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

[Michael Niedermayer]

On Fri, Jan 01, 2016 at 05:55:31PM -0800, Ganesh Ajjanagadde wrote:
> On Wed, Dec 30, 2015 at 1:01 AM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> > 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
> 
> Ping: based on the above, final decision left to you, Michael.

maybe av_log2() could be used, so as to avoid any increase in
complexity

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

The greatest way to live with honor in this world is to be what we pretend
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