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SubjectRe: f@#$ing MMX emulator
   Date: 	Fri, 19 Jun 1998 14:26:51 +0200
From: Erik Corry <erik@arbat.com>

In article <19980619162224.C118@caffeine.ix.net.nz> you wrote:
> On Thu, Jun 18, 1998 at 09:00:46PM -0700, David S. Miller wrote:
>> Not totally true, as Ingo has shown doing IP checksums on MMX was not
>> significantly different in complexity from doing it on Sparc VIS.

Doesn't give much benefit, though, according to Mingo
http://x2.dejanews.com/getdoc.xp?AN=362293483

"but for most RL MTU's the FPU save/restore operation eats
a considerable amount of the saved cycles"

Sore point, because this initially was our problem on sparc64 VIS, and
this gap closed up once we implemented smart FPU saving throughout the
sparc64 port. Now it's a non-issue and the FPU isn't save in most
cases even under high interrupt and overall load.

Read what Ingo says as "It's fast, but it doesn't show because of
problem XXX." and so we should look for a solution for XXX before just
tossing in the towel about this idea.

"this MMX ISA extension thing is not worth to be generated
by a compiler, it's only worth to make it's way into a
few very specialized assembly functions"

Any program that wants to be portable needs a C version
of any MMX code. This is going to be 1000 times faster
than the emulated MMX version, and it is by definition
performance- critical code. There's no point in emulating
MMX.

True and these cases are where you can use the dynamic binding
facilities of procedure calls available in userland or the kernel.
For example, based upon hw capability flags glibc ld.so could decide
to bind memcpy() to an MMX optimized version on capable cpu's. In
fact the infrastructure for this is %95 in glibc right now.

Later,
David S. Miller
davem@dm.cobaltmicro.com

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