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

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Paul B Mahol:
> On Tue, Jun 27, 2023 at 11:05 PM Andreas Rheinhardt <
> andreas.rheinhardt@outlook.com> wrote:
> 
>> Paul B Mahol:
>>> On Tue, Jun 27, 2023 at 9:47 PM Andreas Rheinhardt <
>>> andreas.rheinhardt@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.

- Andreas

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