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.
