https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106081
--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to rsand...@gcc.gnu.org from comment #10)
> (In reply to Richard Biener from comment #9)
[...]
> > 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?
> Yeah, I think so. The patch LGTM. I don't know if it's worth
> caching the “all the same element” result, but probably not.
Yeah, I thought of changing the load_permutation representation to
vec_perm_indices but then as I consider merging lane and load permutes
that would be wrong step.
I've thought of generalizing the above but then change_layout_cost
is always positive but for example when doing { 3, 2, 1, 0 } ontop
an existing load permutation of { 3, 2, 1, 0 } the cost should be
negative?
Anyway, going to test the above patch.
