[FFmpeg-devel] [PATCH 004/114] avcodec/bitstream: Allow static VLC tables to be bigger than needed

[Andreas Rheinhardt]

Paul B Mahol:
> Because it is a bad idea.

And I still like to hear a reason for this.

> No need to change code that worked for ages.
> 

It allows to remove useless tables and it simplifies making more VLCs
that should be static static.

> On Tue, Nov 10, 2020 at 12:26 PM Andreas Rheinhardt <
> andreas.rheinhardt at gmail.com> wrote:
> 
>> Paul B Mahol:
>>> I do not think this is a good direction.
>>>
>>
>> Why?
>>
>>> On Tue, Nov 10, 2020 at 11:50 AM Andreas Rheinhardt <
>>> andreas.rheinhardt at gmail.com> wrote:
>>>
>>>> Right now the allocated size of the VLC table of a static VLC has to
>>>> exactly match the size actually used for the VLC: If it is not enough,
>>>> abort is called; if it is more than enough, an error message is
>>>> emitted. This is no problem when one wants to initialize an individual
>>>> VLC via one of the INIT_VLC macros as one just hardcodes the needed
>>>> size. Yet it is an obstacle when one wants to initialize several VLCs
>>>> in a loop as one then needs to add an array for the sizes/offsets of
>>>> the VLC tables (unless max_depth of all arrays is one in which case
>>>> the sizes are derivable from the number of bits used).
>>>>
>>>> Yet said size array is not necessary if one removes the warning for too
>>>> big buffers. The reason is that the amount of entries needed for the
>>>> table is of course generated as a byproduct of initializing the VLC.
>>>> So one can proceed as follows:
>>>>
>>>> static VLC vlcs[NUM];
>>>> static VLC_TYPE vlc_table[BUF_SIZE][2];
>>>>
>>>> for (int i = 0, offset = 0; i < NUM; i++) {
>>>>     vlcs[i].table           = &vlc_table[offset];
>>>>     vlcs[i].table_allocated = BUF_SIZE - offset;
>>>>     init_vlc();
>>>>     offset += vlcs[i].table_size;
>>>> }
>>>>
>>>> Of course, BUF_SIZE should be equal to the number of entries actually
>>>> needed.
>>>>
>>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>>>> ---
>>>>  libavcodec/bitstream.c | 3 ---
>>>>  1 file changed, 3 deletions(-)
>>>>
>>>> diff --git a/libavcodec/bitstream.c b/libavcodec/bitstream.c
>>>> index 03d39ad88c..4ffec7e17a 100644
>>>> --- a/libavcodec/bitstream.c
>>>> +++ b/libavcodec/bitstream.c
>>>> @@ -281,9 +281,6 @@ static int vlc_common_end(VLC *vlc, int nb_bits, int
>>>> nb_codes, VLCcode *codes,
>>>>      int ret = build_table(vlc, nb_bits, nb_codes, codes, flags);
>>>>
>>>>      if (flags & INIT_VLC_USE_NEW_STATIC) {
>>>> -        if(vlc->table_size != vlc->table_allocated)
>>>> -            av_log(NULL, AV_LOG_ERROR, "needed %d had %d\n",
>>>> vlc->table_size, vlc->table_allocated);
>>>> -
>>>>          av_assert0(ret >= 0);
>>>>          *vlc_arg = *vlc;
>>>>      } else {
>>>> --
>>>> 2.25.1
>>>>
>>