re PR c++/80593 (GCC 7, aligned_storage and “dereferencing type-punned pointer…

re PR c++/80593 (GCC 7, aligned_storage and “dereferencing type-punned pointer will break strict-aliasing rules”)

2017-05-19  Richard Biener  <rguenther@suse.de>

	PR c++/80593
	* c-warn.c (strict_aliasing_warning): Do not warn for accesses
	to alias-set zero memory.

	* g++.dg/warn/Wstrict-aliasing-bogus-char-2.C: New testcase.
	* g++.dg/warn/Wstrict-aliasing-6.C: Adjust expected outcome.

From-SVN: r248269

gcc/c-family/ChangeLog

+2017-05-19  Richard Biener  <rguenther@suse.de>
+
+	PR c++/80593
+	* c-warn.c (strict_aliasing_warning): Do not warn for accesses
+	to alias-set zero memory.
+
 2017-05-18  Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
 	* c-format.c (local_tree_type_node): Add GTY attribute.

gcc/c-family/c-warn.c

@@ -537,10 +537,10 @@ strict_aliasing_warning (tree otype, tree type, tree expr)
 	    = get_alias_set (TREE_TYPE (TREE_OPERAND (expr, 0)));
 	  alias_set_type set2 = get_alias_set (TREE_TYPE (type));
 
-	  if (set1 != set2 && set2 != 0
-	      && (set1 == 0
-		  || (!alias_set_subset_of (set2, set1)
-		      && !alias_sets_conflict_p (set1, set2))))
+	  if (set2 != 0
+	      && set1 != set2
+	      && !alias_set_subset_of (set2, set1)
+	      && !alias_sets_conflict_p (set1, set2))
 	    {
 	      warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
 		       "pointer will break strict-aliasing rules");

gcc/testsuite/ChangeLog

 2017-05-19  Richard Biener  <rguenther@suse.de>
 
+	PR c++/80593
+	* g++.dg/warn/Wstrict-aliasing-bogus-char-2.C: New testcase.
+	* g++.dg/warn/Wstrict-aliasing-6.C: Adjust expected outcome.
+
+2017-05-19  Richard Biener  <rguenther@suse.de>
+
 	PR middle-end/80764
 	* gcc.dg/torture/pr80764.c: New testcase.
 

gcc/testsuite/g++.dg/warn/Wstrict-aliasing-6.C

@@ -4,6 +4,6 @@
 int foo ()
 {
   char buf[8];
-  return *((int *)buf); /* { dg-warning "strict-aliasing" } */
+  return *((int *)buf); /* { dg-bogus "strict-aliasing" } */
 }
 

gcc/testsuite/g++.dg/warn/Wstrict-aliasing-bogus-char-2.C

+// { dg-do compile }
+// { dg-options "-O2 -Wstrict-aliasing" }
+
+template<unsigned _Len, unsigned _Align>
+struct aligned_storage
+{
+  union type
+    {
+      unsigned char __data[_Len];
+      struct __attribute__((__aligned__((_Align)))) { } __align;
+    };
+};
+
+aligned_storage<sizeof(int), __alignof__(int)>::type storage;
+
+int main()
+{
+  *reinterpret_cast<int*>(&storage) = 42; // { dg-bogus "break strict-aliasing" }
+}