PR c++/20408 - unnecessary code for empty struct.

Here initializing the argument from a TARGET_EXPR isn't an empty class
copy even though the type is !TREE_ADDRESSABLE, so we should check
simple_empty_class_p.

	* call.c (build_call_a): Use simple_empty_class_p.

From-SVN: r271523

gcc/cp/ChangeLog

 2019-05-22  Jason Merrill  <jason@redhat.com>
 
+	PR c++/20408 - unnecessary code for empty struct.
+	* call.c (build_call_a): Use simple_empty_class_p.
+
 	PR c++/86485 - -Wmaybe-unused with empty class ?:
 	* cp-gimplify.c (simple_empty_class_p): Also true for MODIFY_EXPR.
 

gcc/cp/call.c

@@ -393,7 +393,7 @@ build_call_a (tree function, int n, tree *argarray)
       {
 	tree arg = CALL_EXPR_ARG (function, i);
 	if (is_empty_class (TREE_TYPE (arg))
-	    && ! TREE_ADDRESSABLE (TREE_TYPE (arg)))
+	    && simple_empty_class_p (TREE_TYPE (arg), arg, INIT_EXPR))
 	  {
 	    tree t = build0 (EMPTY_CLASS_EXPR, TREE_TYPE (arg));
 	    arg = build2 (COMPOUND_EXPR, TREE_TYPE (t), arg, t);

gcc/cp/cp-gimplify.c

@@ -593,7 +593,7 @@ gimplify_must_not_throw_expr (tree *expr_p, gimple_seq *pre_p)
    non-empty CONSTRUCTORs get reduced properly, and we leave the
    return slot optimization alone because it isn't a copy.  */
 
-static bool
+bool
 simple_empty_class_p (tree type, tree op, tree_code code)
 {
   if (TREE_CODE (op) == COMPOUND_EXPR)

gcc/cp/cp-tree.h

@@ -7581,6 +7581,7 @@ extern tree cp_fully_fold_init			(tree);
 extern void clear_fold_cache			(void);
 extern tree lookup_hotness_attribute		(tree);
 extern tree process_stmt_hotness_attribute	(tree, location_t);
+extern bool simple_empty_class_p		(tree, tree, tree_code);
 
 /* in name-lookup.c */
 extern tree strip_using_decl                    (tree);

gcc/testsuite/g++.dg/tree-ssa/empty-3.C

+// PR c++/20408
+// { dg-additional-options -fdump-tree-gimple }
+// { dg-final { scan-tree-dump-times "struct Foo" 2 "gimple" } }
+
+struct Foo {};
+void foo(const Foo&);
+void bar(Foo);
+
+void fooc(void)
+{
+        foo(Foo());
+}
+void barc(void)
+{
+        bar(Foo());
+}