[FFmpeg-devel] [PATCH 1/3] avcodec/get_bits: add cached bitstream reader

[foo86]

On Sat, Jul 08, 2017 at 11:12:06AM +0200, Paul B Mahol wrote:
> [...]

>  static inline void skip_bits(GetBitContext *s, int n)
>  {
> +#ifdef CACHED_BITSTREAM_READER
> +    if (n <= s->bits_left)
> +        skip_remaining(s, n);
> +    else {
> +        n -= s->bits_left;
> +        skip_remaining(s, s->bits_left);
This causes undefined behavior if s->bits_left == 64.

> +        if (n >= 64) {
> +            unsigned skip = n;
> +
> +            n -= skip;
> +            s->index += skip;
> +        }
This block looks strange.

> +        refill_32(s);
> +        if (n)
> +            skip_remaining(s, n);
> +    }
> +#else
>      OPEN_READER(re, s);
>      LAST_SKIP_BITS(re, s, n);
>      CLOSE_READER(re, s);
> +#endif
> +}
> +
> +static inline void skip_bits_long(GetBitContext *s, int n)
> +{
> +#ifdef CACHED_BITSTREAM_READER
> +    skip_bits(s, n);
> +#else
> +#if UNCHECKED_BITSTREAM_READER
> +    s->index += n;
> +#else
> +    s->index += av_clip(n, -s->index, s->size_in_bits_plus8 - s->index);
Uncached bitstream reader allows seeking back by passing negative n
here. If cached bitstream reader disallows this, there should be a
comment saying so (and possibly an assert).