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

[Michael Niedermayer]

On Fri, Jul 07, 2017 at 08:48:46PM +0200, Paul B Mahol wrote:
> Signed-off-by: Paul B Mahol <onemda at gmail.com>
> ---
>  libavcodec/get_bits.h | 205 +++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 196 insertions(+), 9 deletions(-)
> 
> diff --git a/libavcodec/get_bits.h b/libavcodec/get_bits.h
> index c530015..8a9021a 100644
> --- a/libavcodec/get_bits.h
> +++ b/libavcodec/get_bits.h
> @@ -1,5 +1,6 @@
>  /*
> - * copyright (c) 2004 Michael Niedermayer <michaelni at gmx.at>
> + * Copyright (c) 2004 Michael Niedermayer <michaelni at gmx.at>
> + * Copyright (c) 2016 Alexandra Hájková
>   *
>   * This file is part of FFmpeg.
>   *
> @@ -54,6 +55,10 @@
>  
>  typedef struct GetBitContext {
>      const uint8_t *buffer, *buffer_end;
> +#ifdef CACHED_BITSTREAM_READER
> +    uint64_t cache;
> +    unsigned bits_left;
> +#endif
>      int index;
>      int size_in_bits;
>      int size_in_bits_plus8;
> @@ -106,7 +111,9 @@ typedef struct GetBitContext {
>   * For examples see get_bits, show_bits, skip_bits, get_vlc.
>   */
>  
> -#ifdef LONG_BITSTREAM_READER
> +#ifdef CACHED_BITSTREAM_READER
> +#   define MIN_CACHE_BITS 64
> +#elif defined LONG_BITSTREAM_READER
>  #   define MIN_CACHE_BITS 32
>  #else
>  #   define MIN_CACHE_BITS 25
> @@ -198,7 +205,11 @@ typedef struct GetBitContext {
>  
>  static inline int get_bits_count(const GetBitContext *s)
>  {
> +#ifdef CACHED_BITSTREAM_READER
> +    return s->index - s->bits_left;
> +#else
>      return s->index;
> +#endif
>  }
>  
>  static inline void skip_bits_long(GetBitContext *s, int n)
> @@ -210,6 +221,68 @@ static inline void skip_bits_long(GetBitContext *s, int n)
>  #endif
>  }
>  
> +static inline void refill_32(GetBitContext *s)
> +{
> +#ifdef CACHED_BITSTREAM_READER

> +    if (s->buffer + (s->index >> 3) >= s->buffer_end)
> +        return;

should be under !UNCHECKED_BITSTREAM_READER

also this looks like it can result in intermediate invalid pointers
this should avoid that:
s->index >> 3 >= s->buffer_end - s->buffer
same issue in refill_64()

patch overall is nice

thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data
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