Jakub's Recommendations for ia32 Support

Warren Togami wtogami at redhat.com
Tue Feb 3 17:11:59 UTC 2009

I talked with Jakub Jelinek to ask his recommendation of how Fedora 11
should proceed.

Jakub recommends i586.rpm for Fedora 11, because it doesn't gain us much
of anything to go with i686 minimum.  The benefits of i586 to i686 are
simply not important because cmov is usually not a worthwhile
optimization on ia32.  You sometimes find some testcase where cmov
improves something, but often you find one where it pessimizes something
as well.  i686 to i686 w/ SSE2 would be a performance win, but that is
clearly not an option for Fedora.

If we go with i586.rpm's in Fedora, then redhat-rpm-config must be
changed to include -mtune=generic for i586, currently it has that tuning
only for i386 and i686.

Compiler flags would need to be different between RHEL6 and Fedora
anyway, because RHEL6 ppc32 userspace will be built with -mcpu=power4
making it 64bit processor capable only, and we definitely don't want
this in Fedora.  They are also considering doing RHEL6 i686 (Pentium 4+)
only, again not an option for Fedora.

Jakub recommended doing comparison tests of -m32 -march=i586
-mtune=generic vs. -m32 -march=i686 -mtune=generic with Spec2k and
Spec2006 benchmark tools.  Anyone interested in trying this?

He also wants gcc-4.4 to go into dist-f11 immediately after Alpha, even
if we are not doing the mass rebuild just yet.  It is ABI compatible so
shouldn't cause any problems on its own.  Meanwhile it will allow
maintainers to fix some problematic packages sooner.

In related news, cebbert wants to do the following to the F-11 kernel:
- Eliminate the current i686 kernel.
- i686 hardware would get i686 PAE by default.
- i586 kernel becomes i686, except without cmov.  This is primarily so
people don't complain when they realize they have the "i586" kernel.

Warren Togami
wtogami at redhat.com

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