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

[Hendrik Leppkes]

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.

> +    return exp2(M_LOG2_10 * x);
> +}
> +#endif /* HAVE_EXP10 */
> +
> +#if !HAVE_EXP10F
> +static av_always_inline float exp10f(float x)
> +{
> +#ifndef M_LOG2_10
> +#define M_LOG2_10      3.32192809488736234787  /* log_2 10 */
> +#endif
> +    return exp2f(M_LOG2_10 * x);
> +}
> +#endif /* HAVE_EXP10F */
> +
>  #if !HAVE_ISINF
>  #undef isinf
>  /* Note: these do not follow the BSD/Apple/GNU convention of returning -1 for
> --
> 2.6.4
>
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