[Bug tree-optimization/48172] [4.5/4.6/4.7 Regression] incorrect vectorization of loop in GCC 4.5.* with -O3

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48172

--- Comment #12 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-12 12:46:10 UTC ---
Like this?

Index: gcc/tree-vect-loop-manip.c
===================================================================
--- gcc/tree-vect-loop-manip.c  (revision 173703)
+++ gcc/tree-vect-loop-manip.c  (working copy)
@@ -2353,23 +2353,19 @@ vect_create_cond_for_align_checks (loop_

    Input:
      DR: The data reference.
-     VECT_FACTOR: vectorization factor.
-     SCALAR_LOOP_NITERS: number of iterations.
+     LENGTH_FACTOR: segment length to consider.

    Return an expression whose value is the size of segment which will be
    accessed by DR.  */

 static tree
-vect_vfa_segment_size (struct data_reference *dr, int vect_factor,
+vect_vfa_segment_size (struct data_reference *dr, tree length_factor,
                       tree scalar_loop_niters)
 {
   tree segment_length;
   segment_length = size_binop (MULT_EXPR,
                               fold_convert (sizetype, DR_STEP (dr)),
-                              size_int (vect_factor));
-  segment_length = size_binop (MULT_EXPR,
-                              segment_length,
-                              fold_convert (sizetype, scalar_loop_niters));
+                              fold_convert (sizetype, length_factor));
   if (vect_supportable_dr_alignment (dr, false)
         == dr_explicit_realign_optimized)
     {
@@ -2465,10 +2461,12 @@ vect_create_cond_for_alias_checks (loop_
         vect_create_addr_base_for_vector_ref (stmt_b, cond_expr_stmt_list,
                                              NULL_TREE, loop);

-      segment_length_a = vect_vfa_segment_size (dr_a, vect_factor,
-                                               scalar_loop_iters);
-      segment_length_b = vect_vfa_segment_size (dr_b, vect_factor,
-                                               scalar_loop_iters);
+      if (!operand_equal_p (DR_STEP (dr_a), DR_STEP (dr_b), 0))
+       length_factor = scalar_loop_iters;
+      else
+       length_factor = size_int (vect_factor);
+      segment_length_a = vect_vfa_segment_size (dr_a, length_factor);
+      segment_length_b = vect_vfa_segment_size (dr_b, length_factor);

       if (vect_print_dump_info (REPORT_DR_DETAILS))
        {

I also think that the re-alignment adjustment needs to be multiplied
by DR_STEP (maybe we only support it for DR_STEP == 1 at the moment).