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Re: [patches] Re: recognize x86 CPU variants and default SSE/MMX/3dNOW support
> Jan Hubicka <firstname.lastname@example.org> writes:
> >> On Tue, Dec 11, 2001 at 11:30:25AM +0100, Jan Hubicka wrote:
> >> > this patch adds the support for the CPU variants as discussed earlier.
> >> We can't do this until SSE math is moved to a different switch.
> > What is wrong with defaulting to SSE math on these CPUs? It is faster so I
> > believe it is similar as when we enable conditional moves or other use of given
> > architecture. We probably can have switch for backward compatibility with
> > i386 80bit temporaries, but I would still like to see SSE math to default
> > on SSE, mainly SSE2 enabled chips.
> We cannot switch to SSE math since for ia32 since it would violate
> some assumptions people make about floating point precission (all
> evolution done in 80-bit) on Linux. This would give different
> numeric results depending on the switch.
Yes, but we already do give different results depending on -O switches
and gcc version, so I still don't think it is much worse.
Gcc is allowed to truncate the value anytime it wants to. Additionally
Windows and some other oses do use 64bit math, so I guess the default
fp i386 app to eighter not worry or be relativly portable in this respect.
We are speaking about 5-10% performance difference in average FP code
that can be shame to miss. One still can disable it if he wants to.
> Andreas Jaeger
> SuSE Labs email@example.com
> private firstname.lastname@example.org