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

[Stefan Gehrer]

On 05/29/2010 12:49 AM, David Conrad wrote:
> 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.

That doesn't count as an argument. Some people write
decoders without having seen any spec :)

Stefan