[Bug tree-optimization/66598] [4.9/5 Regression] With -O3 gcc incorrectly assumes aligned SSE instructions (e.g. movapd) can be used

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66598

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
The relevant hunk is the following but as it requires us to know the
final vectorization factor it requires us to re-order (and split)
the pipeline quite a bit.  r223670 has bits of the prerequesites but not all
and r224598 has the fix below (and some unrelated bits).

Index: gcc/tree-vect-data-refs.c
===================================================================
--- gcc/tree-vect-data-refs.c   (revision 230924)
+++ gcc/tree-vect-data-refs.c   (working copy)
@@ -701,21 +701,22 @@ vect_compute_data_ref_alignment (struct
        }
     }

-  /* Similarly, if we're doing basic-block vectorization, we can only use
-     base and misalignment information relative to an innermost loop if the
-     misalignment stays the same throughout the execution of the loop.
-     As above, this is the case if the stride of the dataref evenly divides
-     by the vector size.  */
-  if (!loop)
+  /* Similarly we can only use base and misalignment information relative to
+     an innermost loop if the misalignment stays the same throughout the
+     execution of the loop.  As above, this is the case if the stride of
+     the dataref evenly divides by the vector size.  */
+  else
     {
       tree step = DR_STEP (dr);
-      HOST_WIDE_INT dr_step = TREE_INT_CST_LOW (step);
+      unsigned vf = loop ? LOOP_VINFO_VECT_FACTOR (loop_vinfo) : 1;

-      if (dr_step % GET_MODE_SIZE (TYPE_MODE (vectype)) != 0)
+      if (tree_fits_shwi_p (step)
+         && ((tree_to_shwi (step) * vf)
+             % GET_MODE_SIZE (TYPE_MODE (vectype)) != 0))
        {
          if (dump_enabled_p ())
            dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
-                            "SLP: step doesn't divide the vector-size.\n");
+                            "step doesn't divide the vector-size.\n");
          misalign = NULL_TREE;
        }
     }