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

[Ganesh Ajjanagadde]

On Tue, Jan 5, 2016 at 7:03 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Tue, Jan 5, 2016 at 6:44 PM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
>> 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
>
> AFAIK none, but I was not around when it was made, and I am not a
> heavy user :). Replaced by av_log2 locally, will push tomorrow in
> absence of further comments.

pushed, thanks.

>
>>
>>
>> [...]
>> --
>> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>>
>> Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
>>
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