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

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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