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Re: Changing the default 32-bit x86 arch for Fedora 12



On Mon, Jun 15, 2009 at 6:01 PM, Bill Nottingham<notting redhat com> wrote:
> Gregory Maxwell (gmaxwell gmail com) said:
>> I doubt having consistently lower FP precision is anything many users
>> are asking for. The few that do can usually take care of themselves.
>
> And yet you say we should push them all to x86_64, which has
> the same lower precision?

You misunderstand me.
I don't think people care. Code that depends on the order of FPU
register spills on it is broken.
I'm saying it's not a feature. It's neutral. Or nearly so.

>> > - More clearly delineates our support set of targets, sticking true
>> >  to forwards innovation, not necessarily legacy support
>>
>> If thats the case why maintain x86 at all?
>
> Because it's 58% of our userbase (source: F11 torrent stats.)

The relevant break down is what percentage is x86 no sse2 vs x86 no
x86_64 vs x86 w/ x86_64.

>> Take your answer and now apply it to why fedora should maintain
>> support for x86 CPUs without SSE2.
>
> Because that's significantly less of our userbase. I'd love to have
> harder numbers, but we're still talking about a set of CPUs that
> (outside of corner cases like the Geode and C3) ceased production
> anywhere from 4 (Athlon) to 6 (P3) to 10 (P2) years ago.

'outside'. Please don't just dismiss these recent systems, they are a
real issue.

I have low power x86 which I purchased *recently* (geode box for audio
conferencing, this year; and a via box from ~two years ago) without
SSE2. Most of my systems are x86_64, but what is x86 is not sse
capable. Anything of mine that needed performance became x86_64 a long
time ago.

On Mon, Jun 15, 2009 at 6:04 PM, Bill Nottingham<notting redhat com> wrote:
>> It would probably be most interesting to perform that test on the
>> x86-only ATOM, since I can see CMOV being a bigger win on an in-order
>> CPU.
>> (I can't personally protest: I think all the x86 stuff I have has CMOV).
> Last time I profiled it (using the bits of openbench that actually worked),
> i586 -> i686 was an improvement of 1% (Core2Duo) to 2% (Atom). Athlon64 was
> essentially equal.
> SPECCPU results showed similar.

That sounds about right to me.


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