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Re: i686 ---> i586, i386 ---> i586, x86_64 stays the same



Will Woods wrote:
On Sun, 2009-04-05 at 23:01 +0100, psmith wrote:

i completely understand the reasoning to move i365 up to i586, but why on earth restrict i686 capable processors to the i586 instruction sets?

Again: the only difference between i586 and i686 is the CMOV family of
instructions, which are generally *slower* than the i586 version.

You can read Linus' detailed explanation here:
http://ondioline.org/mail/cmov-a-bad-idea-on-out-of-order-cpus

Building for i686 gives *no* real performance benefit[1], but breaks
support for i586 machines - Via C7-based netbooks, AMD Geode (e.g. the
OLPC XO-1), and so on.

-w

[1] except on in-order CPUs like Atom and the original Pentium

well i understand why fedora stuck with downgrading to i586 now, saving work by only having to compile 1 kernel version for the x86 arch (plus to keep the olpc folks happy i suppose) and while linus is undoubtably a clever guy i think intel know their arch better than anyone, and if cmov was a performance hit it would be removed ;) looks like i'll need to compile my own i686 for my aspire one as the i586 kernel in F11 beta is showing a noticeable performance hit over F10 i686 :(

phil


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