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Re: Performance degradation on g++ 4.6


My guess is inlining differences. Try more aggressive inline
parameters to see if helps. Also try FDO to see there is any
performance difference between two versions. You will probably need to
do first level triage and file bug reports.

David


On Fri, Jul 29, 2011 at 10:56 AM, 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.
>
> 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.
>


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