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

psmith johnsmithdoe14 at googlemail.com
Mon Apr 6 10:30:47 UTC 2009


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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