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

[Paul B Mahol]

On Tue, Nov 10, 2020 at 12:31 PM Andreas Rheinhardt <
andreas.rheinhardt at gmail.com> wrote:

> Paul B Mahol:
> > Because it is a bad idea.
>
> And I still like to hear a reason for this.
>
>
Code is there so correct intended size is always allocated.


> > 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.
>

What is static static? How does it allows to remove useless tables?


>
> > 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
> >>>>
> >>
>
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