/dev/null

kbuild · 2006-01-13 19:30:44

Rebuilding a few things with GCC 4.0.2 on an Athlon 64. Is there any point in -O3 for general stuff, or is -O2 -march=athlon64 -pipe where everyone ends up? The Gentoo forums are full of people swearing by flags they never measured.

horic · 2006-01-13 20:05:10

-O3 mostly adds more aggressive inlining and -funswitch-loops, so binaries get bigger, and on a lot of code they get slower because of cache pressure. Use -O2 unless you have a benchmark that says otherwise.

segv · 2006-01-13 20:41:29

And -ffast-math is not a free speedup, whatever people say. It drops IEEE semantics and I've watched it break a physics sim in very creative ways.

kbuild · Yesterday 11:17:58

Quick test with lame, same WAV, best of five runs:

Code:

-O2 -march=athlon64                 41.8s
-O3 -march=athlon64                 41.2s
-O3 -march=athlon64 -funroll-loops  42.0s

Within noise. -O2 it is.

horic · Yesterday 15:47:21

Matches what most people find. The one place I've seen -O3 matter is tight numeric loops, and there you should be profiling anyway.

/dev/null

#1 2006-01-13 19:30:44

best compiler flags for -O2 vs -O3

#2 2006-01-13 20:05:10

Re: best compiler flags for -O2 vs -O3

#3 2006-01-13 20:41:29

Re: best compiler flags for -O2 vs -O3

#4 Yesterday 11:17:58

Re: best compiler flags for -O2 vs -O3

Code:

#5 Yesterday 15:47:21

Re: best compiler flags for -O2 vs -O3

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