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

[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.