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.

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

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

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 *,

gcc/cp/cp-objcp-common.h

@@ -39,6 +39,8 @@ extern bool cp_handle_option (size_t, const char *, HOST_WIDE_INT, int,
 extern tree cxx_make_type_hook			(tree_code);
 extern tree cxx_simulate_enum_decl (location_t, const char *,
 				    vec<string_int_pair> *);
+extern tree cxx_simulate_record_decl (location_t, const char *,
+				      array_slice<const tree>);
 
 /* Lang hooks that are shared between C++ and ObjC++ are defined here.  Hooks
    specific to C++ or ObjC++ go in cp/cp-lang.c and objcp/objcp-lang.c,
@@ -139,6 +141,8 @@ extern tree cxx_simulate_enum_decl (location_t, const char *,
 #define LANG_HOOKS_MAKE_TYPE cxx_make_type_hook
 #undef LANG_HOOKS_SIMULATE_ENUM_DECL
 #define LANG_HOOKS_SIMULATE_ENUM_DECL cxx_simulate_enum_decl
+#undef LANG_HOOKS_SIMULATE_RECORD_DECL
+#define LANG_HOOKS_SIMULATE_RECORD_DECL cxx_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

gcc/cp/decl.c

@@ -57,6 +57,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "omp-general.h"
 #include "omp-offload.h"  /* For offload_vars.  */
 #include "opts.h"
+#include "langhooks-def.h"  /* For lhd_simulate_record_decl  */
 
 /* Possible cases of bad specifiers type used by bad_specifiers. */
 enum bad_spec_place {
@@ -16602,6 +16603,42 @@ cxx_simulate_enum_decl (location_t loc, const char *name,
   input_location = saved_loc;
   return enumtype;
 }
+
+/* Implement LANG_HOOKS_SIMULATE_RECORD_DECL.  */
+
+tree
+cxx_simulate_record_decl (location_t loc, const char *name,
+			  array_slice<const tree> fields)
+{
+  iloc_sentinel ils (loc);
+
+  tree ident = get_identifier (name);
+  tree type = xref_tag (/*tag_code=*/record_type, ident);
+  if (type != error_mark_node
+      && (TREE_CODE (type) != RECORD_TYPE || COMPLETE_TYPE_P (type)))
+    {
+      error ("redefinition of %q#T", type);
+      type = error_mark_node;
+    }
+  if (type == error_mark_node)
+    return lhd_simulate_record_decl (loc, name, fields);
+
+  xref_basetypes (type, NULL_TREE);
+  type = begin_class_definition (type);
+  if (type == error_mark_node)
+    return lhd_simulate_record_decl (loc, name, fields);
+
+  for (tree field : fields)
+    finish_member_declaration (field);
+
+  type = finish_struct (type, NULL_TREE);
+
+  tree decl = build_decl (loc, TYPE_DECL, ident, type);
+  set_underlying_type (decl);
+  lang_hooks.decls.pushdecl (decl);
+
+  return type;
+}
 
 /* We're defining DECL.  Make sure that its type is OK.  */
 

gcc/langhooks-def.h

@@ -56,6 +56,8 @@ extern void lhd_overwrite_decl_assembler_name (tree decl, tree name);
 extern bool lhd_warn_unused_global_decl (const_tree);
 extern tree lhd_simulate_enum_decl (location_t, const char *,
 				    vec<string_int_pair> *);
+extern tree lhd_simulate_record_decl (location_t, const char *,
+				      array_slice<const tree>);
 extern tree lhd_type_for_size (unsigned precision, int unsignedp);
 extern void lhd_incomplete_type_error (location_t, const_tree, const_tree);
 extern tree lhd_type_promotes_to (tree);
@@ -183,6 +185,7 @@ extern tree lhd_unit_size_without_reusable_padding (tree);
 
 #define LANG_HOOKS_MAKE_TYPE lhd_make_node
 #define LANG_HOOKS_SIMULATE_ENUM_DECL	lhd_simulate_enum_decl
+#define LANG_HOOKS_SIMULATE_RECORD_DECL	lhd_simulate_record_decl
 #define LANG_HOOKS_CLASSIFY_RECORD	NULL
 #define LANG_HOOKS_TYPE_FOR_SIZE	lhd_type_for_size
 #define LANG_HOOKS_INCOMPLETE_TYPE_ERROR lhd_incomplete_type_error
@@ -217,6 +220,7 @@ extern tree lhd_unit_size_without_reusable_padding (tree);
 #define LANG_HOOKS_FOR_TYPES_INITIALIZER { \
   LANG_HOOKS_MAKE_TYPE, \
   LANG_HOOKS_SIMULATE_ENUM_DECL, \
+  LANG_HOOKS_SIMULATE_RECORD_DECL, \
   LANG_HOOKS_CLASSIFY_RECORD, \
   LANG_HOOKS_TYPE_FOR_MODE, \
   LANG_HOOKS_TYPE_FOR_SIZE, \

gcc/langhooks.c

@@ -516,6 +516,25 @@ lhd_simulate_enum_decl (location_t loc, const char *name,
   return enumtype;
 }
 
+/* Default implementation of LANG_HOOKS_SIMULATE_RECORD_DECL.
+   Just create a normal RECORD_TYPE and a TYPE_DECL for it.  */
+tree
+lhd_simulate_record_decl (location_t loc, const char *name,
+			  array_slice<const tree> fields)
+{
+  for (unsigned int i = 1; i < fields.size (); ++i)
+    /* Reversed by finish_builtin_struct.  */
+    DECL_CHAIN (fields[i]) = fields[i - 1];
+
+  tree type = lang_hooks.types.make_type (RECORD_TYPE);
+  finish_builtin_struct (type, name, fields.back (), NULL_TREE);
+
+  tree decl = build_decl (loc, TYPE_DECL, get_identifier (name), type);
+  lang_hooks.decls.pushdecl (decl);
+
+  return type;
+}
+
 /* Default implementation of LANG_HOOKS_TYPE_FOR_SIZE.
    Return an integer type with PRECISION bits of precision,
    that is unsigned if UNSIGNEDP is nonzero, otherwise signed.  */

gcc/langhooks.h

@@ -68,6 +68,16 @@ struct lang_hooks_for_types
      them all with the given source location.  */
   tree (*simulate_enum_decl) (location_t, const char *, vec<string_int_pair> *);
 
+  /* Do the equivalent of:
+
+       typedef struct NAME { FIELDS; } NAME;
+
+     associating it with location LOC.  Return the associated RECORD_TYPE.
+
+     FIELDS is a list of FIELD_DECLs, in layout order.  */
+  tree (*simulate_record_decl) (location_t loc, const char *name,
+				array_slice<const tree> fields);
+
   /* Return what kind of RECORD_TYPE this is, mainly for purposes of
      debug information.  If not defined, record types are assumed to
      be structures.  */