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

[Paul B Mahol]

Because it is a bad idea.
No need to change code that worked for ages.

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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