re PR tree-optimization/79955 (GLIBC build fails after r245840)

2017-03-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/79955
	* tree-ssa-uninit.c (warn_uninitialized_vars): Do not warn
	for accesses that are completely outside of the variable.

	* gcc.dg/uninit-24.c: New testcase.

From-SVN: r245976

gcc/ChangeLog

+2017-03-08  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/79955
+	* tree-ssa-uninit.c (warn_uninitialized_vars): Do not warn
+	for accesses that are completely outside of the variable.
+
 2017-03-08  Andrew Haley  <aph@redhat.com>
 
 	PR tree-optimization/79943

gcc/testsuite/ChangeLog

@@ -9,6 +9,11 @@
 
 2017-03-08  Richard Biener  <rguenther@suse.de>
 
+	PR tree-optimization/79955
+	* gcc.dg/uninit-24.c: New testcase.
+
+2017-03-08  Richard Biener  <rguenther@suse.de>
+
 	PR tree-optimization/79920
 	* gcc.dg/vect/pr79920.c: New testcase.
 

gcc/testsuite/gcc.dg/uninit-24.c

+/* { dg-do compile } */
+/* { dg-options "-O -Wmaybe-uninitialized" } */
+
+int foo (int x)
+{
+  int y;
+  if (x)
+    return *(&y + 1); /* { dg-bogus "may be used uninitialized" } */
+  return 0;
+}

gcc/tree-ssa-uninit.c

@@ -287,6 +287,17 @@ warn_uninitialized_vars (bool warn_possibly_uninitialized)
 		  || TREE_NO_WARNING (base))
 		continue;
 
+	      /* Do not warn if the access is fully outside of the
+	         variable.  */
+	      if (ref.size != -1
+		  && ref.max_size == ref.size
+		  && (ref.offset + ref.size <= 0
+		      || (ref.offset >= 0
+			  && TREE_CODE (DECL_SIZE (base)) == INTEGER_CST
+			  && compare_tree_int (DECL_SIZE (base),
+					       ref.offset) <= 0)))
+		continue;
+
 	      /* Limit the walking to a constant number of stmts after
 	         we overcommit quadratic behavior for small functions
 		 and O(n) behavior.  */