On Tue, 7 May 2019, Jakub Jelinek wrote:
> Hi!
>
> fold_real_zero_addition_p will fold x + (-0.0) or x - 0.0 to x
> when not -frounding-math, but not the rest of the options when
> -fsigned-zeros, and not when -fsignaling-nans.
> If we have (x + 0.0) + 0.0, we can fold that to just x + 0.0 even
> when honoring signed zeros, and IMNSHO even when honoring sNaNs,
> of course unless -frounding-math, then we can't do anything.
> For x other than 0.0, -0.0 and sNaN it is obviously correct, for sNaN
> sNaN + 0.0 will raise an exception and turn the result into qNaN, which
> will not raise further exception on the second addition, so IMHO it is ok
> too (unless we want to say special case -fnon-call-exceptions and the
> exception handler changing the result back to sNaN and expecting yet another
> exception). For 0.0/-0.0 if we can assume rounding other than towards
> negative infinity, the results are:
> x x
> (0.0 + 0.0) + 0.0 = 0.0 = (0.0 + 0.0)
> (-0.0 + 0.0) + 0.0 = 0.0 = (-0.0 + 0.0)
> (0.0 - 0.0) - 0.0 = 0.0 = (0.0 - 0.0)
> (-0.0 - 0.0) - 0.0 = -0.0 = (-0.0 - 0.0)
> (0.0 + 0.0) - 0.0 = 0.0 = (0.0 + 0.0)
> (-0.0 + 0.0) - 0.0 = 0.0 = (-0.0 + 0.0)
> For the above ones, the two operations are always equal to the inner operation
> (0.0 - 0.0) + 0.0 = 0.0 = 0.0 + 0.0
> (-0.0 - 0.0) + 0.0 = 0.0 = -0.0 + 0.0
> For the above cases, the two operations are always equal to the outer
> operation
>
> If it is y + (-0.0), it is equivalent to y - 0.0 and if it is y - (-0.0),
> it is equivalent to y + 0.0 in the above.
>
> For rounding towards negative infinity, 0.0 - 0.0 is -0.0 rather than 0.0
> and so some of the above equivalencies are not true.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2019-05-07 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/90356
> * match.pd ((X +/- 0.0) +/- 0.0): Optimize into X +/- 0.0 if possible.
>
> * gcc.dg/tree-ssa/pr90356-1.c: New test.
> * gcc.dg/tree-ssa/pr90356-2.c: New test.
> * gcc.dg/tree-ssa/pr90356-3.c: New test.
> * gcc.dg/tree-ssa/pr90356-4.c: New test.
>
> --- gcc/match.pd.jj 2019-05-03 15:22:07.370401908 +0200
> +++ gcc/match.pd 2019-05-06 11:26:04.701663020 +0200
> @@ -152,6 +152,28 @@ (define_operator_list COND_TERNARY
> (if (fold_real_zero_addition_p (type, @1, 1))
> (non_lvalue @0)))
>
> +/* Even if the fold_real_zero_addition_p can't simplify X + 0.0
> + into X, we can optimize (X + 0.0) + 0.0 or (X + 0.0) - 0.0
> + or (X - 0.0) + 0.0 into X + 0.0 and (X - 0.0) - 0.0 into X - 0.0
> + if not -frounding-math. For sNaNs the first operation would raise
> + exceptions but turn the result into qNan, so the second operation
> + would not raise it. */
> +(for inner_op (plus minus)
> + (for outer_op (plus minus)
> + (simplify
> + (outer_op (inner_op @0 real_zerop@1) real_zerop@2)
> + (if (TREE_CODE (@1) == REAL_CST
> + && TREE_CODE (@2) == REAL_CST
Will leave the "correctness check" for other folks but the above is
better written as
+ (outer_op (inner_op @0 REAL_CST@1) REAL_CST@2)
+ (if (real_zerop (@1)
+ && real_zerop (@2)
because that gets code-generated better. Btw, for -fsignalling-nans
can we have a literal sNaN? Then you need :c on the inner_op since
I'm not sure we canonicalize to sNaN + 0.0 rather than 0.0 + sNaN.
Maybe not worth optimizing though (since we rule out -frounding-math
a similar case there doesn't need to be considered).
> + && HONOR_SIGNED_ZEROS (element_mode (type))
> + && !HONOR_SIGN_DEPENDENT_ROUNDING (element_mode (type)))
You can write HONOR_SIGNED_ZEROS (type) here for brevity.
> + (with { bool plus1 = ((inner_op == PLUS_EXPR)
> + ^ REAL_VALUE_MINUS_ZERO (TREE_REAL_CST (@1)));
> + bool plus2 = ((outer_op == PLUS_EXPR)
> + ^ REAL_VALUE_MINUS_ZERO (TREE_REAL_CST (@2))); }
> + (if (plus2 && !plus1)
> + (outer_op @0 @2)
> + (inner_op @0 @1)))))))
> +
> /* Simplify x - x.
> This is unsafe for certain floats even in non-IEEE formats.
> In IEEE, it is unsafe because it does wrong for NaNs.
> --- gcc/testsuite/gcc.dg/tree-ssa/pr90356-1.c.jj 2019-05-06
> 11:39:58.998288472 +0200
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr90356-1.c 2019-05-06 11:42:53.597489688
> +0200
> @@ -0,0 +1,23 @@
> +/* PR tree-optimization/90356 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-rounding-math -fsignaling-nans -fsigned-zeros
> -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump-times "x_\[0-9]*.D. \\+ 0.0;" 12 "optimized"
> } } */
> +/* { dg-final { scan-tree-dump-times "y_\[0-9]*.D. - 0.0;" 4 "optimized" } }
> */
> +/* { dg-final { scan-tree-dump-times " \[+-] 0.0;" 16 "optimized" } } */
> +
> +double f1 (double x) { return (x + 0.0) + 0.0; }
> +double f2 (double y) { return (y + (-0.0)) + (-0.0); }
> +double f3 (double y) { return (y - 0.0) - 0.0; }
> +double f4 (double x) { return (x - (-0.0)) - (-0.0); }
> +double f5 (double x) { return (x + 0.0) - 0.0; }
> +double f6 (double x) { return (x + (-0.0)) - (-0.0); }
> +double f7 (double x) { return (x - 0.0) + 0.0; }
> +double f8 (double x) { return (x - (-0.0)) + (-0.0); }
> +double f9 (double x) { double t = x + 0.0; return t + 0.0; }
> +double f10 (double y) { double t = y + (-0.0); return t + (-0.0); }
> +double f11 (double y) { double t = y - 0.0; return t - 0.0; }
> +double f12 (double x) { double t = x - (-0.0); return t - (-0.0); }
> +double f13 (double x) { double t = x + 0.0; return t - 0.0; }
> +double f14 (double x) { double t = x + (-0.0); return t - (-0.0); }
> +double f15 (double x) { double t = x - 0.0; return t + 0.0; }
> +double f16 (double x) { double t = x - (-0.0); return t + (-0.0); }
> --- gcc/testsuite/gcc.dg/tree-ssa/pr90356-2.c.jj 2019-05-06
> 11:43:07.232271129 +0200
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr90356-2.c 2019-05-06 11:45:41.145803937
> +0200
> @@ -0,0 +1,8 @@
> +/* PR tree-optimization/90356 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-rounding-math -fno-signaling-nans -fsigned-zeros
> -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump-times "x_\[0-9]*.D. \\+ 0.0;" 12 "optimized"
> } } */
> +/* { dg-final { scan-tree-dump-times "y_\[0-9]*.D. - 0.0;" 0 "optimized" } }
> */
> +/* { dg-final { scan-tree-dump-times " \[+-] 0.0;" 12 "optimized" } } */
> +
> +#include "pr90356-1.c"
> --- gcc/testsuite/gcc.dg/tree-ssa/pr90356-3.c.jj 2019-05-06
> 11:45:05.056382441 +0200
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr90356-3.c 2019-05-06 11:47:19.779222871
> +0200
> @@ -0,0 +1,6 @@
> +/* PR tree-optimization/90356 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -frounding-math -fsignaling-nans -fsigned-zeros
> -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump-times " \[+-] 0.0;" 32 "optimized" } } */
> +
> +#include "pr90356-1.c"
> --- gcc/testsuite/gcc.dg/tree-ssa/pr90356-4.c.jj 2019-05-06
> 11:46:02.140467400 +0200
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr90356-4.c 2019-05-06 11:47:28.175088284
> +0200
> @@ -0,0 +1,6 @@
> +/* PR tree-optimization/90356 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -frounding-math -fno-signaling-nans -fsigned-zeros
> -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump-times " \[+-] 0.0;" 32 "optimized" } } */
> +
> +#include "pr90356-1.c"
>
> Jakub
>
--
Richard Biener <rguent...@suse.de>
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