[FFmpeg-devel] [PATCH] Faster CABAC H.264 residual decoding

[Alexander Strange]

On May 4, 2008, at 3:11 PM, Guillaume POIRIER wrote:

> Hello,
>
> On Sun, May 4, 2008 at 5:46 PM, Alexander Strange <astrange at ithinksw.com 
> > wrote:
>>
>> On May 4, 2008, at 3:07 AM, Guillaume POIRIER wrote:
>
>>> I can test on PPC970 to have an idea of how this new code behaves  
>>> on a
>>> RISC machine.
>>>
>>>> I'll apply it tomorrow
>>>> unless someone thinks it's still worse on Athlon.
>>>
>>> Well, I guess it's better to measure speed on Athlon rather than
>>> making a wild guess ;-)
>>
>> Yeah, if I had one. The asm is a lot shorter with this version,
>> though, so I really hope it's not slower.
>
> On a PPC970, the new code is 0.12% faster on my sample (a 1100.0 kbps
> 672x560 video). It's really not much, but at least, it doesn't slow
> down decoding, which is all I ask when a change is done on a codec I
> use regularly.

Alright, applied after changing the comment to say "decoding".


>> If it is, then:
>> uint8_t *ctx = (abslevelgt1 != 0 ? 0 : abslevel1) +
>>
>> abs_level_m1_ctx_base;
>> ...
>> abslevel1 = FFMIN( 4, abslevel1+1 );
>> ..
>> ctx = 5 + abslevelgt1 + abs_level_m1_ctx_base;
>> abslevelgt1 = FFMIN( 4, abslevelgt1+1 );
>>
>> should also be better than the current code on x86, but it's not
>> really better on PPC where you don't have cmov.
>
> While trying to write an smart reply to your mail, I asked Google what
> it knew about PPC and predicates (I vaguely remembered that PPC had
> some support for writing assmebly with predicates).
> The best source I found was this piece: http://www.warthman.com/ex-CWG.htm
>
> I'm sorry to see that this time around both x86 and Itanium has a much
> more useful instructions to optimize the code snippets you pasted.
>
> :-(
>
> Guillaume

I thought they'd added 'isel' (same as cmov) to PPCs newer than the  
G5, but it doesn't seem to be in Cell. That's kind of weird.