Re: Performance degradation on g++ 4.6

[Richard Guenther <richard dot guenther at gmail dot com>]

On Fri, Jul 29, 2011 at 7:56 PM, Oleg Smolsky <oleg.smolsky@riverbed.com> wrote:
> Hi there, I have compiled and run a set of C++ benchmarks on a CentOS4/64
> box using the following compilers:
> ? ?a) g++4.1 that is available for this distro (GCC version 4.1.2 20071124
> (Red Hat 4.1.2-42)
> ? ?b) g++4.6 that I built (stock version 4.6.1)
>
> The machine has two Intel quad core processors in x86_64 mode (/proc/cpuinfo
> attached)
>
> Benchmarks were taken from this page:
> ? ?http://stlab.adobe.com/performance/
>
> Results:
> ? ?- some of these tests showed 20..30% performance degradation
> ? ? ?(eg the second section in the simple_types_constant_folding test: 30s
> -> 44s)
> ? ?- a few were quicker
> ? ?- full reports are attached
>
> I would assume that performance of the generated code is closely monitored
> by the dev community and obvious blunders should not sneak in... However, my
> findings are reproducible with these synthetic benchmarks as well as
> production code at work. The latter shows approximately 25% degradation on
> CPU bound tests.

We do monitor quite an extensive set of benchmarks - but as always certain kind
of applications may slip through - I'll have a look at the mentioned benchmark
and see whether it's easy to integrate with our testing at gcc.opensuse.org.

Richard.

> Is there a trick to building the compiler or using a specific -mtune/-march
> flag for my CPU? I built the compiler with all the default options (it just
> has a distinct installation path):
> ? ?../gcc-%{version}/configure --prefix=/work/tools/gcc46
> --enable-languages=c,c++ --with-system-zlib --with-mpfr=/work/tools/mpfr24
> --with-gmp=/work/tools/gmp --with-mpc=/work/tools/mpc
> LD_LIBRARY_PATH=/work/tools/mpfr/lib24:/work/tools/gmp/lib:/work/tools/mpc/lib
>
> Are there any published benchmarks? I'd appreciate any advice or pointers.
>
> Thanks in advance,
> Oleg.
>