[PATCH] Defer pow (C, x) folding until after vectorization always (PR middle-end/82004)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

While I've over-simplified the testcase and so this patch doesn't help
the 628.pop2_s miscompare, I still believe it is beneficial to defer this
folding until late for these reasons:
1) if we propagate a constant into the second pow argument too, it will
   be likely more precise than going through the exp (cst * x) way
2) except when C is M_E, pow is fewer operations and thus smaller IL

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2018-02-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/82004
	* match.pd (pow(C,x) -> exp(log(C)*x)): Delay all folding until
	after vectorization.

	* gfortran.dg/pr82004.f90: New test.

--- gcc/match.pd.jj	2018-02-15 12:15:51.655780636 +0100
+++ gcc/match.pd	2018-02-19 17:38:06.390763194 +0100
@@ -4006,7 +4006,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (simplify
    (pows REAL_CST@0 @1)
    (if (real_compare (GT_EXPR, TREE_REAL_CST_PTR (@0), &dconst0)
-	&& real_isfinite (TREE_REAL_CST_PTR (@0)))
+	&& real_isfinite (TREE_REAL_CST_PTR (@0))
+	/* As libmvec doesn't have a vectorized exp2, defer optimizing
+	   the use_exp2 case until after vectorization.  It seems actually
+	   beneficial for all constants to postpone this until later,
+	   because exp(log(C)*x), while faster, will have worse precision
+	   and if x folds into a constant too, that is unnecessary
+	   pessimization.  */
+	&& canonicalize_math_after_vectorization_p ())
     (with {
        const REAL_VALUE_TYPE *const value = TREE_REAL_CST_PTR (@0);
        bool use_exp2 = false;
@@ -4021,10 +4028,7 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
      }
      (if (!use_exp2)
       (exps (mult (logs @0) @1))
-      /* As libmvec doesn't have a vectorized exp2, defer optimizing
-	 this until after vectorization.  */
-      (if (canonicalize_math_after_vectorization_p ())
-	(exp2s (mult (log2s @0) @1))))))))
+      (exp2s (mult (log2s @0) @1)))))))
 
  (for sqrts (SQRT)
       cbrts (CBRT)
--- gcc/testsuite/gfortran.dg/pr82004.f90.jj	2018-02-19 17:58:57.435682156 +0100
+++ gcc/testsuite/gfortran.dg/pr82004.f90	2018-02-19 17:58:34.127684892 +0100
@@ -0,0 +1,18 @@
+! PR middle-end/82004
+! { dg-do run }
+! { dg-options "-Ofast" }
+
+  integer, parameter :: r8 = selected_real_kind(13), i4 = kind(1)
+  integer (i4), parameter :: a = 400, b = 2
+  real (r8), parameter, dimension(b) :: c = (/ .001_r8, 10.00_r8 /)
+  real (r8) :: d, e, f, g, h
+  real (r8), parameter :: j &
+    = 10**(log10(c(1))-(log10(c(b))-log10(c(1)))/real(a))
+
+  d = c(1)
+  e = c(b)
+  f = (log10(e)-log10(d))/real(a)
+  g = log10(d) - f
+  h = 10**(g)
+  if (h.ne.j) stop 1
+end

	Jakub