[FFmpeg-devel] [PATCH] lavu/libm: add exp10 support

[Ganesh Ajjanagadde]

On Tue, Dec 22, 2015 at 4:17 PM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> On Wed, Dec 23, 2015 at 12:29 AM, Ganesh Ajjanagadde
> <gajjanagadde at gmail.com> wrote:
>> exp10 is a function available in GNU libm. Looks like no other common
>> libm has it. This adds support for it to FFmpeg.
>>
>> There are essentially 2 ways of handling the fallback:
>> 1. Using pow(10, x)
>> 2. Using exp2(M_LOG2_10 * x).
>>
>> First one represents a Pareto improvement, with no speed or accuracy
>> regression anywhere, but speed improvement limited to GNU libm.
>>
>> Second one represents a slight accuracy loss (relative error ~ 1e-13)
>> for non GNU libm. Speedup of > 2x is obtained on non GNU libm platforms,
>> ~30% on GNU libm. These are "average case numbers", another benefit is
>> the lack of triggering of the well-known terrible worst case paths
>> through pow.
>>
>> Based on reviews, second one chosen. Comment added accordingly.
>>
>> Reviewed-by: Michael Niedermayer <michael at niedermayer.cc>
>> Reviewed-by: Ronald S. Bultje <rsbultje at gmail.com>
>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>> ---
>>  configure        |  2 ++
>>  libavutil/libm.h | 24 ++++++++++++++++++++++++
>>  2 files changed, 26 insertions(+)
>>
>> diff --git a/configure b/configure
>> index 54c9789..ea717a5 100755
>> --- a/configure
>> +++ b/configure
>> @@ -1815,6 +1815,8 @@ MATH_FUNCS="
>>      copysign
>>      cosf
>>      erf
>> +    exp10
>> +    exp10f
>>      exp2
>>      exp2f
>>      expf
>> diff --git a/libavutil/libm.h b/libavutil/libm.h
>> index 146768a..9705755 100644
>> --- a/libavutil/libm.h
>> +++ b/libavutil/libm.h
>> @@ -292,6 +292,30 @@ static inline double erf(double z)
>>  #define exp2f(x) ((float)exp2(x))
>>  #endif /* HAVE_EXP2F */
>>
>> +/* Somewhat inaccurate fallbacks, relative error ~ 1e-13 concentrated on very
>> +small and very large values. For perfection accuracy-wise, should use pow.
>> +Speed benefits (>2x average, with no super slow paths) deemed to be worth the
>> +accuracy tradeoff */
>> +#if !HAVE_EXP10
>> +static av_always_inline double exp10(double x)
>> +{
>> +#ifndef M_LOG2_10
>> +#define M_LOG2_10      3.32192809488736234787  /* log_2 10 */
>> +#endif
>
> Should include mathematic.h for this constant, it already defines
> common constants if not present.

Did not know if it was safe or not. Changed locally, thanks.

[...]