On Wed, Aug 27, 2025 at 03:50:26PM +0200, Richard Biener wrote:
> Ah, thanks for the write-up how to compute the number of uses.
>
> I'll test the following.
> --- a/gcc/tree-vect-patterns.cc
> +++ b/gcc/tree-vect-patterns.cc
> @@ -4303,6 +4303,8 @@ vect_synth_mult_by_constant (vec_info *vinfo, tree op,
> tree val,
> /* Targets that don't support vector shifts but support vector additions
> can synthesize shifts that way. */
> bool synth_shift_p = !vect_supportable_shift (vinfo, LSHIFT_EXPR,
> multtype);
> + if (synth_shift_p)
> + return NULL;
This is wrong, that will punt no matter what, for reductions/non-reductions,
and even when not really used. Just the other hunk should do it.
Also, I'd probably do it only after the target_supports_mult_synth_alg
check, not before it.
> HOST_WIDE_INT hwval = tree_to_shwi (val);
> /* Use MAX_COST here as we don't want to limit the sequence on rtx costs.
> @@ -4314,6 +4316,35 @@ vect_synth_mult_by_constant (vec_info *vinfo, tree op,
> tree val,
> if (!possible)
> return NULL;
>
> + if (vect_is_reduction (stmt_vinfo))
> + {
> + int op_uses = alg.op[0] != alg_zero;
> + for (int i = 1; i < alg.ops; i++)
> + switch (alg.op[i])
> + {
> + case alg_add_t_m2:
> + case alg_sub_t_m2:
> + if (synth_shift_p && alg.log[i])
> + return NULL;
> + else
> + op_uses++;
> + break;
> + case alg_add_t2_m:
> + case alg_sub_t2_m:
> + op_uses++;
> + break;
> + default:
> + break;
> + }
> + if (variant == add_variant)
> + op_uses++;
> + /* When we'll synthesize more than a single use of the reduction
> + operand the reduction constraints are violated. Avoid this
> + situation. */
> + if (op_uses > 1)
> + return NULL;
> + }
> +
> if (!target_supports_mult_synth_alg (&alg, variant, vectype,
> synth_shift_p))
> return NULL;
>
> --
> 2.43.0
Jakub
