Best compilation flags (CFLAGS, LDFLAGS)

tomee · n00b Joined: 05 Sep 2006 Posts: 38 Location: Poland

Hi,

I think this is a crucial issue for any Gentoo fan - how to tweak the CFLAGS and LDFLAGS for the programs to run faster?
This is such a basic question but I found contradictory reports concerning the use of -Ox, in particular some recommend -O2, some -O3 and some -Os. I have never gotten myself to do a real benchmark of the difference, maybe one of you have?

As far as my system goes, I am using CFLAGS="-march=pentium-m -Os -mfpmath=sse -fomit-frame-pointer -pipe -ffast-math"
(apart from python and openoffice, where -ffast-math breaks things) and am wondering if this is the optimal flag set for my processor.
For the LDFLAGS I use "-Wl,-O1 -Wl,--as-needed -Wl,--hash-style=both" and prelinking, but I can't say that the result is spectacular. Maybe I am missing something?

I was also wondering which packages it is possible to compile with Intel C/C++ Compiler, as this one seems to give a ~20% boost in typical algorithm run time. bzip2/gzip and perl might be a good starting point, I guess it's impossible to compile KDE or OpenOffice that way. Has anyone tried ICC with any bigger packages? Yes, I have read the wiki.

jmbsvicetto · Posted: Fri Oct 20, 2006 3:42 am Post subject:

Moved from Documentation, Tips & Tricks to Portage & Programming.

[mod]As you're asking some support questions, I've moved this thread to P&P.[/mod]
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synss · Posted: Fri Oct 20, 2006 12:50 pm Post subject:

Briefly, I believe all pentium-m have a large cache (mine has 2MB L2 cache) so -Os is useless and -O2 is better. For the rest, "-pipe -fomit-frame-pointer" is enough.

leaving

RoundsToZero · Guru Joined: 17 Nov 2003 Posts: 478 Location: New York, NY

Could someone speak on the performance/stability issues with -mfpmath=sse? On the one hand, the gcc manpage doesn't describe it as an "experimental" feature, but on the other hand, it's not automatically enabled with -march=pentium3 and higher, or with -mssex, which seem like the only things it depends on.

Edit: That looked less like "sex" when I typed it originally.

yngwin · Posted: Wed Oct 25, 2006 8:54 pm Post subject:

You should not set the -mfpmath option. It's best to let the compiler figure it out by itself.
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bubbl07 · Posted: Thu Oct 26, 2006 3:54 am Post subject:

-fomit-frame-pointer is redundant (at least in GCC 4) when using the -O* flags.

Dlareh · Advocate Joined: 06 Aug 2005 Posts: 2102

-fomit-frame-pointer does not appear to have changed from previous versions of gcc.

it's always had an effect on x86 beyond the -O's, because it breaks debugging
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yngwin · Posted: Thu Oct 26, 2006 12:24 pm Post subject:

geekounet · Posted: Thu Oct 26, 2006 12:33 pm Post subject:

tboloo · Posted: Thu Oct 26, 2006 6:46 pm Post subject:

After some time of playing with C/LDFLAGS you'll try some Ubuntu or other distro and you'll that it's only maybe 10percent slower and hell more stable. I used to experience a lot with various flags and ended with -O2 -march=pentium-m -pipe which is quite sufficient and what's more everything comiles flawlessly.
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stahlsau · Posted: Sun Nov 05, 2006 9:05 am Post subject: