c++: Avoid redundant copy in {} init [PR98642]

Here, initializing from { } implies a call to the default constructor for
base.  We were then seeing that we're initializing a base subobject, so we
tried to copy the result of that call.  This is clearly wrong; we should
initialize the base directly from its default constructor.

This patch does a lot of refactoring of unsafe_copy_elision_p and adds
make_safe_copy_elision that will also try to do the base constructor
rewriting from the last patch.

gcc/cp/ChangeLog:

	PR c++/98642
	* call.c (unsafe_return_slot_p): Return int.
	(init_by_return_slot_p): Split out from...
	(unsafe_copy_elision_p): ...here.
	(unsafe_copy_elision_p_opt): New name for old meaning.
	(build_over_call): Adjust.
	(make_safe_copy_elision): New.
	* typeck2.c (split_nonconstant_init_1): Elide copy from safe
	list-initialization.
	* cp-tree.h: Adjust.

gcc/testsuite/ChangeLog:

	PR c++/98642
	* g++.dg/cpp1z/elide5.C: New test.

gcc/cp/call.c

@@ -8439,7 +8439,7 @@ base_ctor_for (tree complete_ctor)
 
 /* Try to make EXP suitable to be used as the initializer for a base subobject,
    and return whether we were successful.  EXP must have already been cleared
-   by unsafe_copy_elision_p.  */
+   by unsafe_copy_elision_p{,_opt}.  */
 
 static bool
 make_base_init_ok (tree exp)
@@ -8463,7 +8463,7 @@ make_base_init_ok (tree exp)
     /* A trivial copy is OK.  */
     return true;
   if (!AGGR_INIT_VIA_CTOR_P (exp))
-    /* unsafe_copy_elision_p must have said this is OK.  */
+    /* unsafe_copy_elision_p_opt must have said this is OK.  */
     return true;
   tree fn = cp_get_callee_fndecl_nofold (exp);
   if (DECL_BASE_CONSTRUCTOR_P (fn))
@@ -8479,20 +8479,20 @@ make_base_init_ok (tree exp)
   return true;
 }
 
-/* Return true iff T refers to a base or potentially-overlapping field, which
-   cannot be used for return by invisible reference.  We avoid doing C++17
-   mandatory copy elision when this is true.
+/* Return 2 if T refers to a base, 1 if a potentially-overlapping field,
+   neither of which can be used for return by invisible reference.  We avoid
+   doing C++17 mandatory copy elision for either of these cases.
 
-   This returns true even if the type of T has no tail padding that other data
-   could be allocated into, because that depends on the particular ABI.
-   unsafe_copy_elision_p, below, does consider whether there is padding.  */
+   This returns non-zero even if the type of T has no tail padding that other
+   data could be allocated into, because that depends on the particular ABI.
+   unsafe_copy_elision_p_opt does consider whether there is padding.  */
 
-bool
+int
 unsafe_return_slot_p (tree t)
 {
   /* Check empty bases separately, they don't have fields.  */
   if (is_empty_base_ref (t))
-    return true;
+    return 2;
 
   STRIP_NOPS (t);
   if (TREE_CODE (t) == ADDR_EXPR)
@@ -8504,28 +8504,21 @@ unsafe_return_slot_p (tree t)
   if (!CLASS_TYPE_P (TREE_TYPE (t)))
     /* The middle-end will do the right thing for scalar types.  */
     return false;
-  return (DECL_FIELD_IS_BASE (t)
-	  || lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (t)));
+  if (DECL_FIELD_IS_BASE (t))
+    return 2;
+  if (lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (t)))
+    return 1;
+  return 0;
 }
 
-/* We can't elide a copy from a function returning by value to a
-   potentially-overlapping subobject, as the callee might clobber tail padding.
-   Return true iff this could be that case.  */
+/* True IFF EXP is a prvalue that represents return by invisible reference.  */
 
 static bool
-unsafe_copy_elision_p (tree target, tree exp)
+init_by_return_slot_p (tree exp)
 {
   /* Copy elision only happens with a TARGET_EXPR.  */
   if (TREE_CODE (exp) != TARGET_EXPR)
     return false;
-  tree type = TYPE_MAIN_VARIANT (TREE_TYPE (exp));
-  /* It's safe to elide the copy for a class with no tail padding.  */
-  if (!is_empty_class (type)
-      && tree_int_cst_equal (TYPE_SIZE (type), CLASSTYPE_SIZE (type)))
-    return false;
-  /* It's safe to elide the copy if we aren't initializing a base object.  */
-  if (!unsafe_return_slot_p (target))
-    return false;
   tree init = TARGET_EXPR_INITIAL (exp);
   /* build_compound_expr pushes COMPOUND_EXPR inside TARGET_EXPR.  */
   while (TREE_CODE (init) == COMPOUND_EXPR)
@@ -8533,16 +8526,58 @@ unsafe_copy_elision_p (tree target, tree exp)
   if (TREE_CODE (init) == COND_EXPR)
     {
       /* We'll end up copying from each of the arms of the COND_EXPR directly
-	 into the target, so look at them. */
+	 into the target, so look at them.  */
       if (tree op = TREE_OPERAND (init, 1))
-	if (unsafe_copy_elision_p (target, op))
+	if (init_by_return_slot_p (op))
 	  return true;
-      return unsafe_copy_elision_p (target, TREE_OPERAND (init, 2));
+      return init_by_return_slot_p (TREE_OPERAND (init, 2));
     }
   return (TREE_CODE (init) == AGGR_INIT_EXPR
 	  && !AGGR_INIT_VIA_CTOR_P (init));
 }
 
+/* We can't elide a copy from a function returning by value to a
+   potentially-overlapping subobject, as the callee might clobber tail padding.
+   Return true iff this could be that case.
+
+   Places that use this function (or _opt) to decide to elide a copy should
+   probably use make_safe_copy_elision instead.  */
+
+static bool
+unsafe_copy_elision_p (tree target, tree exp)
+{
+  return unsafe_return_slot_p (target) && init_by_return_slot_p (exp);
+}
+
+/* As above, but for optimization allow more cases that are actually safe.  */
+
+static bool
+unsafe_copy_elision_p_opt (tree target, tree exp)
+{
+  tree type = TYPE_MAIN_VARIANT (TREE_TYPE (exp));
+  /* It's safe to elide the copy for a class with no tail padding.  */
+  if (!is_empty_class (type)
+      && tree_int_cst_equal (TYPE_SIZE (type), CLASSTYPE_SIZE (type)))
+    return false;
+  return unsafe_copy_elision_p (target, exp);
+}
+
+/* Try to make EXP suitable to be used as the initializer for TARGET,
+   and return whether we were successful.  */
+
+bool
+make_safe_copy_elision (tree target, tree exp)
+{
+  int uns = unsafe_return_slot_p (target);
+  if (!uns)
+    return true;
+  if (init_by_return_slot_p (exp))
+    return false;
+  if (uns == 1)
+    return true;
+  return make_base_init_ok (exp);
+}
+
 /* True IFF the result of the conversion C is a prvalue.  */
 
 static bool
@@ -9188,7 +9223,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 		    /* See unsafe_copy_elision_p.  */
 		    || unsafe_return_slot_p (fa));
 
-      bool unsafe = unsafe_copy_elision_p (fa, arg);
+      bool unsafe = unsafe_copy_elision_p_opt (fa, arg);
       bool eliding_temp = (TREE_CODE (arg) == TARGET_EXPR && !unsafe);
 
       /* [class.copy]: the copy constructor is implicitly defined even if the

gcc/cp/cp-tree.h

@@ -6471,7 +6471,8 @@ extern bool is_std_init_list			(tree);
 extern bool is_list_ctor			(tree);
 extern void validate_conversion_obstack		(void);
 extern void mark_versions_used			(tree);
-extern bool unsafe_return_slot_p		(tree);
+extern int unsafe_return_slot_p			(tree);
+extern bool make_safe_copy_elision		(tree, tree);
 extern bool cp_warn_deprecated_use		(tree, tsubst_flags_t = tf_warning_or_error);
 extern void cp_warn_deprecated_use_scopes	(tree);
 extern tree get_function_version_dispatcher	(tree);

gcc/cp/typeck2.c

@@ -569,17 +569,30 @@ split_nonconstant_init_1 (tree dest, tree init, bool nested)
 		    sub = build3 (COMPONENT_REF, inner_type, dest, field_index,
 				  NULL_TREE);
 
+		  /* We may need to add a copy constructor call if
+		     the field has [[no_unique_address]].  */
 		  if (unsafe_return_slot_p (sub))
 		    {
-		      /* We may need to add a copy constructor call if
-			 the field has [[no_unique_address]].  */
+		      /* But not if the initializer is an implicit ctor call
+			 we just built in digest_init.  */
+		      if (TREE_CODE (value) == TARGET_EXPR
+			  && TARGET_EXPR_LIST_INIT_P (value)
+			  && make_safe_copy_elision (sub, value))
+			goto build_init;
+
+		      tree name = (DECL_FIELD_IS_BASE (field_index)
+				   ? base_ctor_identifier
+				   : complete_ctor_identifier);
 		      releasing_vec args = make_tree_vector_single (value);
 		      code = build_special_member_call
-			(sub, complete_ctor_identifier, &args, inner_type,
+			(sub, name, &args, inner_type,
 			 LOOKUP_NORMAL, tf_warning_or_error);
 		    }
 		  else
-		    code = build2 (INIT_EXPR, inner_type, sub, value);
+		    {
+		    build_init:
+		      code = build2 (INIT_EXPR, inner_type, sub, value);
+		    }
 		  code = build_stmt (input_location, EXPR_STMT, code);
 		  code = maybe_cleanup_point_expr_void (code);
 		  add_stmt (code);

gcc/testsuite/g++.dg/cpp1z/elide5.C

@@ -0,0 +1,15 @@
+// PR c++/98642
+// { dg-do compile { target c++11 } }
+
+struct base {
+  base(void) {}
+  base(base &&) = delete;
+};
+
+struct foo : public base { };
+
+static foo c1 { };
+
+#if __cpp_aggregate_bases
+static foo c2 { {} };
+#endif