[PATCH] Simplify pow with constant

[Wilco Dijkstra]

This patch simplifies pow (C, x) into exp (x * C1), where C1 = log (C).
Do this only for fast-math as accuracy is reduced.  This is much faster
since pow is more complex than exp - with a current GLIBC the speedup
is more than 7 times for this transformation.

ChangeLog:
2017-08-04  Wilco Dijkstra  <wdijk...@arm.com>

        * match.pd: Add pow (C, x) simplification.

--

diff --git a/gcc/match.pd b/gcc/match.pd
index 
e98db52af84946cf579c6434e06d450713a47162..96486aa1f512fe32d85a1de95c46523263ea1b6d
 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3548,6 +3548,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
    (logs (pows @0 @1))
    (mult @1 (logs @0))))
 
+ /* pow(C,x) -> exp(log(C)*x).  */
+ (for pows (POW)
+      exps (EXP)
+      logs (LOG)
+  (simplify
+   (pows REAL_CST@0 @1)
+   (exps (mult (logs @0) @1))))
+
  (for sqrts (SQRT)
       cbrts (CBRT)
       pows (POW)