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

[Ganesh Ajjanagadde]

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
+    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