[FFmpeg-devel] [PATCH 100/114] avcodec/vp3: Use symbols table for VP3 motion vectors

[Andreas Rheinhardt]

Peter Ross:
> On Tue, Nov 10, 2020 at 11:58:22AM +0100, Andreas Rheinhardt wrote:
>> Expressions like array[get_vlc2()] can be optimized by using a symbols
>> table if the array is always the same for a given VLC. This requirement
>> is fulfilled for the VLC used for VP3 motion vectors. The reason it
>> hasn't been done before is probably that the array in this case
>> contained entries in the range -31..31; but this is no problem with
>> ff_init_vlc_from_lengths(): Just apply an offset of 31 to the symbols
>> before storing them in the table used to initialize VP3 motion vectors
>> and apply an offset of -31 when initializing the actual VLC.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>>  libavcodec/vp3.c     | 20 ++++++++++++-------
>>  libavcodec/vp3data.h | 46 +++++++++++---------------------------------
>>  2 files changed, 24 insertions(+), 42 deletions(-)
>>
>> diff --git a/libavcodec/vp3.c b/libavcodec/vp3.c
>> index 7037d03a98..f288a53fe1 100644
>> --- a/libavcodec/vp3.c
>> +++ b/libavcodec/vp3.c
>> @@ -48,6 +48,7 @@
>>  #include "vp3dsp.h"
>>  #include "xiph.h"
>>  
>> +#define VP3_MV_VLC_BITS     6
>>  #define VP4_MV_VLC_BITS     6
>>  #define SUPERBLOCK_VLC_BITS 6
> 
> or just use '#define MV_VLC_BITS 6'
> 
This patch has to modify all the lines that use the motion_vector_vlc
VLC table, so using a named constant can be easily done while at it;
this is no longer true when also changing VP4. Furthermore, there is no
a-priori reason why it should forever be so that both of these are six
(the longest VP3 MV code is only eight bits long, which is not
excessive; I pondered changing it, but decided that it is outside the
remit of this patchset). But if you want to, I can use one constant for
both.

- Andreas