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

[Michael Niedermayer]

On Mon, Jan 04, 2016 at 05:38:46PM -0800, Ganesh Ajjanagadde wrote:
> On Sat, Jan 2, 2016 at 7:59 AM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> > 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: final thoughts?

is there any loss of usefullness of the output with av_log2() ?
i dont think we need to maintain exactly identical output for
such fancy / debug output


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

Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: Digital signature
URL: <http://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20160106/309779d7/attachment.sig>