[FFmpeg-devel] [PATCH 5/5] avcodec/vc1: store zero MVs for all blocks in a MB

James Almer jamrial at gmail.com
Sun May 20 04:45:12 EEST 2018


On 5/19/2018 9:13 PM, Michael Niedermayer wrote:
> On Sat, May 19, 2018 at 08:23:59PM -0300, James Almer wrote:
>> On 5/19/2018 6:40 PM, Michael Niedermayer wrote:
>>> On Fri, May 18, 2018 at 05:06:36PM +0200, Jerome Borsboom wrote:
>>>> Direct prediction for interlace frame B pictures references the mv in the
>>>> second block in an MB in the backward reference frame for the twomv case.
>>>> When the backward reference frame is an I frame, this value may be unset.
>>>>
>>>> Signed-off-by: Jerome Borsboom <jerome.borsboom at carpalis.nl>
>>>> ---
>>>>  libavcodec/vc1_block.c | 6 ++++--
>>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/libavcodec/vc1_block.c b/libavcodec/vc1_block.c
>>>> index 74935ec9e9..9c170a1e3c 100644
>>>> --- a/libavcodec/vc1_block.c
>>>> +++ b/libavcodec/vc1_block.c
>>>> @@ -2678,8 +2678,10 @@ static void vc1_decode_i_blocks_adv(VC1Context *v)
>>>>              s->bdsp.clear_blocks(block[0]);
>>>>              mb_pos = s->mb_x + s->mb_y * s->mb_stride;
>>>>              s->current_picture.mb_type[mb_pos + v->mb_off]                         = MB_TYPE_INTRA;
>>>> -            s->current_picture.motion_val[1][s->block_index[0] + v->blocks_off][0] = 0;
>>>> -            s->current_picture.motion_val[1][s->block_index[0] + v->blocks_off][1] = 0;
>>>> +            for (int i = 0; i < 4; i++) {
>>>> +                s->current_picture.motion_val[1][s->block_index[i] + v->blocks_off][0] = 0;
>>>> +                s->current_picture.motion_val[1][s->block_index[i] + v->blocks_off][1] = 0;
>>>> +            }
>>>
>>> see AV_ZERO*
>>
>> Those may use mmx instructions, which may require calling emms_c() after
>> the loop.
>> memset() may be a better choice here. For a small amount of bytes the
>> compiler should be able to optimize it into an inlined mov/xor on its own.
> 
> This calls clear_blocks() a few lines before which can use mmx too unless iam
> missing something

I see no emms in either ff_clear_blocks_mmx() or anywhere in
vc1_block.c, so either nobody tried this decoder on machines without
sse, or it's a non issue.

Keep in mind AV_ZERO* will choose to use mmx or not at compile time, not
runtime. So an x86_32 build with -msse (but not -msse2) for example
would use it, regardless of it being run on a PIII or a Haswell.


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