[Bug tree-optimization/49771] [4.7 Regression] wrong code with -ftree-vectorize

[irar at il dot ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49771

Ira Rosen <irar at il dot ibm.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |irar at il dot ibm.com

--- Comment #3 from Ira Rosen <irar at il dot ibm.com> 2011-07-18 15:55:49 UTC ---
(In reply to comment #2)
> That commit looks bogus, the change it made is:
>    # ivtmp.37_45 = PHI <ivtmp.37_43(6), ivtmp.37_46(2)>
>    vect_pa.7_44 = (vector(4) int *) ivtmp.37_45;
> -  D.2731_1 = vect_pa.7_44 < &a;
> -  D.2733_14 = vect_pa.7_44 > &a[1000];
> +  D.2731_1 = vect_pa.7_44 <= &a;
> +  D.2733_14 = vect_pa.7_44 >= &a[1000];
>    D.2734_23 = D.2733_14 | D.2731_1;
>    if (D.2734_23 != 0)
> 
> vect_pa.7_44 is the pointer that goes from &a[0] to &a[999].  While in this
> case it is fine to change the second > to >=, changing the first one is of
> course wrong, because in the first iteration it will overlap, yet this test
> will say that they do not.
> What is weird is that segment_length_a in vect_create_cond_for_alias_checks
> is 0 instead of 4 (sizeof (int)) I'd expect, while segment_length_b is the
> expected 4000.  Apparently DR_STEP in the first case is 0. 

The step of a[i] is indeed 0 in the j loop.

> I wonder whether
> the change of the comparison should only be done if one segment length is empty
> (and thus clearly doesn't include all bytes of the read/store), or something
> similar.

We can create a segment of length sizeof(type) for the zero step case:

Index: tree-vect-loop-manip.c
===================================================================
--- tree-vect-loop-manip.c      (revision 176389)
+++ tree-vect-loop-manip.c      (working copy)
@@ -2356,9 +2356,14 @@ static tree
 vect_vfa_segment_size (struct data_reference *dr, tree length_factor)
 {
   tree segment_length;
-  segment_length = size_binop (MULT_EXPR,
-                              fold_convert (sizetype, DR_STEP (dr)),
-                              fold_convert (sizetype, length_factor));
+
+  if (!compare_tree_int (DR_STEP (dr), 0))
+    segment_length = TYPE_SIZE_UNIT (TREE_TYPE (DR_REF (dr)));
+  else
+    segment_length = size_binop (MULT_EXPR,
+                                fold_convert (sizetype, DR_STEP (dr)),
+                                fold_convert (sizetype, length_factor));
+
   if (vect_supportable_dr_alignment (dr, false)
         == dr_explicit_realign_optimized)
     {


(tested only with the vectorizer testsuite).

Ira