Re: [PATCH 2/2] middle-end: Wrong code for a != b | (a|b) != 0. [PR126742]

On Sun, Aug 9, 2026 at 8:04 AM Kael Andrew Alonzo Franco
<[email protected]> wrote:
>
> Since r17-2886, GCC does a wrong optimize with:
>
> (a == b) & ((a|b) == 0) -> ((a|b) != 0)
> (a != b) | ((a|b) != 0) -> ((a|b) == 0)
>
> Should be:
>
> (a == b) & ((a|b) == 0) -> ((a|b) == 0)
> (a != b) | ((a|b) != 0) -> ((a|b) != 0)
>
> Regtest missed this because gcc.dg/int-bwise-opt-2.c only test:
>
> /* { dg-final { scan-tree-dump-times "a == b" 0 "optimized" } } */
> /* { dg-final { scan-tree-dump-times "a != b" 0 "optimized" } } */
>
> Make this test more rigorous by comparing the final code.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
>         PR middle-end/126742
>
> gcc/ChangeLog:
>
>         * match.pd: Fix wrong code.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/int-bwise-opt-2.c: Also test for PR126742.

Ok.

>
> Signed-off-by: Kael Andrew Franco <[email protected]>
> ---
>  gcc/match.pd                           |  2 +-
>  gcc/testsuite/gcc.dg/int-bwise-opt-2.c | 15 +++++++++------
>  2 files changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index c2f411001a0..efaf0026711 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -7037,7 +7037,7 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>    (neeq @0 @1))
>   (simplify
>    (bitop:c (eqne @0 @1) (eqne (bit_ior@2 @0 @1) integer_zerop@3))
> -  (neeq @2 @3))
> +  (eqne @2 @3))
>   (simplify
>    (bitop (neeq @0 @1) (eqne (bit_ior @0 @1) integer_zerop))
>    { constant_boolean_node (bitop == BIT_IOR_EXPR, type); })
> diff --git a/gcc/testsuite/gcc.dg/int-bwise-opt-2.c b/gcc/testsuite/gcc.dg/int-bwise-opt-2.c
> index cc1a48b061a..e0c065a54f3 100644
> --- a/gcc/testsuite/gcc.dg/int-bwise-opt-2.c
> +++ b/gcc/testsuite/gcc.dg/int-bwise-opt-2.c
> @@ -1,15 +1,18 @@
>  /* { dg-do compile } */
>  /* { dg-options "-O2 -fdump-tree-optimized" } */
>
> -int f1(int a, int b)
> +_Bool
> +a_ne_b_bit_ior (int a, int b)
>  {
> -  return (a != b) | ((a | b) != 0);
> +  _Bool ret = ((a | b) != 0);
> +  return (((a != b) | ret) == ret);
>  }
>
> -int f2(int a, int b)
> +_Bool
> +a_eq_b_bit_and (int a, int b)
>  {
> -  return (a == b) & ((a | b) == 0);
> +  _Bool ret = ((a | b) == 0);
> +  return (((a == b) & ret) == ret);
>  }
>
> - /* { dg-final { scan-tree-dump-times "a == b" 0 "optimized" } } */
> - /* { dg-final { scan-tree-dump-times "a != b" 0 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "return 1;" 2 "optimized" } } */
> --
> 2.55.0
>