tree-ssa-dse.c (compute_trims): Avoid folding away undefined behaviour.

	* tree-ssa-dse.c (compute_trims): Avoid folding away undefined
	behaviour.

From-SVN: r263793

gcc/ChangeLog

+2018-08-22  Bernd Edlinger  <bernd.edlinger@hotmail.de>
+
+	* tree-ssa-dse.c (compute_trims): Avoid folding away undefined
+	behaviour.
+
 2018-08-22  Martin Sebor  <msebor@redhat.com>
 
 	PR middle-end/87052

gcc/tree-ssa-dse.c

@@ -248,6 +248,13 @@ compute_trims (ao_ref *ref, sbitmap live, int *trim_head, int *trim_tail,
 	 residual handling in mem* and str* functions is usually
 	 reasonably efficient.  */
       *trim_tail = last_orig - last_live;
+
+      /* But don't trim away out of bounds accesses, as this defeats
+	 proper warnings.  */
+      if (*trim_tail
+	  && compare_tree_int (TYPE_SIZE_UNIT (TREE_TYPE (ref->base)),
+			       last_orig) <= 0)
+	*trim_tail = 0;
     }
   else
     *trim_tail = 0;