Re: [PATCH v2 1/2] match: Fix recent pattern for signed integer overflow dealing with - [PR126418]

On Wed, Aug 12, 2026 at 5:06 AM Andrea Pinski
<[email protected]> wrote:
>
> A recent match patterns were added that introduce signed integer overflow where
> there was none before.  `cmp + (-cmp ^ x)` has no signed integer overflow when
> x is INT_MIN when cmp is 0. This gets translated into cmp ? -x : x.
> But this has now introduced an signed integer overflow for INT_MIN.
> The fix is to use unsigned type for the negative.
>
> Bootstrapped and tested on x86_64-linux-gnu.

OK.

I've commented on 2/2 in v1.

> Changes since v1:
> * v2: Remove the abs patterns since a signed integer overflow would have happened anyways.
>
>         PR tree-optimization/126418
>
> gcc/ChangeLog:
>
>         * match.pd (`(A ^ -cmp) + cmp`): Cast to unsigned type
>         before taking the negative.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/pr126418-1.c: New test.
>
> Signed-off-by: Andrea Pinski <[email protected]>
> ---
>  gcc/match.pd                               |  8 +++++++-
>  gcc/testsuite/gcc.dg/tree-ssa/pr126418-1.c | 11 +++++++++++
>  2 files changed, 18 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr126418-1.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 5da1e372f10..945fcabf956 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -4942,7 +4942,13 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>   (if (INTEGRAL_TYPE_P (type)
>        && !TYPE_SATURATING (type)
>        && (GIMPLE || !TREE_SIDE_EFFECTS (@0)))
> -  (cond (convert:boolean_type_node @1) (negate @0) @0)))
> +  /* Do the negate in unsigned type always; otherwise
> +     we would be introducing an overflow.  */
> +  (with { tree utype = unsigned_type_for (type); }
> +   (cond
> +    (convert:boolean_type_node @1)
> +    (convert:type (negate (convert:utype @0)))
> +    @0))))
>
>  /* Transform A & (B*cmp) into (A&B)*cmp.  */
>  (simplify
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr126418-1.c b/gcc/testsuite/gcc.dg/tree-ssa/pr126418-1.c
> new file mode 100644
> index 00000000000..3714069b43f
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr126418-1.c
> @@ -0,0 +1,11 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-forwprop1" } */
> +
> +int f_cmp_gt_commuted(int x, int y)
> +{
> +  int cmp = x > y;
> +  return cmp + (-cmp ^ x);
> +}
> +
> +/* { dg-final { scan-tree-dump-times "\\(unsigned int\\) " 1 "forwprop1" } } */
> +/* { dg-final { scan-tree-dump-times "\\(int\\) " 1 "forwprop1" } } */
> --
> 2.43.0
>