[Bug tree-optimization/61680] [4.8 Regression] vectorization gives wrong answer for sandybridge target

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61680

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|4.10.0                      |5.0

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
For 4.8 we can't backport the patch as group analysis and dependence analysis
are still the wrong way around.  A simple

Index: gcc/tree-vect-data-refs.c
===================================================================
--- gcc/tree-vect-data-refs.c   (revision 215073)
+++ gcc/tree-vect-data-refs.c   (working copy)
@@ -2307,6 +2307,17 @@ vect_analyze_group_access (struct data_r

       while (next)
         {
+         /* Check that there is no load-store dependencies for this loads
+            to prevent a case of load-store-load to the same location.  */
+         if (GROUP_READ_WRITE_DEPENDENCE (vinfo_for_stmt (next))
+             || GROUP_READ_WRITE_DEPENDENCE (vinfo_for_stmt (prev)))
+           {
+             if (dump_enabled_p ())
+               dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location, 
+                                "READ_WRITE dependence in interleaving.");
+             return false;
+           }
+
           /* Skip same data-refs.  In case that two or more stmts share
              data-ref (supported only for loads), we vectorize only the first
              stmt, and the rest get their vectorized loads from the first
@@ -2323,17 +2334,6 @@ vect_analyze_group_access (struct data_r
                   return false;
                 }

-              /* Check that there is no load-store dependencies for this loads
-                 to prevent a case of load-store-load to the same location. 
*/
-              if (GROUP_READ_WRITE_DEPENDENCE (vinfo_for_stmt (next))
-                  || GROUP_READ_WRITE_DEPENDENCE (vinfo_for_stmt (prev)))
-                {
-                  if (dump_enabled_p ())
-                    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location, 
-                                     "READ_WRITE dependence in
interleaving.");
-                  return false;
-                }
-
               /* For load use the same data-ref load.  */
               GROUP_SAME_DR_STMT (vinfo_for_stmt (next)) = prev;


will regress testcases in vect.exp.

Like with two other 4.8 vectorizer miscompiles it's hard to fix them without
turning the vectorizer upside down (aka backport most of the re-org
from GCC 4.9).