[FFmpeg-devel] [PATCH] ffmpeg: replace log2 by faster variant

[Hendrik Leppkes]

On Wed, Dec 30, 2015 at 4:39 AM, Ganesh Ajjanagadde
<gajjanagadde at gmail.com> wrote:
> The log is anyway rounded to an integer, so one may use an frexp
> based approach. Note that this may be made frexpf; if arguments are less than
> 2^24 there is no loss. Kept as double precision for simplicity; 2^32 is
> exactly representable as a double.
>
> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> ---
>  ffmpeg.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/ffmpeg.c b/ffmpeg.c
> index 6d01987..ee72f91 100644
> --- a/ffmpeg.c
> +++ b/ffmpeg.c
> @@ -1486,6 +1486,17 @@ static void print_final_stats(int64_t total_size)
>      }
>  }
>
> +static inline int log2i(double d)
> +{
> +    int exp;
> +    double mant;
> +
> +    mant = frexp(d, &exp);
> +    if (mant >= M_SQRT1_2)
> +        return exp;
> +    return exp-1;
> +}
> +
>  static void print_report(int is_last_report, int64_t timer_start, int64_t cur_time)
>  {
>      char buf[1024];
> @@ -1559,7 +1570,7 @@ static void print_report(int is_last_report, int64_t timer_start, int64_t cur_ti
>                  if (qp >= 0 && qp < FF_ARRAY_ELEMS(qp_histogram))
>                      qp_histogram[qp]++;
>                  for (j = 0; j < 32; j++)
> -                    snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "%X", (int)lrintf(log2(qp_histogram[j] + 1)));
> +                    snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "%X", log2i(qp_histogram[j] + 1));
>              }
>
>              if ((enc->flags & AV_CODEC_FLAG_PSNR) && (ost->pict_type != AV_PICTURE_TYPE_NONE || is_last_report)) {
> --
> 2.6.4

This isn't exactly a performance critical area, and defining a custom
function just for this seems somewhat like over-optimization.
Just my opinion, of course, I'll leave the decision up to the
maintainers of ffmpeg.c