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

[David Conrad]

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, and codecs can't set the bitexact flag themselves.