* alias.c (read_dependence): Return true for ALIAS_SET_MEMORY_BARRIER.

From-SVN: r190941

gcc/ChangeLog

+2012-09-04  Richard Henderson  <rth@redhat.com>
+
+	* alias.c (read_dependence): Return true for ALIAS_SET_MEMORY_BARRIER.
+
 2012-09-04  Richard Guenther  <rguenther@suse.de>
 
 	* tree-ssa-pre.c (value_expressions): Make it a vector of

gcc/alias.c

@@ -2177,12 +2177,18 @@ memrefs_conflict_p (int xsize, rtx x, int ysize, rtx y, HOST_WIDE_INT c)
    storeqi_unaligned pattern.  */
 
 /* Read dependence: X is read after read in MEM takes place.  There can
-   only be a dependence here if both reads are volatile.  */
+   only be a dependence here if both reads are volatile, or if either is
+   an explicit barrier.  */
 
 int
 read_dependence (const_rtx mem, const_rtx x)
 {
-  return MEM_VOLATILE_P (x) && MEM_VOLATILE_P (mem);
+  if (MEM_VOLATILE_P (x) && MEM_VOLATILE_P (mem))
+    return true;
+  if (MEM_ALIAS_SET (x) == ALIAS_SET_MEMORY_BARRIER
+      || MEM_ALIAS_SET (mem) == ALIAS_SET_MEMORY_BARRIER)
+    return true;
+  return false;
 }
 
 /* Return true if we can determine that the fields referenced cannot