On Fri, Sep 26, 2025 at 1:27 PM Andrew Stubbs <[email protected]> wrote:
>
> When testcases (such as libgomp.c-c++-common/for-11.c) use unsigned types for
> loop counters then the vectorizer will also use unsigned types for the offsets
> passed to gather/scatter instructions. This is a problem if the loop step is
> negative and the offset type is narrower than a pointer.
Hmm, but why did we chose an unsigned type in the first place then? So,
isn't this an issue in vect_truncate_gather_scatter_offset or
vect_use_strided_gather_scatters_p?
> This commit ensures that negative offsets are labelled as signed.
>
> gcc/ChangeLog:
>
> PR target/121393
> * tree-vect-stmts.cc (vect_get_strided_load_store_ops): Use signed
> offsets for negative steps.
> ---
>
> This fixes a problem observed on amdgcn, but there doesn't seem to be
> anything about it that is target-specific.
>
> OK for mainline?
>
> Andrew
>
>
> gcc/tree-vect-stmts.cc | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> index cfc4f323a22..254fe6e6c0f 100644
> --- a/gcc/tree-vect-stmts.cc
> +++ b/gcc/tree-vect-stmts.cc
> @@ -2969,6 +2969,14 @@ vect_get_strided_load_store_ops (stmt_vec_info
> stmt_info, slp_tree node,
> type of the vector instead. */
> tree offset_type = TREE_TYPE (offset_vectype);
>
> + if (TREE_CODE (DR_STEP (dr)) == INTEGER_CST
> + && compare_step_with_zero (loop_vinfo, stmt_info) < 0
> + && TYPE_SIGN (offset_type) == UNSIGNED)
> + {
> + offset_type = signed_type_for (offset_type);
> + offset_vectype = signed_type_for (offset_vectype);
> + }
> +
> /* Calculate X = DR_STEP / SCALE and convert it to the appropriate type.
> */
> tree step = size_binop (EXACT_DIV_EXPR, unshare_expr (DR_STEP (dr)),
> ssize_int (SLP_TREE_GS_SCALE (node)));
> --
> 2.51.0
>
