[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 #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to rsandifo@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.