tree-ssa-alias.c (nonoverlapping_refs_since_match_p): Do not skip non-zero array accesses.

	* tree-ssa-alias.c (nonoverlapping_refs_since_match_p): Do not
	skip non-zero array accesses.

	* gcc.c-torture/execute/alias-access-path-2.c: New testcase.
	* gcc.dg/tree-ssa/alias-access-path-11.c: xfail.

From-SVN: r277214

1 files changed, 22 insertions, 6 deletions

diff --git a/gcc/tree-ssa-alias.c b/gcc/tree-ssa-alias.c
index d6c4438..4cfe3e2 100644
--- a/gcc/tree-ssa-alias.c
+++ b/gcc/tree-ssa-alias.c
@@ -1444,20 +1444,36 @@ nonoverlapping_refs_since_match_p (tree match1, tree ref1,
 	  for (; narray_refs1 > narray_refs2; narray_refs1--)
 	    {
 	      ref1 = component_refs1.pop ();
-	      /* Track whether we possibly introduced partial overlap assuming
-		 that innermost type sizes does not match.  This only can
-		 happen if the offset introduced by the ARRAY_REF
-		 is non-zero.  */
+
+	      /* If index is non-zero we need to check whether the reference
+		 does not break the main invariant that bases are either
+		 disjoint or equal.  Consider the example:
+
+		 unsigned char out[][1];
+		 out[1]="a";
+		 out[i][0];
+
+		 Here bases out and out are same, but after removing the
+		 [i] index, this invariant no longer holds, because
+		 out[i] points to the middle of array out.
+
+		 TODO: If size of type of the skipped reference is an integer
+		 multiply of the size of type of the other reference this
+		 invariant can be verified, but even then it is not completely
+		 safe with !flag_strict_aliasing if the other reference contains
+		 unbounded array accesses.
+		 See   */
+
 	      if (!operand_equal_p (TREE_OPERAND (ref1, 1),
 				    cheap_array_ref_low_bound (ref1), 0))
-	        seen_unmatched_ref_p = true;
+		return 0;
 	    }
 	  for (; narray_refs2 > narray_refs1; narray_refs2--)
 	    {
 	      ref2 = component_refs2.pop ();
 	      if (!operand_equal_p (TREE_OPERAND (ref2, 1),
 				    cheap_array_ref_low_bound (ref2), 0))
-	        seen_unmatched_ref_p = true;
+		return 0;
 	    }
 	  /* Try to disambiguate matched arrays.  */
 	  for (unsigned int i = 0; i < narray_refs1; i++)