[FFmpeg-devel] [PATCH] faster vp6 decoding

[Aurelien Jacobs]

Sebastien Lucas wrote:

> On Tue, Feb 10, 2009 at 2:42 PM, Aurelien Jacobs <aurel at gnuage.org> wrote:
> > Sebastien Lucas wrote:
> >
> >> On Tue, Feb 10, 2009 at 1:14 AM, Aurelien Jacobs <aurel at gnuage.org> wrote:
> >> > I've slightly modified your patch (see attached):
> >> >  - uses a vp6dsp_mmx.c file
> >> >  - some cosmetics (no trailing whitespace, etc...)
> >> >  - gcc 2.95 compat: you can't do for(int i=....)
> >> >  - rough attempt at x86_64 compatibility
> >> >
> >> > Unfortunately it don't work (totally garbled output), but I've only
> >> > tested it on x86_64.
> >>
> >> I tested further, mplayer (latest svn vanilla build) don't manage to
> >> decode properly my vp6 sample (it was working fine 1 month ago). So I
> >> build ffmpeg, applied your modified patch and using -f framecrc (only
> >> ssh access to my dev computer for now) I get 17 frames with changed
> >> CRC out of the 2611 frames of my sample (I was previously testing only
> >> the first 1000 frames with mplayer and there was no difference).
> >> Logically there should be no difference at all.
> >>
> >> But the "totally garbled output" you see could (~should~) be due to
> >> x86_64. Can you point me the sample you used ?
> >
> > http://samples.mplayerhq.hu/FLV/flash8/harrypotter-480x272-450-vp6.flv
> > (Any sample in the same directory should do too)
> >
> 
> I just tested your sample with the following command line :
> ./ffmpeg -i ../../harrypotter-480x272-450-vp6.flv -an -f framecrc - >
> hout_ref.txt
> with or without the patch and CRC always match.
> 
> So either my computer is blessed or the problem lies with X86_64. I'll
> try to test it with another computer at home (no 64bits at home).

OK. I gave it a deeper look and found the culprit. You had a problem
in asm constraints. %0 and %1 are modified by the asm code but they
where using a "r" constraint. They must in fact use a "+r" constraint.
Without it, the second asm bloc was using the same register as the
first asm block, without setting it again to a correct value (because
it assumed its value didn't changed in between).
Attached is an updated patch which works fine on x86_64 too. I verified
it to be bitexact. I also cleaned it up.
I intend to apply it soon, unless some asm guru has other suggestions.

Aurel
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