Re: Performance degradation on g++ 4.6

[Oleg Smolsky <oleg dot smolsky at riverbed dot com>]

Hey David, here are a couple of answers and notes:
    - I built the test suite with -O3 and cannot see anything else 
related to inlining that isn't already ON (except for -finline-limit=n 
which I do not how to use)
    http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
    - FTO looks like a very different kettle of fish, I'd prefer to 
leave it aside to limit the number of data points (at least for the 
initial investigation)
    - I've just rerun the suite with -flto and there are no significant 
differences in performance

What else is there?

Oleg.

On 2011/7/29 11:07, Xinliang David Li wrote:
> 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.