Re: [FFmpeg-devel] [PATCH] avcodec/xbmdec: Use LUT for hex char->number conversion

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Andreas Rheinhardt:
> Improves performance by about 30%.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
> [1] is a patch that would also move the bitreverse into the LUT;
> it didn't make a difference in the benchmarks here, but if it is
> an improvement for someone else, it could still be applied.
> [1]: https://github.com/mkver/FFmpeg/commit/d4fdac5335d2ccf29f7b55fe3147edf388bb7efa
>  libavcodec/xbmdec.c | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/libavcodec/xbmdec.c b/libavcodec/xbmdec.c
> index ecfc957a3f..2d84327e02 100644
> --- a/libavcodec/xbmdec.c
> +++ b/libavcodec/xbmdec.c
> @@ -28,17 +28,22 @@
>  
>  static int get_nibble(uint8_t x)
>  {
> -    int ret = 255;
> -
> -    if (x <= '9') {
> -        if (x >= '0')
> -            ret = x - '0';
> -    } else if (x >= 'a') {
> -        if (x <= 'f')
> -            ret = x - ('a' - 10);
> -    } else if (x >= 'A' && x <= 'F')
> -        ret = x - ('A' - 10);
> -    return ret;
> +#define TIMES256(idx) \
> +TIMES64(4 * (idx)) TIMES64(4 * (idx) + 1) TIMES64(4 * (idx) + 2) TIMES64(4 * (idx) + 3)
> +#define TIMES64(idx) \
> +TIMES16(4 * (idx)) TIMES16(4 * (idx) + 1) TIMES16(4 * (idx) + 2) TIMES16(4 * (idx) + 3)
> +#define TIMES16(idx) \
> +TIMES4(4 * (idx)) TIMES4(4 * (idx) + 1) TIMES4(4 * (idx) + 2) TIMES4(4 * (idx) + 3)
> +#define TIMES4(idx) \
> +ENTRY(4 * (idx)) ENTRY(4 * (idx) + 1) ENTRY(4 * (idx) + 2) ENTRY(4 * (idx) + 3)
> +#define ENTRY(x) [x] = ((x) >= 'a' && (x) <= 'f') ? (x) - ('a' - 10) : \
> +                       ((x) >= 'A' && (x) <= 'F') ? (x) - ('A' - 10) : \
> +                       ((x) >= '0' && (x) <= '9') ? (x) - '0' : 255,
> +
> +    static const uint8_t lut[] = {
> +        TIMES256(0)
> +    };
> +    return lut[x];
>  }
>  
>  static int parse_str_int(const uint8_t *p, const uint8_t *end, const uint8_t *key)

Will apply.

- Andreas

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