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 <ja...@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
