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

[Duncan Sands <baldrick at free dot fr>]

> > > 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.

Note that these are the instructions to follow on linux for LTO with llvm-gcc:
  http://llvm.org/docs/GoldPlugin.html

Ciao, Duncan.

> > 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.