On Sun, 5 Aug 2018, MCC CS wrote:
Besides, if you think optimizing
/* logN(a) + logN(b) -> logN(a * b) */
would be enough, without the coefficients,
here's a simpler patch:
Index: gcc/match.pd
===================================================================
--- gcc/match.pd (revision 263305)
+++ gcc/match.pd (working copy)
@@ -4015,6 +4015,18 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(exps (logs @0))
@0))
+(for logs (LOG LOG2 LOG10)
You seem to be missing some indentation, see neighboring
transformations.
+ /* logN(a) + logN(b) -> logN(a * b). */
+ (simplify
+ (plus:c (logs:s (@0)) (logs:s (@1)))
+ (logs (mult (@0) (@1))))
No parentheses around @0 and @1. No :c here, the pattern is symmetric
and a permutation would be redundant.
+
+(for logs (LOG LOG2 LOG10)
+ /* logN(a) - logN(b) -> logN(a / b). */
+ (simplify
+ (sub (logs:s (@0)) (logs:s (@1)))
+ (logs (rdiv (@0) (@1))))
It would be possible to merge the 2 with a second 'for', but that's not a
requirement.
+
/* Optimize logN(func()) for various exponential functions. We
want to determine the value "x" and the power "exponent" in
order to transform logN(x**exponent) into exponent*logN(x). */
Index: gcc/testsuite/gcc.dg/pr86710.c
===================================================================
--- gcc/testsuite/gcc.dg/pr86710.c (nonexistent)
+++ gcc/testsuite/gcc.dg/pr86710.c (working copy)
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
The transformation should not happen at -O2, it should require
-funsafe-math-optimizations or similar.
+/* { dg-final { scan-assembler-not "log(.*)log(.*)log" } } */
I think I'd rather scan-tree-dump-times the "optimized" dump.
+
+double c;
+
+void f1 (double x, double s)
+{
+ c = __builtin_log(x) + __builtin_log(s);
+}
+
+void f2 (double x, double s)
+{
+ c = __builtin_log(x) - __builtin_log(s);
+}
You need to specify how you tested the patch (bootstrap on what platform,
run make check, whatever). And then we need to see if other people think
-funsafe-math-optimization is enough protection or the transformation is
too dangerous...
