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

[Paul B Mahol]

I do not think this is a good direction.

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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