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[PATCH] Improve pow (C, x) -> exp (log (C) * x) optimization (PR middle-end/84309)
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Richard Biener <rguenther at suse dot de>, Jeff Law <law at redhat dot com>, Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Sat, 10 Feb 2018 00:37:38 +0100
- Subject: [PATCH] Improve pow (C, x) -> exp (log (C) * x) optimization (PR middle-end/84309)
- Authentication-results: sourceware.org; auth=none
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
Hi!
Apparently the new pow(C,x) -> exp(log(C)*x) if C > 0 optimization
breaks some package (Marek should know which), as it has 7ulp error.
Generally one should be prepared for some errors with -ffast-math.
Though, in this case, if the target has c99 runtime and C is
a positive 0x1pNN it seems much better to use exp2 over exp, for
C being 2 pow (2, x) is optimized into exp2 (x) and even for other
values log2(C) will still be some positive or negative integer, so
in many cases there won't be any rounding errors in the multiplication.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Perhaps we should do something similar if C is a power of 10 (use exp10
and log10).
2018-02-10 Jakub Jelinek <jakub@redhat.com>
PR middle-end/84309
* match.pd (pow(C,x) -> exp(log(C)*x)): Optimize instead into
exp2(log2(C)*x) if C is a power of 2 and c99 runtime is available.
* gcc.dg/pr84309.c: New test.
--- gcc/match.pd.jj 2018-01-26 12:43:23.208922420 +0100
+++ gcc/match.pd 2018-02-09 18:48:26.412021408 +0100
@@ -3992,15 +3992,33 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(logs (pows @0 @1))
(mult @1 (logs @0))))
- /* pow(C,x) -> exp(log(C)*x) if C > 0. */
+ /* pow(C,x) -> exp(log(C)*x) if C > 0,
+ or if C is a positive power of 2,
+ pow(C,x) -> exp2(log2(C)*x). */
(for pows (POW)
exps (EXP)
logs (LOG)
+ exp2s (EXP2)
+ log2s (LOG2)
(simplify
(pows REAL_CST@0 @1)
- (if (real_compare (GT_EXPR, TREE_REAL_CST_PTR (@0), &dconst0)
- && real_isfinite (TREE_REAL_CST_PTR (@0)))
- (exps (mult (logs @0) @1)))))
+ (if (real_compare (GT_EXPR, TREE_REAL_CST_PTR (@0), &dconst0)
+ && real_isfinite (TREE_REAL_CST_PTR (@0)))
+ (with {
+ const REAL_VALUE_TYPE *const value = TREE_REAL_CST_PTR (@0);
+ bool use_exp2 = false;
+ if (targetm.libc_has_function (function_c99_misc)
+ && value->cl == rvc_normal)
+ {
+ REAL_VALUE_TYPE frac_rvt = *value;
+ SET_REAL_EXP (&frac_rvt, 1);
+ if (real_equal (&frac_rvt, &dconst1))
+ use_exp2 = true;
+ }
+ }
+ (if (use_exp2)
+ (exp2s (mult (log2s @0) @1))
+ (exps (mult (logs @0) @1)))))))
(for sqrts (SQRT)
cbrts (CBRT)
--- gcc/testsuite/gcc.dg/pr84309.c.jj 2018-02-09 18:54:52.254787678 +0100
+++ gcc/testsuite/gcc.dg/pr84309.c 2018-02-09 18:59:02.343636178 +0100
@@ -0,0 +1,14 @@
+/* PR middle-end/84309 */
+/* { dg-do run { target c99_runtime } } */
+/* { dg-options "-O2 -ffast-math" } */
+
+int
+main ()
+{
+ unsigned long a = 1024;
+ unsigned long b = 16 * 1024;
+ unsigned long c = __builtin_pow (2, (__builtin_log2 (a) + __builtin_log2 (b)) / 2);
+ if (c != 4096)
+ __builtin_abort ();
+ return 0;
+}
Jakub