Re: [PATCH v14 19/19] vect: Guard against infinite loop in vect_create_constant_vectors

On Thu, Jul 30, 2026 at 4:58 PM Christopher Bazley <[email protected]> wrote:
>
> If we need to replicate the vectors but vec_num ends up with a
> value of 0 because of alterations elsewhere in this function
> then the final loop never terminates because no SLP_TREE_VEC_DEFS
> are pushed by its inner loop.  This change eases debugging.
>
> gcc/ChangeLog:
>
>         * tree-vect-slp.cc (vect_create_constant_vectors):
>         Add an assertion to guard against the outer loop never
>         terminating because the inner loop is never entered.
> ---
>  gcc/tree-vect-slp.cc | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
> index ca9bae97e7e..0880875bcf0 100644
> --- a/gcc/tree-vect-slp.cc
> +++ b/gcc/tree-vect-slp.cc
> @@ -11282,9 +11282,15 @@ vect_create_constant_vectors (vec_info *vinfo, slp_tree op_node)
>       NUMBER_OF_SCALARS/NUNITS or NUNITS/NUMBER_OF_SCALARS, and hence we have
>       to replicate the vectors.  */
>    while (number_of_vectors > SLP_TREE_VEC_DEFS (op_node).length ())
> -    for (i = 0; SLP_TREE_VEC_DEFS (op_node).iterate (i, &vop) && i < vec_num;
> -        i++)
> -      SLP_TREE_VEC_DEFS (op_node).quick_push (vop);
> +    {
> +      /* Guard against the outer loop never terminating because the
> +        inner loop is never entered.  */
> +      gcc_checking_assert (vec_num > 0);

This is loop invariant in the while() loop.  Do we actually have a
valid cause of
not asserting this when !(number_of_vectors > SLP_TREE_VEC_DEFS
(op_node).length ())?
I don't think so.

OK with asserting this right after

  vec_num = voprnds.length ();

You can push this separately from the rest.

Thanks,
Richard.

> +
> +      for (i = 0; SLP_TREE_VEC_DEFS (op_node).iterate (i, &vop) && i < vec_num;
> +          i++)
> +       SLP_TREE_VEC_DEFS (op_node).quick_push (vop);
> +    }
>  }
>
>  /* Get the scalar definition of the Nth lane from SLP_NODE or NULL_TREE
> --
> 2.43.0
>