Add new log2 implementation

Similar algorithm is used as in log, but there are more operations
(and more error) due to the 1/ln2 multiplier.

There is separate code path when fma instruction is not available for
computing x/c - 1 precisely, for which the table size is doubled,
and to compute (x/c - 1)/ln2 precisely.

The worst case error is 0.547 ULP (0.55 without fma), the read only
global data size is 1168 bytes (2192 without fma).  The non-nearest
rounding error is less than 1 ULP.

Improvements on Cortex-A72 compared to current glibc master:
log latency: 2.04x
log thruput: 1.87x

1 files changed, 1 insertions, 0 deletions

diff --git a/math/include/mathlib.h b/math/include/mathlib.h
index 17fdcf4..7a544d1 100644
--- a/math/include/mathlib.h
+++ b/math/include/mathlib.h
@@ -32,3 +32,4 @@ void sincosf(float, float*, float*);
 double exp(double);
 double exp2(double);
 double log(double);
+double log2(double);