[FFmpeg-devel] [PATCH] avcodec/vlc: auto calculate depth

[Paul B Mahol]

On Tue, Jun 27, 2023 at 11:45 PM Andreas Rheinhardt <
andreas.rheinhardt at outlook.com> wrote:

> Paul B Mahol:
> > On Tue, Jun 27, 2023 at 11:05 PM Andreas Rheinhardt <
> > andreas.rheinhardt at outlook.com> wrote:
> >
> >> Paul B Mahol:
> >>> On Tue, Jun 27, 2023 at 9:47 PM Andreas Rheinhardt <
> >>> andreas.rheinhardt at outlook.com> wrote:
> >>>
> >>>> Paul B Mahol:
> >>>>> Patch attached.
> >>>>>
> >>>>
> >>>> Where do you intend to use this? What is the point of it?
> >>>> After all, using this value in GET_VLC makes no sense; only
> compile-time
> >>>> constants do.
> >>>>
> >>>
> >>> It works when used in ac-4 as get_vlc2.
> >>>
> >>
> >> Could you please define "works"? Using a non-compile-time constant will
> >> not benefit at all; it will only lead to more runtime checks.
> >>
> >
> > I do not follow your worries.
> > I can not use compile time constant as its very complicated code.
> >
>
> Let's take a look at GET_VLC:
> #define GET_VLC(code, name, gb, table, bits, max_depth)         \
>     do {                                                        \
>         int n, nb_bits;                                         \
>         unsigned int index;                                     \
>                                                                 \
>         index = SHOW_UBITS(name, gb, bits);                     \
>         code  = table[index].sym;                               \
>         n     = table[index].len;                               \
>                                                                 \
>         if (max_depth > 1 && n < 0) {                           \
>             LAST_SKIP_BITS(name, gb, bits);                     \
>             UPDATE_CACHE(name, gb);                             \
>                                                                 \
>             nb_bits = -n;                                       \
>                                                                 \
>             index = SHOW_UBITS(name, gb, nb_bits) + code;       \
>             code  = table[index].sym;                           \
>             n     = table[index].len;                           \
>             if (max_depth > 2 && n < 0) {                       \
>                 LAST_SKIP_BITS(name, gb, nb_bits);              \
>                 UPDATE_CACHE(name, gb);                         \
>                                                                 \
>                 nb_bits = -n;                                   \
>                                                                 \
>                 index = SHOW_UBITS(name, gb, nb_bits) + code;   \
>                 code  = table[index].sym;                       \
>                 n     = table[index].len;                       \
>             }                                                   \
>         }                                                       \
>         SKIP_BITS(name, gb, n);                                 \
>     } while (0)
>
> If max_depth is not a compile-time constant, then the compiler will have
> to perform both of the max_depth > 1 && n < 0 checks; yet, this is not
> useful: If the depth of a particular VLC is (say) 1, then none of the
> possible bits read will lead to reloading at all, because the n < 0
> condition will never be true; the only reason this condition exists is
> to use a compile-time upper bound, so that one can eliminate the reload
> code (and in particular, avoid the runtime checks).
>
> > Works means that vlc code is extracted correctly.
> >
>
> If you have no upper bound about max_depth and it works, then use 3.
>

It does not work to use 3 all the time. And that one never worked in any
codec so far.


>
> - Andreas
>
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