diff --git a/iseq.c b/iseq.c
index d985b57239..ee78a918f1 100644
--- a/iseq.c
+++ b/iseq.c
@@ -708,7 +708,7 @@ rb_iseq_compile_with_option(VALUE src, VALUE file, VALUE realpath, VALUE line, c
 	ast = (*parse)(parser, file, src, ln);
     }
 
-    if (!ast->root) {
+    if (!ast->body.root) {
 	rb_ast_dispose(ast);
 	rb_exc_raise(GET_EC()->errinfo);
     }
@@ -716,7 +716,7 @@ rb_iseq_compile_with_option(VALUE src, VALUE file, VALUE realpath, VALUE line, c
 	INITIALIZED VALUE label = parent ?
 	    parent->body->location.label :
 	    rb_fstring_cstr("<compiled>");
-	iseq = rb_iseq_new_with_opt(ast->root, label, file, realpath, line,
+	iseq = rb_iseq_new_with_opt(ast->body.root, label, file, realpath, line,
 				    parent, type, &option);
 	rb_ast_dispose(ast);
     }
@@ -927,17 +927,17 @@ iseqw_s_compile_file(int argc, VALUE *argv, VALUE self)
     parser = rb_parser_new();
     rb_parser_set_context(parser, NULL, FALSE);
     ast = rb_parser_compile_file_path(parser, file, f, NUM2INT(line));
-    if (!ast->root) exc = GET_EC()->errinfo;
+    if (!ast->body.root) exc = GET_EC()->errinfo;
 
     rb_io_close(f);
-    if (!ast->root) {
+    if (!ast->body.root) {
 	rb_ast_dispose(ast);
 	rb_exc_raise(exc);
     }
 
     make_compile_option(&option, opt);
 
-    ret = iseqw_new(rb_iseq_new_with_opt(ast->root, rb_fstring_cstr("<main>"),
+    ret = iseqw_new(rb_iseq_new_with_opt(ast->body.root, rb_fstring_cstr("<main>"),
 					 file,
 					 rb_realpath_internal(Qnil, file, 1),
 					 line, NULL, ISEQ_TYPE_TOP, &option));
diff --git a/load.c b/load.c
index cfc8b1f032..e127eb1b68 100644
--- a/load.c
+++ b/load.c
@@ -604,7 +604,7 @@ rb_load_internal0(rb_execution_context_t *ec, VALUE fname, int wrap)
 	    VALUE parser = rb_parser_new();
 	    rb_parser_set_context(parser, NULL, FALSE);
 	    ast = (rb_ast_t *)rb_parser_load_file(parser, fname);
-	    iseq = rb_iseq_new_top(ast->root, rb_fstring_cstr("<top (required)>"),
+	    iseq = rb_iseq_new_top(ast->body.root, rb_fstring_cstr("<top (required)>"),
 			    fname, rb_realpath_internal(Qnil, fname, 1), NULL);
 	    rb_ast_dispose(ast);
 	}
diff --git a/node.c b/node.c
index 4254fd7425..072f4f57b3 100644
--- a/node.c
+++ b/node.c
@@ -1150,7 +1150,7 @@ rb_ast_delete_node(rb_ast_t *ast, NODE *n)
 rb_ast_t *
 rb_ast_new(void)
 {
-    return (rb_ast_t *)rb_imemo_new(imemo_ast, 0, (VALUE)rb_node_buffer_new(), rb_ary_tmp_new(0), 0);
+    return (rb_ast_t *)rb_imemo_new(imemo_ast, rb_ary_tmp_new(0), 0, 0, (VALUE)rb_node_buffer_new());
 }
 
 void
diff --git a/node.h b/node.h
index aa16a1f93f..f0ddf58d57 100644
--- a/node.h
+++ b/node.h
@@ -467,12 +467,15 @@ RUBY_SYMBOL_EXPORT_BEGIN
 
 typedef struct node_buffer_struct node_buffer_t;
 /* T_IMEMO/ast */
+typedef struct rb_ast_body_struct {
+    const NODE *root;
+    VALUE reserved;
+} rb_ast_body_t;
 typedef struct rb_ast_struct {
     VALUE flags;
-    VALUE reserved1;
-    const NODE *root;
     node_buffer_t *node_buffer;
     VALUE mark_ary;
+    rb_ast_body_t body;
 } rb_ast_t;
 rb_ast_t *rb_ast_new();
 void rb_ast_mark(rb_ast_t*);
diff --git a/parse.y b/parse.y
index a036c808b6..6df4a3b2ca 100644
--- a/parse.y
+++ b/parse.y
@@ -5675,7 +5675,7 @@ yycompile(VALUE vparser, struct parser_params *parser, VALUE fname, int line)
     ruby_sourceline = line - 1;
 
     parser->ast = ast = rb_ast_new();
-    ast->root = (NODE *)rb_suppress_tracing(yycompile0, (VALUE)parser);
+    ast->body.root = (NODE *)rb_suppress_tracing(yycompile0, (VALUE)parser);
     parser->ast = 0;
     RB_GC_GUARD(vparser); /* prohibit tail call optimization */
 
diff --git a/ruby.c b/ruby.c
index 452d0a0e3c..83dcc4093b 100644
--- a/ruby.c
+++ b/ruby.c
@@ -1704,7 +1704,7 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 	rb_enc_set_default_internal(Qnil);
     rb_stdio_set_default_encoding();
 
-    if (!ast->root) {
+    if (!ast->body.root) {
 	rb_ast_dispose(ast);
 	return Qfalse;
     }
@@ -1726,7 +1726,7 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
     }
 
     if (dump & (DUMP_BIT(parsetree)|DUMP_BIT(parsetree_with_comment))) {
-	rb_io_write(rb_stdout, rb_parser_dump_tree(ast->root, dump & DUMP_BIT(parsetree_with_comment)));
+	rb_io_write(rb_stdout, rb_parser_dump_tree(ast->body.root, dump & DUMP_BIT(parsetree_with_comment)));
 	rb_io_flush(rb_stdout);
 	dump &= ~DUMP_BIT(parsetree)&~DUMP_BIT(parsetree_with_comment);
 	if (!dump) {
@@ -1749,7 +1749,7 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 	    }
 	}
 	base_block = toplevel_context(toplevel_binding);
-	iseq = rb_iseq_new_main(ast->root, opt->script_name, path, vm_block_iseq(base_block));
+	iseq = rb_iseq_new_main(ast->body.root, opt->script_name, path, vm_block_iseq(base_block));
 	rb_ast_dispose(ast);
     }
 
diff --git a/template/prelude.c.tmpl b/template/prelude.c.tmpl
index 3cf74eda12..b144f8ab0a 100644
--- a/template/prelude.c.tmpl
+++ b/template/prelude.c.tmpl
@@ -161,11 +161,11 @@ prelude_eval(VALUE code, VALUE name, int line)
     };
 
     rb_ast_t *ast = rb_parser_compile_string_path(rb_parser_new(), name, code, line);
-    if (!ast->root) {
+    if (!ast->body.root) {
 	rb_ast_dispose(ast);
 	rb_exc_raise(rb_errinfo());
     }
-    rb_iseq_eval(rb_iseq_new_with_opt(ast->root, name, name, Qnil, INT2FIX(line),
+    rb_iseq_eval(rb_iseq_new_with_opt(ast->body.root, name, name, Qnil, INT2FIX(line),
 				      NULL, ISEQ_TYPE_TOP, &optimization));
     rb_ast_dispose(ast);
 }
diff --git a/vm_eval.c b/vm_eval.c
index 87fee5aa64..d9b2ec06b3 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -1272,8 +1272,8 @@ eval_make_iseq(VALUE src, VALUE fname, int line, const rb_binding_t *bind,
 
     rb_parser_set_context(parser, base_block, FALSE);
     ast = rb_parser_compile_string_path(parser, fname, src, line);
-    if (ast->root) {
-	iseq = rb_iseq_new_with_opt(ast->root,
+    if (ast->body.root) {
+	iseq = rb_iseq_new_with_opt(ast->body.root,
 				    parent->body->location.label,
 				    fname, realpath, INT2FIX(line),
 				    parent, ISEQ_TYPE_EVAL, NULL);