[Bug middle-end/106081] missed vectorization

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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?