Changing the default 32-bit x86 arch for Fedora 12 (#2)

[Gregory Maxwell]

On Wed, Jun 17, 2009 at 1:52 PM, Bill Nottingham<notting at redhat.com> wrote:
> Given the loud feedback, I've updated the proposal at:
>        https://fedoraproject.org/wiki/Features/F12X86Support
>
> The revised proposal:
>
> - Build all packages for i686 (this requires cmov)
> - Optimize for Atom
>
> Why?
>
> - We don't really support i586 in any meaningful matter
> - OLPC still works with base i686
> - We are likely doing a mass rebuild for F-12 anyways, might as well switch
>  while we're doing it
> - Atom is the only currently produced 32-bit x86 chip of note; optimize
>  for what's currently available
>
> If you want numbers, I did some benchmarking of code [1] with various
> build options on a variety of processors, with the F-11 gcc code. All
> of these results are relative to a F-11 baseline of "-march=i586
> -mtune=generic".
>
>                P4 2.4Ghz       Athlon 3400+    Core2Duo E6850  Atom N270
> march=i686/     -1.1%           +2.0%           +0.9%           +0.6%
>  mtune=generic
> march=i586/     +0.3%           -0.3%           -0.2%           +1.3%
>  mtune=atom
> march=i686/     -1.5%           +1.2%           +0.5%           +1.7%
>  mtune=atom

If there is a mass rebuild…

Consider:

-Os on the x86 build?
-O3 on x86_64?

(Back in 2007 I would have screamed loudly that the auto-vectorizer
produces broken code; but today it appears to work fine.)