% when using -mcpu=athlon-tbird
instead of the more generic -mcpu=pentiumpro. Nice work guys !


MSB
-- 
If Barbie is so popular, why do you have to buy her friends?

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