[FFmpeg-devel] [PATCH] Make MMX2 put_no_rnd_pixels _x2 and _y2 bitexact

[David Conrad]

On May 28, 2010, at 6:34 PM, Stefan Gehrer wrote:

> On 05/29/2010 12:27 AM, David Conrad wrote:
>> On May 28, 2010, at 6:25 PM, Stefan Gehrer wrote:
>> 
>>> On 05/28/2010 11:49 PM, David Conrad wrote:
>>>> Hi,
>>>> 
>>>> The mmx2/3dnow put_no_rnd functions don't always round correctly, since they compensate for the +1 in pavgb by subtracting 1 from one of the inputs. This causes our theora decoder to not be bitexact to libtheora, though I haven't found any real source where the error accumulates enough to be visible.
>>>> 
>>>> This fixes it by using the property that (a+b)>>1 is equivalent to ~(~a+~b+1)>>1. This makes these functions 5 cycles slower on my penryn, but on my atom the additional instructions appear to be free probably due to load stalls.
>>> 
>>> Wouldn't it be worth creating new bitexact functions, but still
>>> override them with the old/faster ones if BITEXACT is not set?
>> 
>> The problem is that theora must always be bitexact,
> 
> Why is that? If I don't see the difference I would think
> I can decode it with deviation.

Same reason H.264 must be: it's part of the spec.