vect: Fix load costs for SLP permutes

For the following test case (compiled with load/store lanes
disabled locally):

  void
  f (uint32_t *restrict x, uint8_t *restrict y, int n)
  {
    for (int i = 0; i < n; ++i)
      {
	x[i * 2] = x[i * 2] + y[i * 2];
	x[i * 2 + 1] = x[i * 2 + 1] + y[i * 2];
      }
  }

we have a redundant no-op permute on the x[] load node:

   node 0x4472350 (max_nunits=8, refcnt=2)
          stmt 0 _5 = *_4;
          stmt 1 _13 = *_12;
          load permutation { 0 1 }

Then, when costing it, we pick a cost of 1, even though we need 4 copies
of the x[] load to match a single y[] load:

   ==> examining statement: _5 = *_4;
   Vectorizing an unaligned access.
   vect_model_load_cost: unaligned supported by hardware.
   vect_model_load_cost: inside_cost = 1, prologue_cost = 0 .

The problem is that the code only considers the permutation for
the first scalar iteration, rather than for all VF iterations.

This patch tries to fix that by making vect_transform_slp_perm_load
calculate the value instead.

gcc/
	* tree-vectorizer.h (vect_transform_slp_perm_load): Take an
	optional extra parameter.
	* tree-vect-slp.c (vect_transform_slp_perm_load): Calculate
	the number of loads as well as the number of permutes, taking
	the counting loop from...
	* tree-vect-stmts.c (vect_model_load_cost): ...here.  Use the
	value computed by vect_transform_slp_perm_load for ncopies.

1 files changed, 2 insertions, 1 deletions

diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index 13a02cd..fbf5291 100644
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -1952,7 +1952,8 @@ extern tree cse_and_gimplify_to_preheader (loop_vec_info, tree);
 extern void vect_free_slp_instance (slp_instance);
 extern bool vect_transform_slp_perm_load (vec_info *, slp_tree, vec<tree>,
 					  gimple_stmt_iterator *, poly_uint64,
-					  bool, unsigned *);
+					  bool, unsigned *,
+					  unsigned * = nullptr);
 extern bool vect_slp_analyze_operations (vec_info *);
 extern void vect_schedule_slp (vec_info *, vec<slp_instance>);
 extern opt_result vect_analyze_slp (vec_info *, unsigned);