On Tue, Aug 3, 2021 at 2:09 PM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> When the vectoriser scalarises a strided store, it counts one
> scalar_store for each element plus one vec_to_scalar extraction
> for each element. However, extracting element 0 is free on AArch64,
> so it should have zero cost.
>
> I don't have a testcase that requires this for existing -mtune
> options, but it becomes more important with a later patch.
>
> gcc/
> * config/aarch64/aarch64.c (aarch64_is_store_elt_extraction): New
> function, split out from...
> (aarch64_detect_vector_stmt_subtype): ...here.
> (aarch64_add_stmt_cost): Treat extracting element 0 as free.
> ---
> gcc/config/aarch64/aarch64.c | 22 +++++++++++++++++++---
> 1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index 36f11808916..084f8caa0da 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -14622,6 +14622,18 @@ aarch64_builtin_vectorization_cost (enum
> vect_cost_for_stmt type_of_cost,
> }
> }
>
> +/* Return true if an operaton of kind KIND for STMT_INFO represents
> + the extraction of an element from a vector in preparation for
> + storing the element to memory. */
> +static bool
> +aarch64_is_store_elt_extraction (vect_cost_for_stmt kind,
> + stmt_vec_info stmt_info)
> +{
> + return (kind == vec_to_scalar
> + && STMT_VINFO_DATA_REF (stmt_info)
> + && DR_IS_WRITE (STMT_VINFO_DATA_REF (stmt_info)));
> +}
It would be nice to put functions like this in tree-vectorizer.h in some
section marked with a comment to contain helpers for the target
add_stmt_cost.
> /* Return true if STMT_INFO represents part of a reduction. */
> static bool
> aarch64_is_reduction (stmt_vec_info stmt_info)
> @@ -14959,9 +14971,7 @@ aarch64_detect_vector_stmt_subtype (vec_info *vinfo,
> vect_cost_for_stmt kind,
> /* Detect cases in which vec_to_scalar is describing the extraction of a
> vector element in preparation for a scalar store. The store itself is
> costed separately. */
> - if (kind == vec_to_scalar
> - && STMT_VINFO_DATA_REF (stmt_info)
> - && DR_IS_WRITE (STMT_VINFO_DATA_REF (stmt_info)))
> + if (aarch64_is_store_elt_extraction (kind, stmt_info))
> return simd_costs->store_elt_extra_cost;
>
> /* Detect SVE gather loads, which are costed as a single scalar_load
> @@ -15382,6 +15392,12 @@ aarch64_add_stmt_cost (class vec_info *vinfo, void
> *data, int count,
> if (vectype && aarch64_sve_only_stmt_p (stmt_info, vectype))
> costs->saw_sve_only_op = true;
>
> + /* If we scalarize a strided store, the vectorizer costs one
> + vec_to_scalar for each element. However, we can store the first
> + element using an FP store without a separate extract step. */
> + if (aarch64_is_store_elt_extraction (kind, stmt_info))
> + count -= 1;
> +
> stmt_cost = aarch64_detect_scalar_stmt_subtype
> (vinfo, kind, stmt_info, stmt_cost);
>
