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

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

Paul B Mahol:
> On Tue, Jun 27, 2023 at 11:45 PM Andreas Rheinhardt <
> andreas.rheinhardt@outlook.com> wrote:
> 
>> 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.
>>
> 
> It does not work to use 3 all the time. And that one never worked in any
> codec so far.
> 

I just ran FATE with the check for max_depth removed from GET_VLC and
from read_vlc for the cached API (effectively setting max_depth to 3
everywhere). It passed. So it "works" (but in a suboptimal way). At
least it does if you have ordinary VLCs (created by the vlc.c
functions). Are you doing anything special with them or so?

- Andreas

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