[MPlayer-dev-eng] [PATCH] AltiVec: dct64 for mp3lib, IMDCT for liba52, detection code

[Daniel Egger]

Am Don, 2003-01-16 um 16.22 schrieb Romain Dolbeau:

> 1) An altivec-optimized DCT64 for mp3lib
>     (partially, it seems roughly three times as fast as
>     the C code according to quick-n-dirty gprof tests)
>     This one is bit-perfect.

Cool, my code started breaking a few months ago and never recovered
so I couldn't contribute it. :( Not that vectorized mp3 decoding
has ever been one of my top priorities, but your implementation
is really nice (though Darwin specific).

> 2) An AltiVec-enhanced IMDCT for liba52 (liba52/imdct.c)
>      It's nearly bit-perfect, I have a couple of lsb
>      changed in a 128 frames sample. I can't hear the
>      differences :-)

This is *really* cool.

> 3) AltiVec detection code ("borrowed" from FFmpeg) &
>     enough code to enable the AltiVec IMDCT in liba52
>     and mp3lib.

Hm, this code is certainly suboptimal because it'll only work
with Darwin; what about simply executing an altivec instruction
and catching the trap, I think libmpeg2 does that. An alternative
would be checking the CPU capabilities directly but this requires
root privileges.

> If there's any problem with this patch please tell me.
> (and in particular how to add cpu-specific
>   optimisation to mp3lib, it seems a bit x86-centric...)

There's one problem: Depending on the compiler you're using
which in turn depends on the version of Darwin you have, you
might need a -mabi=altivec to ensure proper register saving
over a context switch (not so much because of the other changes
because you don't seem to pass vector registers as parameters
or use varargs).

-- 
Servus,
       Daniel
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