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

[Ganesh Ajjanagadde]

On Sat, Jan 2, 2016 at 6:24 AM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> 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

Not really, or at least not without additional code. The lrint rounds
the log; av_log2 either returns the log of the next higher power of 2,
or the one before it. Thus they are quite different things. I don't
know if this is wrong or not; I created this to preserve identical
behavior.

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