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

[Ganesh Ajjanagadde]

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?

>
>>
>> [...]
>> --
>> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>>
>> The greatest way to live with honor in this world is to be what we pretend
>> to be. -- Socrates
>>
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