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

[Richard Biener]

On February 19, 2018 11:02:50 PM GMT+01:00, Jakub Jelinek <jakub@redhat.com> wrote:
>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?

OK. 

Richard. 

>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