re PR c++/70259 (-flifetime-dse=2 bug with empty bases)

	PR c++/70259
	* decl.c (start_preparsed_function): Don't clobber an empty base.

From-SVN: r234267

gcc/cp/ChangeLog

+2016-03-16  Jason Merrill  <jason@redhat.com>
+
+	PR c++/70259
+	* decl.c (start_preparsed_function): Don't clobber an empty base.
+
 2016-03-16  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/70147

gcc/cp/decl.c

@@ -14121,6 +14121,8 @@ start_preparsed_function (tree decl1, tree attrs, int flags)
       && (flag_lifetime_dse > 1)
       && DECL_CONSTRUCTOR_P (decl1)
       && !DECL_CLONED_FUNCTION_P (decl1)
+      /* Clobbering an empty base is harmful if it overlays real data.  */
+      && !is_empty_class (current_class_type)
       /* We can't clobber safely for an implicitly-defined default constructor
 	 because part of the initialization might happen before we enter the
 	 constructor, via AGGR_INIT_ZERO_FIRST (c++/68006).  */

gcc/testsuite/g++.dg/opt/flifetime-dse5.C

+// PR c++/70259
+// { dg-options -O2 }
+// { dg-do run }
+
+struct Empty { };
+struct A { A() : a(true) { } bool a; };
+struct B : Empty { B() : Empty() { } };
+struct C : A, B { C() : A(), B() { } };
+int main() {
+  C c;
+  if ( c.a == false )
+    __builtin_abort();
+};