[PATCH] match.pd: Fold x == ~x to false [PR96782]

[Richard Biener]

On Sun, 3 Jan 2021, Jakub Jelinek wrote:

> Hi!
> 
> x is never equal to ~x, so we can fold such comparisons to constants.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

Richard.

> 2021-01-03  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/96782
> 	* match.pd (x == ~x -> false, x != ~x -> true): New simplifications.
> 
> 	* gcc.dg/tree-ssa/pr96782.c: New test.
> 
> --- gcc/match.pd.jj	2021-01-01 00:03:14.987440788 +0100
> +++ gcc/match.pd	2021-01-02 10:40:34.289209121 +0100
> @@ -4045,6 +4045,13 @@ (define_operator_list COND_TERNARY
>   (if (!flag_trapping_math)
>    { constant_boolean_node (false, type); }))
>  
> +/* x == ~x -> false */
> +/* x != ~x -> true */
> +(for cmp (eq ne)
> + (simplify
> +  (cmp:c @0 (bit_not @0))
> +  { constant_boolean_node (cmp == NE_EXPR, type); }))
> +
>  /* Fold ~X op ~Y as Y op X.  */
>  (for cmp (simple_comparison)
>   (simplify
> --- gcc/testsuite/gcc.dg/tree-ssa/pr96782.c.jj	2021-01-02 10:39:50.740704373 +0100
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr96782.c	2021-01-02 10:39:36.268868954 +0100
> @@ -0,0 +1,17 @@
> +/* PR tree-optimization/96782 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump-times "return 0;" 1 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "return 1;" 1 "optimized" } } */
> +
> +int
> +foo (int a)
> +{
> +  return a == ~a;
> +}
> +
> +int
> +bar (int b)
> +{
> +  return ~b != b;
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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