Re: Comparison of GCC-4.6.1 and LLVM-2.9 on x86/x86-64 targets

[Vladimir Makarov <vmakarov at redhat dot com>]

On 09/08/2011 04:47 AM, Jakub Jelinek wrote:
> On Wed, Sep 07, 2011 at 11:15:39AM -0400, Vladimir Makarov wrote:
> >    This year I used -Ofast -flto -fwhole-program instead of
> > -O3 for GCC and -O3 -ffast-math for LLVM for comparison of peak
> > performance.  I could improve GCC performance even more by using
> > other GCC possibilities (like support of AVX insns, Graphite optimizations
> > and even some experimental stuff like LIPO) but I wanted to give LLVM
> > some chances too.  Probably an experienced user in LLVM could improve
> > LLVM performance too.  So I think it is a fair comparison.
> -march=native in addition would be nice to see, that can make significant
> difference, especially on AVX capable CPUs.  I guess LLVM equivalent would
> be -march=corei7 -mtune=corei7 and, if it works, -mavx too (though, the only
> time I've tried LLVM 2.9 it crashed on almost anything with -mavx).
Yes, Jakub.  It would be better to use corei7 with avx for GCC.  
Unfortunately, the last tuning which llvm 2.9 supports is core2 
therefore I used -march=core2 for comparison on x86-64.  So I think it 
would be unfair to use corei7 and avx for GCC without using it for LLVM.

I mostly tried to compare state of general optimizations of GCC and 
LLVM.  There are a lot of other aspects of the compilers which we could 
compare and I did not do it.  But for me it is obvious that Apple loses 
a lot not using modern versions of GCC.