Add a simulate_record_decl lang hook

This patch adds a lang hook for defining a struct/RECORD_TYPE
“as if” it had appeared directly in the source code.  It follows
the similar existing hook for enums.

It's the caller's responsibility to create the fields
(as FIELD_DECLs) but the hook's responsibility to create
and declare the associated RECORD_TYPE.

For now the hook is hard-coded to do the equivalent of:

  typedef struct NAME { FIELDS } NAME;

but this could be controlled by an extra parameter if some callers
want a different behaviour in future.

The motivating use case is to allow the long list of struct
definitions in arm_neon.h to be provided by the compiler,
which in turn unblocks various arm_neon.h optimisations.

gcc/
	* langhooks.h (lang_hooks_for_types::simulate_record_decl): New hook.
	* langhooks-def.h (lhd_simulate_record_decl): Declare.
	(LANG_HOOKS_SIMULATE_RECORD_DECL): Define.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Include it.
	* langhooks.c (lhd_simulate_record_decl): New function.

gcc/c/
	* c-tree.h (c_simulate_record_decl): Declare.
	* c-objc-common.h (LANG_HOOKS_SIMULATE_RECORD_DECL): Override.
	* c-decl.c (c_simulate_record_decl): New function.

gcc/cp/
	* decl.c: Include langhooks-def.h.
	(cxx_simulate_record_decl): New function.
	* cp-objcp-common.h (cxx_simulate_record_decl): Declare.
	(LANG_HOOKS_SIMULATE_RECORD_DECL): Override.

3 files changed, 34 insertions, 0 deletions

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 771efa3..186fa16 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -9436,6 +9436,36 @@ c_simulate_enum_decl (location_t loc, const char *name,
   input_location = saved_loc;
   return enumtype;
 }
+
+/* Implement LANG_HOOKS_SIMULATE_RECORD_DECL.  */
+
+tree
+c_simulate_record_decl (location_t loc, const char *name,
+			array_slice<const tree> fields)
+{
+  location_t saved_loc = input_location;
+  input_location = loc;
+
+  class c_struct_parse_info *struct_info;
+  tree ident = get_identifier (name);
+  tree type = start_struct (loc, RECORD_TYPE, ident, &struct_info);
+
+  for (unsigned int i = 0; i < fields.size (); ++i)
+    {
+      DECL_FIELD_CONTEXT (fields[i]) = type;
+      if (i > 0)
+	DECL_CHAIN (fields[i - 1]) = fields[i];
+    }
+
+  finish_struct (loc, type, fields[0], NULL_TREE, struct_info);
+
+  tree decl = build_decl (loc, TYPE_DECL, ident, type);
+  set_underlying_type (decl);
+  lang_hooks.decls.pushdecl (decl);
+
+  input_location = saved_loc;
+  return type;
+}
 
 /* Create the FUNCTION_DECL for a function definition.
    DECLSPECS, DECLARATOR and ATTRIBUTES are the parts of
diff --git a/gcc/c/c-objc-common.h b/gcc/c/c-objc-common.h
index 7d35a06..f4e8271 100644
--- a/gcc/c/c-objc-common.h
+++ b/gcc/c/c-objc-common.h
@@ -81,6 +81,8 @@ along with GCC; see the file COPYING3.  If not see
 
 #undef LANG_HOOKS_SIMULATE_ENUM_DECL
 #define LANG_HOOKS_SIMULATE_ENUM_DECL c_simulate_enum_decl
+#undef LANG_HOOKS_SIMULATE_RECORD_DECL
+#define LANG_HOOKS_SIMULATE_RECORD_DECL c_simulate_record_decl
 #undef LANG_HOOKS_TYPE_FOR_MODE
 #define LANG_HOOKS_TYPE_FOR_MODE c_common_type_for_mode
 #undef LANG_HOOKS_TYPE_FOR_SIZE
diff --git a/gcc/c/c-tree.h b/gcc/c/c-tree.h
index a046c6b..f1dbbd5 100644
--- a/gcc/c/c-tree.h
+++ b/gcc/c/c-tree.h
@@ -598,6 +598,8 @@ extern tree finish_struct (location_t, tree, tree, tree,
 			   class c_struct_parse_info *);
 extern tree c_simulate_enum_decl (location_t, const char *,
 				  vec<string_int_pair> *);
+extern tree c_simulate_record_decl (location_t, const char *,
+				    array_slice<const tree>);
 extern struct c_arg_info *build_arg_info (void);
 extern struct c_arg_info *get_parm_info (bool, tree);
 extern tree grokfield (location_t, struct c_declarator *,