https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106081
--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> --- So I can adjust change_layout_cost in a bit awkward way, but it seems that internal_node_cost would already work out that a permute can be merged into an existing permute. It seems that existing permutes are not recorded in the "layout". Also vectorizable_slp_permutation_1 doesn't try to elide permutes that are noop based on knowledge of the layout of 'node', say a permute { 1 0 3 2 } of a { _1, _1, _2, _2 } node would be noop. But change_layout_cost does MAX (count, 1) on its result anyway. The following elides the unnecessary permutation for this special case (but not the general case): diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc index e4430248ab5..e9048a61891 100644 --- a/gcc/tree-vect-slp.cc +++ b/gcc/tree-vect-slp.cc @@ -4389,6 +4389,19 @@ vect_optimize_slp_pass::change_layout_cost (slp_tree node, if (from_layout_i == to_layout_i) return 0; + /* When there's a uniform load permutation permutating that in any + way is free. */ + if (SLP_TREE_LOAD_PERMUTATION (node).exists ()) + { + unsigned l = SLP_TREE_LOAD_PERMUTATION (node)[0]; + unsigned i; + for (i = 1; i < SLP_TREE_LOAD_PERMUTATION (node).length (); ++i) + if (SLP_TREE_LOAD_PERMUTATION (node)[i] != l) + break; + if (i == SLP_TREE_LOAD_PERMUTATION (node).length ()) + return 0; + } + auto_vec<slp_tree, 1> children (1); children.quick_push (node); auto_lane_permutation_t perm (SLP_TREE_LANES (node)); I'm not sure this is the correct place to factor in cost savings materialization would give. Is it? Are explicit VEC_PERM nodes also still there in the optimization process or are they turned into sth implicit?