[FFmpeg-devel] [PATCH 13/21] avcodec/smacker: Remove redundant checks when reading VLC codes

[Andreas Rheinhardt]

Paul B Mahol:
> On 8/1/20, Andreas Rheinhardt <andreas.rheinhardt at gmail.com> wrote:
>> The VLC codes in question originate from a Huffmann tree and so every
>> sequence of bits that is longer than the longest code contains an
>> initial sequence that is a valid code. Given that it has been checked
>> during reading said tree (and once again in ff_init_vlc_sparse()) that
>> the length of each code is <= 3 * the number of bits read at once when
>> reading codes, get_vlc2() will always find a matching entry.
>>
>> These checks have been added in 71d3c25a7ef442ac2dd7b6fbf7c489ebc0b58e9b
>> at a time when the length of the codes had not been checked when parsing
>> the tree.
>>
>> For GCC 9 and the sample from ticket #2425 this led to a slight
>> performance regression: The time for one call to smka_decode_frame()
>> increased from 2053671 to 2064529 decicycles; for Clang 9, performance
>> improved from 1521288 to 1508459 decicycles.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>>  libavcodec/smacker.c | 32 --------------------------------
>>  1 file changed, 32 deletions(-)
>>
> 
> Sure this does not continue under bitstream errors?
> 
> Try some smart fuzzers.
> 
I have now finally fuzzed my patches here and found no bugs at all
(neither with ASAN nor with UBSan). I'll therefore apply it.

- Andreas