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