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Re: Comparison of GCC-4.6.1 and LLVM-2.9 on x86/x86-64 targets
On Wed, Sep 7, 2011 at 6:23 PM, Vladimir Makarov <vmakarov@redhat.com> wrote:
> On 09/07/2011 11:55 AM, Xinliang David Li wrote:
>>
>> Why is lto/whole program mode not used in LLVM for peak performance
>> comparison? (of course, peak performance should really use FDO..)
>>
> Thanks for the feedback. ?I did not manage to use LTO for LLVM as it
> described on
>
> http://llvm.org/docs/LinkTimeOptimization.html#lto
>
> I am getting 'file not recognized: File format not recognized' ?during the
> linkage pass.
>
> You probably right that I should use -Ofast without -flto for gcc then.
> ?Although I don't think that it significantly change GCC peak performance.
> ?Still I am going to run SPEC2000 without -flto and post the data (probably
> on the next week).
Note that due to a bug in 4.6.x -Ofast is not equivalent to -O3 -ffast-math
(it doesn't use crtfastmath.o). I'll backport the fix.
> As for FDO, unfortunately for some tests SPEC uses different training sets
> and it gives sometimes wrong info for the further optimizations.
>
> I do not look at this comparison as finished work and am going to run more
> SPEC2000 tests and change the results if I have serious reasonable
> objections for the current comparison.
>