Index: gcc/langhooks.h
===================================================================
--- gcc/langhooks.h	2019-03-08 18:15:33.660751905 +0000
+++ gcc/langhooks.h	2019-09-26 13:02:42.543520465 +0100
@@ -494,6 +494,15 @@ struct lang_hooks
      backend must add all of the builtins at program initialization time.  */
   tree (*builtin_function_ext_scope) (tree decl);
 
+  /* Do language-specific processing for target-specific built-in
+     function DECL, so that it is defined in the global scope (only)
+     and is available without needing to be explicitly declared.
+
+     This is intended for targets that want to inject declarations of
+     built-in functions into the source language (such as in response
+     to a pragma) rather than providing them in the source language itself.  */
+  tree (*simulate_builtin_function_decl) (tree decl);
+
   /* Used to set up the tree_contains_structure array for a frontend. */
   void (*init_ts) (void);
 
@@ -562,6 +571,8 @@ extern tree add_builtin_function_ext_sco
 					    enum built_in_class cl,
 					    const char *library_name,
 					    tree attrs);
+extern tree simulate_builtin_function_decl (location_t, const char *, tree,
+					    int, const char *, tree);
 extern tree add_builtin_type (const char *name, tree type);
 
 /* Language helper functions.  */
Index: gcc/langhooks-def.h
===================================================================
--- gcc/langhooks-def.h	2019-03-08 18:15:39.328730361 +0000
+++ gcc/langhooks-def.h	2019-09-26 13:02:42.543520465 +0100
@@ -122,6 +122,7 @@ #define LANG_HOOKS_TREE_SIZE		lhd_tree_s
 #define LANG_HOOKS_TYPES_COMPATIBLE_P	lhd_types_compatible_p
 #define LANG_HOOKS_BUILTIN_FUNCTION	lhd_builtin_function
 #define LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE	LANG_HOOKS_BUILTIN_FUNCTION
+#define LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL LANG_HOOKS_BUILTIN_FUNCTION
 #define LANG_HOOKS_EXPR_TO_DECL		lhd_expr_to_decl
 #define LANG_HOOKS_TO_TARGET_CHARSET	lhd_to_target_charset
 #define LANG_HOOKS_INIT_TS		lhd_do_nothing
@@ -338,6 +339,7 @@ #define LANG_HOOKS_INITIALIZER { \
   LANG_HOOKS_GIMPLIFY_EXPR, \
   LANG_HOOKS_BUILTIN_FUNCTION, \
   LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE, \
+  LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL, \
   LANG_HOOKS_INIT_TS,          \
   LANG_HOOKS_EXPR_TO_DECL, \
   LANG_HOOKS_EH_PERSONALITY, \
Index: gcc/langhooks.c
===================================================================
--- gcc/langhooks.c	2019-08-25 19:10:35.194159619 +0100
+++ gcc/langhooks.c	2019-09-26 13:02:42.543520465 +0100
@@ -599,19 +599,16 @@ lhd_omp_mappable_type (tree type)
   return true;
 }
 
-/* Common function for add_builtin_function and
-   add_builtin_function_ext_scope.  */
+/* Common function for add_builtin_function, add_builtin_function_ext_scope
+   and simulate_builtin_function_decl.  */
+
 static tree
-add_builtin_function_common (const char *name,
-			     tree type,
-			     int function_code,
-			     enum built_in_class cl,
-			     const char *library_name,
-			     tree attrs,
-			     tree (*hook) (tree))
+build_builtin_function (location_t location, const char *name, tree type,
+			int function_code, enum built_in_class cl,
+			const char *library_name, tree attrs)
 {
   tree   id = get_identifier (name);
-  tree decl = build_decl (BUILTINS_LOCATION, FUNCTION_DECL, id, type);
+  tree decl = build_decl (location, FUNCTION_DECL, id, type);
 
   TREE_PUBLIC (decl)         = 1;
   DECL_EXTERNAL (decl)       = 1;
@@ -632,8 +629,7 @@ add_builtin_function_common (const char
   else
     decl_attributes (&decl, NULL_TREE, 0);
 
-  return hook (decl);
-
+  return decl;
 }
 
 /* Create a builtin function.  */
@@ -646,9 +642,9 @@ add_builtin_function (const char *name,
 		      const char *library_name,
 		      tree attrs)
 {
-  return add_builtin_function_common (name, type, function_code, cl,
-				      library_name, attrs,
-				      lang_hooks.builtin_function);
+  tree decl = build_builtin_function (BUILTINS_LOCATION, name, type,
+				      function_code, cl, library_name, attrs);
+  return lang_hooks.builtin_function (decl);
 }
 
 /* Like add_builtin_function, but make sure the scope is the external scope.
@@ -666,9 +662,40 @@ add_builtin_function_ext_scope (const ch
 				const char *library_name,
 				tree attrs)
 {
-  return add_builtin_function_common (name, type, function_code, cl,
-				      library_name, attrs,
-				      lang_hooks.builtin_function_ext_scope);
+  tree decl = build_builtin_function (BUILTINS_LOCATION, name, type,
+				      function_code, cl, library_name, attrs);
+  return lang_hooks.builtin_function_ext_scope (decl);
+}
+
+/* Simulate a declaration of a target-specific built-in function at
+   location LOCATION, as though it had been declared directly in the
+   source language.  NAME is the name of the function, TYPE is its function
+   type, FUNCTION_CODE is the target-specific function code, LIBRARY_NAME
+   is the name of the underlying library function (NULL if none) and
+   ATTRS is a list of function attributes.
+
+   Return the decl of the declared function.  */
+
+tree
+simulate_builtin_function_decl (location_t location, const char *name,
+				tree type, int function_code,
+				const char *library_name, tree attrs)
+{
+  tree decl = build_builtin_function (location, name, type,
+				      function_code, BUILT_IN_MD,
+				      library_name, attrs);
+  tree new_decl = lang_hooks.simulate_builtin_function_decl (decl);
+
+  /* Give the front end a chance to create a new decl if necessary,
+     but if the front end discards the decl in favour of a conflicting
+     (erroneous) previous definition, return the decl that we tried but
+     failed to add.  This allows the caller to process the returned decl
+     normally, even though the source code won't be able to use it.  */
+  if (TREE_CODE (new_decl) == FUNCTION_DECL
+      && fndecl_built_in_p (new_decl, function_code, BUILT_IN_MD))
+    return new_decl;
+
+  return decl;
 }
 
 tree
Index: gcc/c/c-tree.h
===================================================================
--- gcc/c/c-tree.h	2019-07-10 19:41:20.543944894 +0100
+++ gcc/c/c-tree.h	2019-09-26 13:02:42.539520492 +0100
@@ -579,6 +579,7 @@ extern struct c_declarator *set_array_de
 							struct c_declarator *);
 extern tree c_builtin_function (tree);
 extern tree c_builtin_function_ext_scope (tree);
+extern tree c_simulate_builtin_function_decl (tree);
 extern void shadow_tag (const struct c_declspecs *);
 extern void shadow_tag_warned (const struct c_declspecs *, int);
 extern tree start_enum (location_t, struct c_enum_contents *, tree);
Index: gcc/c/c-decl.c
===================================================================
--- gcc/c/c-decl.c	2019-09-21 13:56:06.983948786 +0100
+++ gcc/c/c-decl.c	2019-09-26 13:02:42.539520492 +0100
@@ -4481,6 +4481,16 @@ c_builtin_function_ext_scope (tree decl)
 
   return decl;
 }
+
+/* Implement LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL.  */
+
+tree
+c_simulate_builtin_function_decl (tree decl)
+{
+  tree type = TREE_TYPE (decl);
+  C_DECL_BUILTIN_PROTOTYPE (decl) = prototype_p (type);
+  return pushdecl (decl);
+}
 
 /* Called when a declaration is seen that contains no names to declare.
    If its type is a reference to a structure, union or enum inherited
Index: gcc/c/c-objc-common.h
===================================================================
--- gcc/c/c-objc-common.h	2019-04-04 08:34:51.661939350 +0100
+++ gcc/c/c-objc-common.h	2019-09-26 13:02:42.539520492 +0100
@@ -60,6 +60,9 @@ #define LANG_HOOKS_MISSING_NORETURN_OK_P
 #define LANG_HOOKS_BUILTIN_FUNCTION c_builtin_function
 #undef  LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE
 #define LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE c_builtin_function_ext_scope
+#undef  LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL
+#define LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL \
+  c_simulate_builtin_function_decl
 #undef LANG_HOOKS_EMITS_BEGIN_STMT
 #define LANG_HOOKS_EMITS_BEGIN_STMT true
 
Index: gcc/cp/cp-tree.h
===================================================================
--- gcc/cp/cp-tree.h	2019-09-26 08:38:00.759713359 +0100
+++ gcc/cp/cp-tree.h	2019-09-26 13:02:42.539520492 +0100
@@ -6461,6 +6461,7 @@ extern tmpl_spec_kind current_tmpl_spec_
 extern tree cp_fname_init			(const char *, tree *);
 extern tree cxx_builtin_function		(tree decl);
 extern tree cxx_builtin_function_ext_scope	(tree decl);
+extern tree cxx_simulate_builtin_function_decl	(tree);
 extern tree check_elaborated_type_specifier	(enum tag_types, tree, bool);
 extern void warn_extern_redeclared_static	(tree, tree);
 extern tree cxx_comdat_group			(tree);
Index: gcc/cp/decl.c
===================================================================
--- gcc/cp/decl.c	2019-09-17 15:27:10.958069743 +0100
+++ gcc/cp/decl.c	2019-09-26 13:02:42.543520465 +0100
@@ -71,7 +71,6 @@ static tree grokvardecl (tree, tree, tre
 			 int, int, int, bool, int, tree, location_t);
 static void check_static_variable_definition (tree, tree);
 static void record_unknown_type (tree, const char *);
-static tree builtin_function_1 (tree, tree, bool);
 static int member_function_or_else (tree, tree, enum overload_flags);
 static tree local_variable_p_walkfn (tree *, int *, void *);
 static const char *tag_name (enum tag_types);
@@ -4567,8 +4566,12 @@ cp_make_fname_decl (location_t loc, tree
   return decl;
 }
 
+/* IS_SIMULATED_DECL is true if the decl should simulate a global
+   declaration in the source language.  */
+
 static tree
-builtin_function_1 (tree decl, tree context, bool is_global)
+builtin_function_1 (tree decl, tree context, bool is_global,
+		    bool is_simulated_decl = false)
 {
   tree          id = DECL_NAME (decl);
   const char *name = IDENTIFIER_POINTER (id);
@@ -4576,7 +4579,10 @@ builtin_function_1 (tree decl, tree cont
   retrofit_lang_decl (decl);
 
   DECL_ARTIFICIAL (decl) = 1;
-  SET_DECL_LANGUAGE (decl, lang_c);
+  if (is_simulated_decl)
+    SET_DECL_LANGUAGE (decl, lang_cplusplus);
+  else
+    SET_DECL_LANGUAGE (decl, lang_c);
   /* Runtime library routines are, by definition, available in an
      external shared object.  */
   DECL_VISIBILITY (decl) = VISIBILITY_DEFAULT;
@@ -4584,21 +4590,24 @@ builtin_function_1 (tree decl, tree cont
 
   DECL_CONTEXT (decl) = context;
 
-  /* A function in the user's namespace should have an explicit
-     declaration before it is used.  Mark the built-in function as
-     anticipated but not actually declared.  */
-  if (name[0] != '_' || name[1] != '_')
-    DECL_ANTICIPATED (decl) = 1;
-  else if (strncmp (name + 2, "builtin_", strlen ("builtin_")) != 0)
-    {
-      size_t len = strlen (name);
-
-      /* Treat __*_chk fortification functions as anticipated as well,
-	 unless they are __builtin_*.  */
-      if (len > strlen ("___chk")
-	  && memcmp (name + len - strlen ("_chk"),
-		     "_chk", strlen ("_chk") + 1) == 0)
+  if (!is_simulated_decl)
+    {
+      /* A function in the user's namespace should have an explicit
+	 declaration before it is used.  Mark the built-in function as
+	 anticipated but not actually declared.  */
+      if (name[0] != '_' || name[1] != '_')
 	DECL_ANTICIPATED (decl) = 1;
+      else if (strncmp (name + 2, "builtin_", strlen ("builtin_")) != 0)
+	{
+	  size_t len = strlen (name);
+
+	  /* Treat __*_chk fortification functions as anticipated as well,
+	     unless they are __builtin_*.  */
+	  if (len > strlen ("___chk")
+	      && memcmp (name + len - strlen ("_chk"),
+			 "_chk", strlen ("_chk") + 1) == 0)
+	    DECL_ANTICIPATED (decl) = 1;
+	}
     }
 
   if (is_global)
@@ -4650,6 +4659,14 @@ cxx_builtin_function_ext_scope (tree dec
   return builtin_function_1 (decl, NULL_TREE, true);
 }
 
+/* Implement LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL.  */
+
+tree
+cxx_simulate_builtin_function_decl (tree decl)
+{
+  return builtin_function_1 (decl, NULL_TREE, true, true);
+}
+
 /* Generate a FUNCTION_DECL with the typical flags for a runtime library
    function.  Not called directly.  */
 
Index: gcc/cp/cp-objcp-common.h
===================================================================
--- gcc/cp/cp-objcp-common.h	2019-03-08 18:15:25.000784821 +0000
+++ gcc/cp/cp-objcp-common.h	2019-09-26 13:02:42.539520492 +0100
@@ -100,6 +100,9 @@ #define LANG_HOOKS_POST_COMPILATION_PARS
 #define LANG_HOOKS_BUILTIN_FUNCTION cxx_builtin_function
 #undef  LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE
 #define LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE cxx_builtin_function_ext_scope
+#undef  LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL
+#define LANG_HOOKS_SIMULATE_BUILTIN_FUNCTION_DECL \
+  cxx_simulate_builtin_function_decl
 #undef	LANG_HOOKS_TYPE_HASH_EQ
 #define LANG_HOOKS_TYPE_HASH_EQ	cxx_type_hash_eq
 #undef	LANG_HOOKS_COPY_LANG_QUALIFIERS
