From e29db08cf7f68fefe18f043250e2292aaf91ef3b Mon Sep 17 00:00:00 2001 From: Ganesh Ajjanagadde Date: Mon, 21 Dec 2015 19:05:00 -0800 Subject: lavu/libm: add exp10 support 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: Hendrik Leppkes Reviewed-by: Michael Niedermayer Reviewed-by: Ronald S. Bultje Signed-off-by: Ganesh Ajjanagadde --- libavutil/libm.h | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) (limited to 'libavutil/libm.h') diff --git a/libavutil/libm.h b/libavutil/libm.h index 146768aac6..6f9ac1b349 100644 --- a/libavutil/libm.h +++ b/libavutil/libm.h @@ -29,6 +29,7 @@ #include "config.h" #include "attributes.h" #include "intfloat.h" +#include "mathematics.h" #if HAVE_MIPSFPU && HAVE_INLINE_ASM #include "libavutil/mips/libm_mips.h" @@ -292,6 +293,24 @@ 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) +{ + return exp2(M_LOG2_10 * x); +} +#endif /* HAVE_EXP10 */ + +#if !HAVE_EXP10F +static av_always_inline float exp10f(float x) +{ + 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 -- cgit v1.2.3