> Am 14.12.2025 um 13:39 schrieb Andrew Pinski <[email protected]>:
>
> Right now forwprop calls into fold to do some folding of comparisons.
> But fold is designed for generic which does not have a limitation on
> vector comparisons in many cases. So fold turns `a >> signbit != 0`
> into `a < 0` but `a < 0` does not exist for the vector type (with a
> resulting bool).
I believe fold needs to be fixed instead. Either by not folding vector
compared or validating operations similar how match.pd does. The specific
folding could be also moved to match.pd of course.
Richard
> Really forwprop should not call into fold but that is an on going
> project and should be fixed for GCC 17 (see PR 120206). For now
> the way to fix this is to reject the vector comparison if
> expand_vec_cmp_expr_p says the comparison is not valid.
>
> Bootstrapped and tested on x86_64-linux-gnu.
>
> PR tree-optimization/123107
>
> gcc/ChangeLog:
>
> * tree-ssa-forwprop.cc (combine_cond_expr_cond): Reject vector
> comparisons in some cases.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/torture/pr123107-1.c: New test.
>
> Signed-off-by: Andrew Pinski <[email protected]>
> ---
> gcc/testsuite/gcc.dg/torture/pr123107-1.c | 14 ++++++++++++++
> gcc/tree-ssa-forwprop.cc | 12 ++++++++++++
> 2 files changed, 26 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/torture/pr123107-1.c
>
> diff --git a/gcc/testsuite/gcc.dg/torture/pr123107-1.c
> b/gcc/testsuite/gcc.dg/torture/pr123107-1.c
> new file mode 100644
> index 00000000000..cf70abe9451
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/torture/pr123107-1.c
> @@ -0,0 +1,14 @@
> +/* { dg-do compile } */
> +/* { dg-additional-options "-mavx2" { target i?86-*-* x86_64-*-* } } */
> +
> +/* PR tree-optimization/123107 */
> +/* Forwardprop should check what fold gets back to be valid. */
> +
> +typedef int vec8short __attribute__((vector_size(8 * sizeof(int))));
> +int f( vec8short t)
> +{
> + t = t < 0;
> + return !(t[0] | t[1] | t[2] | t[3]
> + | t[4] | t[5] | t[6] | t[7]);
> +}
> +
> diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
> index 2200fc04918..7f5820be9c4 100644
> --- a/gcc/tree-ssa-forwprop.cc
> +++ b/gcc/tree-ssa-forwprop.cc
> @@ -428,6 +428,18 @@ combine_cond_expr_cond (gimple *stmt, enum tree_code
> code, tree type,
> return NULL_TREE;
> }
>
> + /* For vector comparisons, check to make sure fold
> + returns a valid coparison for this target. */
> + if (COMPARISON_CLASS_P (t)
> + && TREE_CODE (TREE_TYPE (TREE_OPERAND (t, 0))) == VECTOR_TYPE
> + && !expand_vec_cmp_expr_p (TREE_TYPE (TREE_OPERAND (t, 0)),
> + TREE_TYPE (t),
> + TREE_CODE (t)))
> + {
> + fold_undefer_overflow_warnings (false, NULL, 0);
> + return NULL_TREE;
> + }
> +
> bool nowarn = warning_suppressed_p (stmt, OPT_Wstrict_overflow);
> fold_undefer_overflow_warnings (!nowarn, stmt, 0);
>
> --
> 2.43.0
>
