Bruce Cran wrote:
> I'm afraid you're wrong - the V2SI datatype and MMX functions
> automatically become available after -march=pentium2, while
> with other processor types you've got to explicitly add -mmmx.
> -msse is presumed with -march=pentium3 and up.
I'm afraid I'm not; I was talking strictly about -march=pentiumpro,
not -march=pentium2 et cetera.

> ... I've seen code which runs 40x faster when compiled for
> athlon-xp than for i386, and I would guess that a lot of that
> is because of clever use of sse and mmx.   That wasn't an
> audio/video program, it was the libgmp arbitrary precision maths
> package.
MMX won't help you even a bit in precise mathematical calculations,
unlike SSE. Also, if you're familiar with the way how MMX works, you
won't enable it when compiling any application that requires heavy
floating-point computations, because constant switching between MMX
unit and FPU results in significant latency.

> Also, I'm sure most people wouldn't say no to 50% more processing
> speed for free!
I didn't mean to stop using MMX at all; I only intended to note that
there is no need to enable it for _all_ applications.

> So, if you've got a pentium, k6 or pentiumpro which supports MMX,
> you _do_ need to explicitly add -mmmx, but for other processors
> it's implied.
You didn't pay enough attention to my original post; PPro doesn't
support MMX. Though, you can specify -mmmx anyway, if you feel
comfortable with SIGILLs.


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