match.pd: Add std::pow folding optimizations.

This patch adds the following two simplifications in match.pd for
POW_ALL and POWI:
- pow (1.0/x, y) to pow (x, -y), avoiding the division
- pow (0.0, x) to 0.0, avoiding the call to pow.
The patterns are guarded by flag_unsafe_math_optimizations,
!flag_trapping_math, and !HONOR_INFINITIES.
The POW_ALL patterns are also gated under !flag_errno_math.
The second pattern is also guarded by !HONOR_NANS and
!HONOR_SIGNED_ZEROS.

Tests were added to confirm the application of the transform for
builtins pow, powf, powl, powi, powif, powil, and powf16.

The patch was bootstrapped and regtested on aarch64-linux-gnu and
x86_64-linux-gnu, no regression.
OK for mainline?

Signed-off-by: Jennifer Schmitz <jschmitz@nvidia.com>

gcc/
	* match.pd: Fold pow (1.0/x, y) -> pow (x, -y) and
	pow (0.0, x) -> 0.0.

gcc/testsuite/
	* gcc.dg/tree-ssa/pow_fold_1.c: New test.

2 files changed, 70 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index f16b733..809c717 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -8285,6 +8285,21 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
    (rdiv @0 (exps:s @1))
     (mult @0 (exps (negate @1)))))
 
+ (for pow (POW_ALL)
+  (if (! HONOR_INFINITIES (type)
+       && ! flag_trapping_math
+       && ! flag_errno_math)
+   /* Simplify pow(1.0/x, y) into pow(x, -y).  */
+   (simplify
+    (pow (rdiv:s real_onep@0 @1) @2)
+     (pow @1 (negate @2)))
+
+   /* Simplify pow(0.0, x) into 0.0.  */
+   (if (! HONOR_NANS (type) && ! HONOR_SIGNED_ZEROS (type))
+    (simplify
+     (pow real_zerop@0 @1)
+      @0))))
+
  (if (! HONOR_SIGN_DEPENDENT_ROUNDING (type)
       && ! HONOR_NANS (type) && ! HONOR_INFINITIES (type)
       && ! flag_trapping_math
@@ -8643,6 +8658,19 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (mult (POW:s @0 @1) (POW:s @2 @1))
    (POW (mult @0 @2) @1))
 
+ (if (! HONOR_INFINITIES (type) && ! flag_trapping_math)
+  /* Simplify powi(1.0/x, y) into powi(x, -y).  */
+  (simplify
+   (POWI (rdiv@3 real_onep@0 @1) @2)
+   (if (single_use (@3))
+    (POWI @1 (negate @2))))
+
+  /* Simplify powi(0.0, x) into 0.0.  */
+  (if (! HONOR_NANS (type) && ! HONOR_SIGNED_ZEROS (type))
+   (simplify
+    (POWI real_zerop@0 @1)
+     @0)))
+
  /* Simplify powi(x,y) * powi(z,y) -> powi(x*z,y). */
  (simplify
   (mult (POWI:s @0 @1) (POWI:s @2 @1))
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pow_fold_1.c b/gcc/testsuite/gcc.dg/tree-ssa/pow_fold_1.c
new file mode 100644
index 0000000..d98bcb0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pow_fold_1.c
@@ -0,0 +1,42 @@
+/* { dg-do compile } */
+/* { dg-options "-Ofast -fdump-tree-optimized -fexcess-precision=16" } */
+/* { dg-add-options float16 } */
+/* { dg-require-effective-target float16_runtime } */
+/* { dg-require-effective-target c99_runtime } */
+
+extern void link_error (void);
+
+#define POW1OVER(TYPE1, TYPE2, CTY, TY)			\
+  void							\
+  pow1over_##TY (TYPE1 x, TYPE2 y)			\
+  {							\
+    TYPE1 t1 = 1.0##CTY / x;				\
+    TYPE1 t2 = __builtin_pow##TY (t1, y);		\
+    TYPE2 t3 = -y;					\
+    TYPE1 t4 = __builtin_pow##TY (x, t3);		\
+    if (t2 != t4)					\
+      link_error ();					\
+  }							\
+
+#define POW0(TYPE1, TYPE2, CTY, TY)			\
+  void							\
+  pow0_##TY (TYPE2 x)					\
+  {							\
+    TYPE1 t1 = __builtin_pow##TY (0.0##CTY, x);		\
+    if (t1 != 0.0##CTY)					\
+      link_error ();					\
+  }							\
+
+#define TEST_ALL(TYPE1, TYPE2, CTY, TY)			\
+  POW1OVER (TYPE1, TYPE2, CTY, TY)			\
+  POW0 (TYPE1, TYPE2, CTY, TY)
+
+TEST_ALL (double, double, , )
+TEST_ALL (float, float, f, f)
+TEST_ALL (_Float16, _Float16, f16, f16)
+TEST_ALL (long double, long double, L, l)
+TEST_ALL (double, int, , i)
+TEST_ALL (float, int, f, if)
+TEST_ALL (long double, int, L, il)
+
+/* { dg-final { scan-tree-dump-not "link_error" "optimized" } } */