[FFmpeg-devel] [PATCH] avcodec/put_bits: Always check buffer end before writing

[Paul B Mahol]

On 1/1/16, Michael Niedermayer <michaelni at gmx.at> wrote:
> From: Michael Niedermayer <michael at niedermayer.cc>
>
> This causes a overall slowdown of 0.1 % (tested with mpeg4 single thread
> encoding of matrixbench at QP=3)
>
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
>  libavcodec/put_bits.h |   16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/libavcodec/put_bits.h b/libavcodec/put_bits.h
> index 5b1bc8b..69a3049 100644
> --- a/libavcodec/put_bits.h
> +++ b/libavcodec/put_bits.h
> @@ -163,9 +163,11 @@ static inline void put_bits(PutBitContext *s, int n,
> unsigned int value)
>  #ifdef BITSTREAM_WRITER_LE
>      bit_buf |= value << (32 - bit_left);
>      if (n >= bit_left) {
> -        av_assert2(s->buf_ptr+3<s->buf_end);
> -        AV_WL32(s->buf_ptr, bit_buf);
> -        s->buf_ptr += 4;
> +        if (3 < s->buf_end - s->buf_ptr) {
> +            AV_WL32(s->buf_ptr, bit_buf);
> +            s->buf_ptr += 4;
> +        } else
> +            av_assert0(0);
>          bit_buf     = value >> bit_left;
>          bit_left   += 32;
>      }
> @@ -177,9 +179,11 @@ static inline void put_bits(PutBitContext *s, int n,
> unsigned int value)
>      } else {
>          bit_buf   <<= bit_left;
>          bit_buf    |= value >> (n - bit_left);
> -        av_assert2(s->buf_ptr+3<s->buf_end);
> -        AV_WB32(s->buf_ptr, bit_buf);
> -        s->buf_ptr += 4;
> +        if (3 < s->buf_end - s->buf_ptr) {
> +            AV_WB32(s->buf_ptr, bit_buf);
> +            s->buf_ptr += 4;
> +        } else
> +            av_assert0(0);
>          bit_left   += 32 - n;
>          bit_buf     = value;
>      }

If this can happen, using assert0 is bad idea.
If this should not happen add if under assert2.