> -----Original Message-----
> From: Richard Biener <[email protected]>
> Sent: 17 June 2026 13:29
> To: [email protected]
> Cc: Tamar Christina <[email protected]>; [email protected]
> Subject: [PATCH 2/5] Add vec_deconstruct costing kind
>
> The following adds vec_decostruct to replace nunits * vec_to_scalar
> which allows for more precise costing whenever vectorization
> needs to decompose a vector to pieces, like for emulated gather/scatter
> but also for strided [SLP] loads/stores.
>
> This requires changes across targets, both for the different kind
> but also for the difference in expected count.
>
> I've done x86 adjustments but refrained from trying to understand
> the complex aarch64 and riscv cost models (nor arm or ppc). I'd
> welcome patches or a simple "OK, we'll fix it" though ;)
>
> That is, does this look OK?
I'll update the AArch64 one, The only thing I'm wondering about is if
I can tell what the consumer of the de-constructed vector is.
AArch64 and Arm have by lane operations, e.g. deconstructing a vector
into a store or something like FMA is free since we just pass the element
Along.
I think we already don't cost that correctly with the current hooks so not
a blocker, but to cost this correctly I'll probably have to follow the uses?
Thanks,
Tamar
>
> The series bootstrapped / tested ok on x86_64-unknown-linux-gnu.
>
> Richard.
>
> * target.h (vect_cost_for_stmt::vec_deconstruct): New kind.
> * tree-vect-loop.cc (vect_model_reduction_cost): Use it.
> * tree-vect-stmts.cc (vectorizable_store): Likewise.
> (vectorizable_load): Likewise.
> * tree-vectorizer.cc (dump_stmt_cost): Handle vec_deconstruct.
> ---
> gcc/target.h | 3 ++-
> gcc/tree-vect-loop.cc | 11 +++++------
> gcc/tree-vect-stmts.cc | 15 +++++++--------
> gcc/tree-vectorizer.cc | 3 +++
> 4 files changed, 17 insertions(+), 15 deletions(-)
>
> diff --git a/gcc/target.h b/gcc/target.h
> index 68ba626da6a..67b04f90f45 100644
> --- a/gcc/target.h
> +++ b/gcc/target.h
> @@ -212,7 +212,8 @@ enum vect_cost_for_stmt
> cond_branch_taken,
> vec_perm,
> vec_promote_demote,
> - vec_construct
> + vec_construct,
> + vec_deconstruct
> };
>
> /* Separate locations for which the vectorizer cost model should
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 4d9b691c329..0167d52b28b 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -4752,10 +4752,10 @@ vect_model_reduction_cost (loop_vec_info
> loop_vinfo,
> node, 0, vect_body);
> else
> {
> - /* Use NELEMENTS extracts and NELEMENTS scalar ops. */
> + /* Use NCOPIES deconstructs and NELEMENTS scalar ops. */
> unsigned int nelements = ncopies * vect_nunits_for_cost (vectype);
> - inside_cost = record_stmt_cost (cost_vec, nelements,
> - vec_to_scalar, node, 0,
> + inside_cost = record_stmt_cost (cost_vec, ncopies,
> + vec_deconstruct, node, 0,
> vect_body);
> inside_cost += record_stmt_cost (cost_vec, nelements,
> scalar_stmt, node, 0,
> @@ -4817,9 +4817,8 @@ vect_model_reduction_cost (loop_vec_info
> loop_vinfo,
> {
> unsigned estimated_nunits = vect_nunits_for_cost (vectype);
> /* Extraction of scalar elements. */
> - epilogue_cost += record_stmt_cost (cost_vec,
> - 2 * estimated_nunits,
> - vec_to_scalar, node, 0,
> + epilogue_cost += record_stmt_cost (cost_vec, 2,
> + vec_deconstruct, node, 0,
> vect_epilogue);
> /* Scalar max reductions via COND_EXPR / MAX_EXPR. */
> epilogue_cost += record_stmt_cost (cost_vec,
> diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> index 4e7b45f5a6c..e991e861525 100644
> --- a/gcc/tree-vect-stmts.cc
> +++ b/gcc/tree-vect-stmts.cc
> @@ -8654,12 +8654,12 @@ vectorizable_store (vec_info *vinfo,
> inside_cost
> += record_stmt_cost (cost_vec, n_adjacent_stores,
> scalar_store, slp_node, 0, vect_body);
> - /* Only need vector extracting when there are more
> - than one stores. */
> + /* Only need vector deconstruction when there is more
> + than one store. */
> if (nstores > 1)
> inside_cost
> - += record_stmt_cost (cost_vec, n_adjacent_stores,
> - vec_to_scalar, slp_node, 0, vect_body);
> + += record_stmt_cost (cost_vec, ncopies,
> + vec_deconstruct, slp_node, 0, vect_body);
> }
> if (dump_enabled_p ())
> dump_printf_loc (MSG_NOTE, vect_location,
> @@ -9222,11 +9222,11 @@ vectorizable_store (vec_info *vinfo,
> (we assume the scalar scaling and ptr + offset add is
> consumed by the load). */
> inside_cost
> - += record_stmt_cost (cost_vec, cnunits, vec_to_scalar,
> + += record_stmt_cost (cost_vec, 1, vec_deconstruct,
> slp_node, 0, vect_body);
> /* N scalar stores plus extracting the elements. */
> inside_cost
> - += record_stmt_cost (cost_vec, cnunits, vec_to_scalar,
> + += record_stmt_cost (cost_vec, 1, vec_deconstruct,
> slp_node, 0, vect_body);
> inside_cost
> += record_stmt_cost (cost_vec, cnunits, scalar_store,
> @@ -11201,8 +11201,7 @@ vectorizable_load (vec_info *vinfo,
> {
> /* For emulated gathers N offset vector element
> offset add is consumed by the load). */
> - inside_cost = record_stmt_cost (cost_vec, const_nunits,
> - vec_to_scalar,
> + inside_cost = record_stmt_cost (cost_vec, 1,
> vec_deconstruct,
> slp_node, 0, vect_body);
> /* N scalar loads plus gathering them into a
> vector. */
> diff --git a/gcc/tree-vectorizer.cc b/gcc/tree-vectorizer.cc
> index ab455c08349..8c824e4ee33 100644
> --- a/gcc/tree-vectorizer.cc
> +++ b/gcc/tree-vectorizer.cc
> @@ -169,6 +169,9 @@ dump_stmt_cost (FILE *f, int count, enum
> vect_cost_for_stmt kind,
> case vec_construct:
> ks = "vec_construct";
> break;
> + case vec_deconstruct:
> + ks = "vec_deconstruct";
> + break;
> }
> fprintf (f, "%s ", ks);
> if (kind == unaligned_load || kind == unaligned_store)
> --
> 2.51.0
