diff --git a/common.mk b/common.mk
index 937d6bf9bcd769e34f37e1dbacadaf7d53a0a5be..6353b247b3f1b657380c8caa6917bf3b7f745fb9 100644
--- a/common.mk
+++ b/common.mk
@@ -13168,6 +13168,7 @@ struct.$(OBJEXT): {$(VPATH)}backward/2/r_cast.h
 struct.$(OBJEXT): {$(VPATH)}backward/2/rmodule.h
 struct.$(OBJEXT): {$(VPATH)}backward/2/stdalign.h
 struct.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
+struct.$(OBJEXT): {$(VPATH)}builtin.h
 struct.$(OBJEXT): {$(VPATH)}config.h
 struct.$(OBJEXT): {$(VPATH)}defines.h
 struct.$(OBJEXT): {$(VPATH)}encoding.h
diff --git a/compile.c b/compile.c
index 9fba24f0fa9e26597d6406f2d11bf7d22ea71197..f522954fb8d4a5d06cfaee4ddb1d9e9c7c85bbb2 100644
--- a/compile.c
+++ b/compile.c
@@ -9681,19 +9681,19 @@ caller_location(VALUE *path, VALUE *realpath)
 
 typedef struct {
     VALUE arg;
-    rb_insn_func_t func;
+    VALUE func;
     int line;
 } accessor_args;
 
 static const rb_iseq_t *
-method_for_self(VALUE name, VALUE arg, rb_insn_func_t func,
+method_for_self(VALUE name, VALUE arg, const struct rb_builtin_function *func,
                 void (*build)(rb_iseq_t *, LINK_ANCHOR *, const void *))
 {
     VALUE path, realpath;
     accessor_args acc;
 
     acc.arg = arg;
-    acc.func = func;
+    acc.func = (VALUE)func;
     acc.line = caller_location(&path, &realpath);
     struct rb_iseq_new_with_callback_callback_func *ifunc =
         rb_iseq_new_with_callback_new_callback(build, &acc);
@@ -9714,7 +9714,7 @@ for_self_aref(rb_iseq_t *iseq, LINK_ANCHOR *ret, const void *a)
     body->param.size = 0;
 
     ADD_INSN1(ret, line, putobject, args->arg);
-    ADD_INSN1(ret, line, opt_call_c_function, (VALUE)args->func);
+    ADD_INSN1(ret, line, invokebuiltin, args->func);
 }
 
 static void
@@ -9731,24 +9731,23 @@ for_self_aset(rb_iseq_t *iseq, LINK_ANCHOR *ret, const void *a)
 
     ADD_GETLOCAL(ret, line, numberof(vars)-1, 0);
     ADD_INSN1(ret, line, putobject, args->arg);
-    ADD_INSN1(ret, line, opt_call_c_function, (VALUE)args->func);
-    ADD_INSN(ret, line, pop);
+    ADD_INSN1(ret, line, invokebuiltin, args->func);
 }
 
 /*
  * func (index) -> (value)
  */
 const rb_iseq_t *
-rb_method_for_self_aref(VALUE name, VALUE arg, rb_insn_func_t func)
+rb_method_for_self_aref(VALUE name, VALUE arg, const struct rb_builtin_function *func)
 {
     return method_for_self(name, arg, func, for_self_aref);
 }
 
 /*
- * func (index, value) -> (index, value)
+ * func (index, value) -> (value)
  */
 const rb_iseq_t *
-rb_method_for_self_aset(VALUE name, VALUE arg, rb_insn_func_t func)
+rb_method_for_self_aset(VALUE name, VALUE arg, const struct rb_builtin_function *func)
 {
     return method_for_self(name, arg, func, for_self_aset);
 }
diff --git a/struct.c b/struct.c
index be742fbee5eec86f62f3d727c775aa5e0558f38c..41349f71f59650b71d2d259eb60fd96dca1a694f 100644
--- a/struct.c
+++ b/struct.c
@@ -20,6 +20,7 @@
 #include "internal/symbol.h"
 #include "transient_heap.h"
 #include "vm_core.h"
+#include "builtin.h"
 
 /* only for struct[:field] access */
 enum {
@@ -27,8 +28,8 @@ enum {
     AREF_HASH_THRESHOLD = 10
 };
 
-const rb_iseq_t *rb_method_for_self_aref(VALUE name, VALUE arg, rb_insn_func_t func);
-const rb_iseq_t *rb_method_for_self_aset(VALUE name, VALUE arg, rb_insn_func_t func);
+const rb_iseq_t *rb_method_for_self_aref(VALUE name, VALUE arg, const struct rb_builtin_function *func);
+const rb_iseq_t *rb_method_for_self_aset(VALUE name, VALUE arg, const struct rb_builtin_function *func);
 
 VALUE rb_cStruct;
 static ID id_members, id_back_members, id_keyword_init;
@@ -288,11 +289,41 @@ new_struct(VALUE name, VALUE super)
     return rb_define_class_id_under(super, id, super);
 }
 
+NORETURN(static void invalid_struct_pos(VALUE s, VALUE idx));
+
+static inline long
+struct_pos_num(VALUE s, VALUE idx)
+{
+    long i = NUM2INT(idx);
+    if (i < 0 || i >= RSTRUCT_LEN(s)) invalid_struct_pos(s, idx);
+    return i;
+}
+
+static VALUE
+opt_struct_aref(rb_execution_context_t *ec, VALUE self, VALUE idx)
+{
+    long i = struct_pos_num(self, idx);
+    return RSTRUCT_GET(self, i);
+}
+
+static VALUE
+opt_struct_aset(rb_execution_context_t *ec, VALUE self, VALUE val, VALUE idx)
+{
+    long i = struct_pos_num(self, idx);
+    rb_struct_modify(self);
+    RSTRUCT_SET(self, i, val);
+    return val;
+}
+
+static const struct rb_builtin_function struct_aref_builtin =
+    RB_BUILTIN_FUNCTION(0, struct_aref, opt_struct_aref, 1);
+static const struct rb_builtin_function struct_aset_builtin =
+    RB_BUILTIN_FUNCTION(1, struct_aref, opt_struct_aset, 2);
+
 static void
 define_aref_method(VALUE nstr, VALUE name, VALUE off)
 {
-    rb_control_frame_t *FUNC_FASTCALL(rb_vm_opt_struct_aref)(rb_execution_context_t *, rb_control_frame_t *);
-    const rb_iseq_t *iseq = rb_method_for_self_aref(name, off, rb_vm_opt_struct_aref);
+    const rb_iseq_t *iseq = rb_method_for_self_aref(name, off, &struct_aref_builtin);
 
     rb_add_method_iseq(nstr, SYM2ID(name), iseq, NULL, METHOD_VISI_PUBLIC);
 }
@@ -300,8 +331,7 @@ define_aref_method(VALUE nstr, VALUE name, VALUE off)
 static void
 define_aset_method(VALUE nstr, VALUE name, VALUE off)
 {
-    rb_control_frame_t *FUNC_FASTCALL(rb_vm_opt_struct_aset)(rb_execution_context_t *, rb_control_frame_t *);
-    const rb_iseq_t *iseq = rb_method_for_self_aset(name, off, rb_vm_opt_struct_aset);
+    const rb_iseq_t *iseq = rb_method_for_self_aset(name, off, &struct_aset_builtin);
 
     rb_add_method_iseq(nstr, SYM2ID(name), iseq, NULL, METHOD_VISI_PUBLIC);
 }
@@ -1023,7 +1053,6 @@ rb_struct_pos(VALUE s, VALUE *name)
     }
 }
 
-NORETURN(static void invalid_struct_pos(VALUE s, VALUE idx));
 static void
 invalid_struct_pos(VALUE s, VALUE idx)
 {
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 611739e9570a01e59fab3d3809d44e6cb9a8fc26..8f1b6e73074eef25fc549b704116d1dd8cfb8aca 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -3621,20 +3621,6 @@ vm_once_clear(VALUE data)
     return Qnil;
 }
 
-rb_control_frame_t *
-FUNC_FASTCALL(rb_vm_opt_struct_aref)(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp)
-{
-    TOPN(0) = rb_struct_aref(GET_SELF(), TOPN(0));
-    return reg_cfp;
-}
-
-rb_control_frame_t *
-FUNC_FASTCALL(rb_vm_opt_struct_aset)(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp)
-{
-    rb_struct_aset(GET_SELF(), TOPN(0), TOPN(1));
-    return reg_cfp;
-}
-
 /* defined insn */
 
 static enum defined_type