Revert https://github.com/ruby/ruby/pull/2486

This reverts commits: 10d6a3aca7 8ba48c1b85 fba8627dc1 dd883de5ba
6c6a25feca 167e6b48f1 7cb96d41a5 3207979278 595b3c4fdd 1521f7cf89
c11c5e69ac cf33608203 3632a812c0 f56506be0d 86427a3219 .

The reason for the revert is that we observe ABA problem around
inline method cache.  When a cache misshits, we search for a
method entry.  And if the entry is identical to what was cached
before, we reuse the cache.  But the commits we are reverting here
introduced situations where a method entry is freed, then the
identical memory region is used for another method entry.  An
inline method cache cannot detect that ABA.

Here is a code that reproduce such situation:

```ruby
require 'prime'

class << Integer
  alias org_sqrt sqrt
  def sqrt(n)
    raise
  end

  GC.stress = true
  Prime.each(7*37){} rescue nil # <- Here we populate CC
  class << Object.new; end

  # These adjacent remove-then-alias maneuver
  # frees a method entry, then immediately
  # reuses it for another.
  remove_method :sqrt
  alias sqrt org_sqrt
end

Prime.each(7*37).to_a # <- SEGV
```

14 files changed, 391 insertions, 501 deletions

diff --git a/class.c b/class.c
index aaf754146d..b4aeb59e25 100644
--- a/class.c
+++ b/class.c
@@ -956,41 +956,25 @@ include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
     return method_changed;
 }
 
-typedef struct tuple {
-    struct RClass *klass;
-    struct RClass *origin;
-} tuple;
-
-static enum rb_id_table_iterator_result
-inject_refined_method(ID *key, VALUE *value, void *data, int _)
-{
-    const tuple             *ptr     = data;
-    const rb_method_entry_t *me      = *(const rb_method_entry_t **) value;
-    const rb_method_entry_t *orig_me = me->def->body.refined.orig_me;
-    const rb_method_entry_t *new_me  =
-        rb_method_entry_from_template(
-            me, &(rb_method_refined_t) {
-                .orig_me = NULL,
-                .owner   = me->def->body.refined.owner, });
-    rb_id_table_insert(RCLASS_M_TBL(ptr->klass), *key, (VALUE)new_me);
-    RB_OBJ_WRITTEN(ptr->klass, Qundef, new_me);
-    *value = (VALUE)rb_method_entry_clone(orig_me);
-    RB_OBJ_WRITTEN(ptr->origin, Qundef, orig_me);
-    return ID_TABLE_CONTINUE;
-}
-
 static enum rb_id_table_iterator_result
 move_refined_method(ID key, VALUE value, void *data)
 {
-    const tuple             *ptr = data;
-    const rb_method_entry_t *me  = (const rb_method_entry_t *) value;
+    rb_method_entry_t *me = (rb_method_entry_t *) value;
+    VALUE klass = (VALUE)data;
+    struct rb_id_table *tbl = RCLASS_M_TBL(klass);
 
     if (me->def->type == VM_METHOD_TYPE_REFINED) {
 	if (me->def->body.refined.orig_me) {
-            return ID_TABLE_REPLACE;
+	    const rb_method_entry_t *orig_me = me->def->body.refined.orig_me, *new_me;
+	    RB_OBJ_WRITE(me, &me->def->body.refined.orig_me, NULL);
+	    new_me = rb_method_entry_clone(me);
+	    rb_id_table_insert(tbl, key, (VALUE)new_me);
+	    RB_OBJ_WRITTEN(klass, Qundef, new_me);
+	    rb_method_entry_copy(me, orig_me);
+	    return ID_TABLE_CONTINUE;
 	}
 	else {
-            rb_id_table_insert(RCLASS_M_TBL(ptr->klass), key, (VALUE)me);
+	    rb_id_table_insert(tbl, key, (VALUE)me);
 	    return ID_TABLE_DELETE;
 	}
     }
@@ -1016,12 +1000,7 @@ rb_prepend_module(VALUE klass, VALUE module)
 	RCLASS_SET_ORIGIN(klass, origin);
 	RCLASS_M_TBL(origin) = RCLASS_M_TBL(klass);
 	RCLASS_M_TBL_INIT(klass);
-        rb_id_table_foreach_with_replace_with_key(
-            RCLASS_M_TBL(origin),
-            move_refined_method,
-            inject_refined_method,
-            &(tuple) { RCLASS(klass), RCLASS(origin), },
-            true);
+	rb_id_table_foreach(RCLASS_M_TBL(origin), move_refined_method, (void *)klass);
     }
     changed = include_modules_at(klass, klass, module, FALSE);
     if (changed < 0)
diff --git a/ext/coverage/coverage.c b/ext/coverage/coverage.c
index cb571caa4b..8503c9d6c6 100644
--- a/ext/coverage/coverage.c
+++ b/ext/coverage/coverage.c
@@ -123,7 +123,7 @@ method_coverage_i(void *vstart, void *vend, size_t stride, void *data)
 
     for (v = (VALUE)vstart; v != (VALUE)vend; v += stride) {
 	if (RB_TYPE_P(v, T_IMEMO) && imemo_type(v) == imemo_ment) {
-            const rb_method_entry_t *me = (const rb_method_entry_t *) v;
+	    const rb_method_entry_t *me = (rb_method_entry_t *) v;
 	    VALUE path, first_lineno, first_column, last_lineno, last_column;
 	    VALUE data[5], ncoverage, methods;
 	    VALUE methods_id = ID2SYM(rb_intern("methods"));
diff --git a/gc.c b/gc.c
index 52b3a66b6c..c9f26cc174 100644
--- a/gc.c
+++ b/gc.c
@@ -7829,9 +7829,9 @@ void rb_update_st_references(struct st_table *ht)
 }
 
 static void
-gc_ref_update_method_entry(rb_objspace_t *objspace, const rb_method_entry_t *me)
+gc_ref_update_method_entry(rb_objspace_t *objspace, rb_method_entry_t *me)
 {
-    const rb_method_definition_t *def = me->def;
+    rb_method_definition_t *def = me->def;
 
     UPDATE_IF_MOVED(objspace, me->owner);
     UPDATE_IF_MOVED(objspace, me->defined_class);
diff --git a/id_table.c b/id_table.c
index b383fcf81d..f566582479 100644
--- a/id_table.c
+++ b/id_table.c
@@ -269,62 +269,57 @@ rb_id_table_delete(struct rb_id_table *tbl, ID id)
 void
 rb_id_table_foreach_with_replace(struct rb_id_table *tbl, rb_id_table_foreach_func_t *func, rb_id_table_update_callback_func_t *replace, void *data)
 {
-    rb_id_table_foreach_with_replace_with_key(tbl, func, replace, data, false);
+    int i, capa = tbl->capa;
+
+    for (i=0; i<capa; i++) {
+        if (ITEM_KEY_ISSET(tbl, i)) {
+            const id_key_t key = ITEM_GET_KEY(tbl, i);
+            enum rb_id_table_iterator_result ret = (*func)(Qundef, tbl->items[i].val, data);
+            assert(key != 0);
+
+            if (ret == ID_TABLE_REPLACE) {
+                VALUE val = tbl->items[i].val;
+                ret = (*replace)(NULL, &val, data, TRUE);
+                tbl->items[i].val = val;
+            }
+            else if (ret == ID_TABLE_STOP)
+                return;
+        }
+    }
 }
 
 void
 rb_id_table_foreach(struct rb_id_table *tbl, rb_id_table_foreach_func_t *func, void *data)
 {
-    rb_id_table_foreach_with_replace_with_key(tbl, func, 0, data, true);
-}
-
-typedef struct tuple {
-    rb_id_table_foreach_values_func_t *const func;
-    void *const data;
-} tuple;
+    int i, capa = tbl->capa;
 
-static enum rb_id_table_iterator_result
-cdr(ID car, VALUE cdr, void *data)
-{
-    const tuple *ptr = data;
-    return ptr->func(cdr, ptr->data);
+    for (i=0; i<capa; i++) {
+	if (ITEM_KEY_ISSET(tbl, i)) {
+	    const id_key_t key = ITEM_GET_KEY(tbl, i);
+	    enum rb_id_table_iterator_result ret = (*func)(key2id(key), tbl->items[i].val, data);
+	    assert(key != 0);
+
+	    if (ret == ID_TABLE_DELETE)
+		hash_delete_index(tbl, i);
+	    else if (ret == ID_TABLE_STOP)
+		return;
+	}
+    }
 }
 
 void
 rb_id_table_foreach_values(struct rb_id_table *tbl, rb_id_table_foreach_values_func_t *func, void *data)
 {
-    rb_id_table_foreach_with_replace(
-        tbl, cdr, 0, &(tuple) { func, data, });
-}
+    int i, capa = tbl->capa;
 
-void
-rb_id_table_foreach_with_replace_with_key(
-    struct rb_id_table *tbl,
-    rb_id_table_foreach_func_t *func,
-    rb_id_table_update_callback_func_t *replace,
-    void *data,
-    bool needkey)
-{
-    for (int i = 0; i < tbl->capa; i++) {
-        if (ITEM_KEY_ISSET(tbl, i)) {
-            const id_key_t key = ITEM_GET_KEY(tbl, i);
-            assert(key != 0);
-            ID k = needkey ? key2id(key) : 0;
-            VALUE v = tbl->items[i].val;
-            switch (func(k, v, data)) {
-              case ID_TABLE_DELETE:
-                hash_delete_index(tbl, i);
-                /* FALLTHROUGH */
-              case ID_TABLE_CONTINUE:
-                continue;
-              case ID_TABLE_STOP:
-                return;
-              case ID_TABLE_REPLACE:
-                if (replace) {
-                    replace(&k, &v, data, true);
-                    tbl->items[i].val = v;
-                }
-            }
-        }
+    for (i=0; i<capa; i++) {
+	if (ITEM_KEY_ISSET(tbl, i)) {
+	    enum rb_id_table_iterator_result ret = (*func)(tbl->items[i].val, data);
+
+	    if (ret == ID_TABLE_DELETE)
+		hash_delete_index(tbl, i);
+	    else if (ret == ID_TABLE_STOP)
+		return;
+	}
     }
 }
diff --git a/id_table.h b/id_table.h
index 19181190ea..abd9eb5f38 100644
--- a/id_table.h
+++ b/id_table.h
@@ -10,6 +10,7 @@ enum rb_id_table_iterator_result {
     ID_TABLE_STOP     = ST_STOP,
     ID_TABLE_DELETE   = ST_DELETE,
     ID_TABLE_REPLACE  = ST_REPLACE,
+    ID_TABLE_ITERATOR_RESULT_END
 };
 
 struct rb_id_table *rb_id_table_create(size_t size);
@@ -28,7 +29,6 @@ typedef enum rb_id_table_iterator_result rb_id_table_foreach_func_t(ID id, VALUE
 typedef enum rb_id_table_iterator_result rb_id_table_foreach_values_func_t(VALUE val, void *data);
 void rb_id_table_foreach(struct rb_id_table *tbl, rb_id_table_foreach_func_t *func, void *data);
 void rb_id_table_foreach_with_replace(struct rb_id_table *tbl, rb_id_table_foreach_func_t *func, rb_id_table_update_callback_func_t *replace, void *data);
-void rb_id_table_foreach_with_replace_with_key(struct rb_id_table *tbl, rb_id_table_foreach_func_t *func, rb_id_table_update_callback_func_t *replace, void *data, bool needkey);
 void rb_id_table_foreach_values(struct rb_id_table *tbl, rb_id_table_foreach_values_func_t *func, void *data);
 
 #endif	/* RUBY_ID_TABLE_H */
diff --git a/insns.def b/insns.def
index e3edc5e29f..f365106e2c 100644
--- a/insns.def
+++ b/insns.def
@@ -911,7 +911,7 @@ invokeblock
 // attr rb_snum_t sp_inc = sp_inc_of_invokeblock(ci);
 {
     static struct rb_call_cache cc = {
-        0, 0, NULL, vm_invokeblock_i,
+        0, 0, NULL, NULL, vm_invokeblock_i,
     };
 
     VALUE bh = VM_BLOCK_HANDLER_NONE;
diff --git a/internal.h b/internal.h
index dee7775d95..2346703d68 100644
--- a/internal.h
+++ b/internal.h
@@ -2330,6 +2330,7 @@ enum method_missing_reason {
     MISSING_NONE      = 0x40
 };
 struct rb_callable_method_entry_struct;
+struct rb_method_definition_struct;
 struct rb_execution_context_struct;
 struct rb_control_frame_struct;
 struct rb_calling_info;
@@ -2341,6 +2342,7 @@ struct rb_call_cache {
 
     /* inline cache: values */
     const struct rb_callable_method_entry_struct *me;
+    const struct rb_method_definition_struct *def;
 
     VALUE (*call)(struct rb_execution_context_struct *ec,
                   struct rb_control_frame_struct *cfp,
diff --git a/method.h b/method.h
index b203f7112d..84ade7fef3 100644
--- a/method.h
+++ b/method.h
@@ -49,23 +49,54 @@ typedef struct rb_cref_struct {
 /* method data type */
 
 typedef struct rb_method_entry_struct {
-    const VALUE flags;
-    const VALUE defined_class;
+    VALUE flags;
+    VALUE defined_class;
     struct rb_method_definition_struct * const def;
-    const ID called_id;
-    const VALUE owner;
+    ID called_id;
+    VALUE owner;
 } rb_method_entry_t;
 
 typedef struct rb_callable_method_entry_struct { /* same fields with rb_method_entry_t */
-    const VALUE flags;
+    VALUE flags;
     const VALUE defined_class;
     struct rb_method_definition_struct * const def;
-    const ID called_id;
+    ID called_id;
     const VALUE owner;
 } rb_callable_method_entry_t;
 
 #define METHOD_ENTRY_VISI(me)  (rb_method_visibility_t)(((me)->flags & (IMEMO_FL_USER0 | IMEMO_FL_USER1)) >> (IMEMO_FL_USHIFT+0))
 #define METHOD_ENTRY_BASIC(me) (int)                   (((me)->flags & (IMEMO_FL_USER2                 )) >> (IMEMO_FL_USHIFT+2))
+#define METHOD_ENTRY_COMPLEMENTED(me)     ((me)->flags & IMEMO_FL_USER3)
+#define METHOD_ENTRY_COMPLEMENTED_SET(me) ((me)->flags = (me)->flags | IMEMO_FL_USER3)
+
+static inline void
+METHOD_ENTRY_VISI_SET(rb_method_entry_t *me, rb_method_visibility_t visi)
+{
+    VM_ASSERT((int)visi >= 0 && visi <= 3);
+    me->flags = (me->flags & ~(IMEMO_FL_USER0 | IMEMO_FL_USER1)) | (visi << (IMEMO_FL_USHIFT+0));
+}
+static inline void
+METHOD_ENTRY_BASIC_SET(rb_method_entry_t *me, unsigned int basic)
+{
+    VM_ASSERT(basic <= 1);
+    me->flags = (me->flags & ~(IMEMO_FL_USER2                 )) | (basic << (IMEMO_FL_USHIFT+2));
+}
+static inline void
+METHOD_ENTRY_FLAGS_SET(rb_method_entry_t *me, rb_method_visibility_t visi, unsigned int basic)
+{
+    VM_ASSERT((int)visi >= 0 && visi <= 3);
+    VM_ASSERT(basic <= 1);
+    me->flags =
+      (me->flags & ~(IMEMO_FL_USER0|IMEMO_FL_USER1|IMEMO_FL_USER2)) |
+	((visi << (IMEMO_FL_USHIFT+0)) | (basic << (IMEMO_FL_USHIFT+2)));
+}
+static inline void
+METHOD_ENTRY_FLAGS_COPY(rb_method_entry_t *dst, const rb_method_entry_t *src)
+{
+    dst->flags =
+      (dst->flags & ~(IMEMO_FL_USER0|IMEMO_FL_USER1|IMEMO_FL_USER2)) |
+	(src->flags & (IMEMO_FL_USER0|IMEMO_FL_USER1|IMEMO_FL_USER2));
+}
 
 typedef enum {
     VM_METHOD_TYPE_ISEQ,      /*!< Ruby method */
@@ -93,32 +124,32 @@ typedef struct rb_iseq_struct rb_iseq_t;
 #endif
 
 typedef struct rb_method_iseq_struct {
-    const rb_iseq_t *const iseqptr; /*!< iseq pointer, should be separated from iseqval */
-    rb_cref_t *const cref;          /*!< class reference, should be marked */
-} rb_method_iseq_t;
+    rb_iseq_t * iseqptr; /*!< iseq pointer, should be separated from iseqval */
+    rb_cref_t * cref;          /*!< class reference, should be marked */
+} rb_method_iseq_t; /* check rb_add_method_iseq() when modify the fields */
 
 typedef struct rb_method_cfunc_struct {
-    VALUE (*const func)(ANYARGS);
-    VALUE (*const invoker)(VALUE recv, int argc, const VALUE *argv, VALUE (*func)(ANYARGS));
-    const int argc;
+    VALUE (*func)(ANYARGS);
+    VALUE (*invoker)(VALUE recv, int argc, const VALUE *argv, VALUE (*func)(ANYARGS));
+    int argc;
 } rb_method_cfunc_t;
 
 typedef struct rb_method_attr_struct {
-    const ID id;
-    const VALUE location; /* should be marked */
+    ID id;
+    VALUE location; /* should be marked */
 } rb_method_attr_t;
 
 typedef struct rb_method_alias_struct {
-    const struct rb_method_entry_struct *const original_me; /* original_me->klass is original owner */
+    struct rb_method_entry_struct * original_me; /* original_me->klass is original owner */
 } rb_method_alias_t;
 
 typedef struct rb_method_refined_struct {
-    const struct rb_method_entry_struct *const orig_me;
-    const VALUE owner;
+    struct rb_method_entry_struct * orig_me;
+    VALUE owner;
 } rb_method_refined_t;
 
 typedef struct rb_method_bmethod_struct {
-    const VALUE proc; /* should be marked */
+    VALUE proc; /* should be marked */
     struct rb_hook_list_struct *hooks;
 } rb_method_bmethod_t;
 
@@ -130,22 +161,22 @@ enum method_optimized_type {
 };
 
 struct rb_method_definition_struct {
-    BITFIELD(rb_method_type_t, const type, VM_METHOD_TYPE_MINIMUM_BITS);
+    BITFIELD(rb_method_type_t, type, VM_METHOD_TYPE_MINIMUM_BITS);
     int alias_count : 28;
     int complemented_count : 28;
 
     union {
-        const rb_method_iseq_t iseq;
-        const rb_method_cfunc_t cfunc;
-        const rb_method_attr_t attr;
-        const rb_method_alias_t alias;
-        const rb_method_refined_t refined;
+	rb_method_iseq_t iseq;
+	rb_method_cfunc_t cfunc;
+	rb_method_attr_t attr;
+	rb_method_alias_t alias;
+	rb_method_refined_t refined;
         rb_method_bmethod_t bmethod;
 
-        const enum method_optimized_type optimize_type;
+        enum method_optimized_type optimize_type;
     } body;
 
-    const ID original_id;
+    ID original_id;
 };
 
 typedef struct rb_method_definition_struct rb_method_definition_t;
@@ -161,9 +192,8 @@ void rb_add_method_iseq(VALUE klass, ID mid, const rb_iseq_t *iseq, rb_cref_t *c
 void rb_add_refined_method_entry(VALUE refined_class, ID mid);
 void rb_add_method(VALUE klass, ID mid, rb_method_type_t type, void *option, rb_method_visibility_t visi);
 
-const rb_method_entry_t *rb_method_entry_set(VALUE klass, ID mid, const rb_method_entry_t *, rb_method_visibility_t noex);
-const rb_method_entry_t *rb_method_entry_from_template(const rb_method_entry_t *template, const void *opts);
-const rb_method_entry_t *rb_method_entry_for_missing(ID mid, VALUE klass);
+rb_method_entry_t *rb_method_entry_set(VALUE klass, ID mid, const rb_method_entry_t *, rb_method_visibility_t noex);
+rb_method_entry_t *rb_method_entry_create(ID called_id, VALUE klass, rb_method_visibility_t visi, const rb_method_definition_t *def);
 
 const rb_method_entry_t *rb_method_entry_at(VALUE obj, ID id);
 
@@ -193,6 +223,7 @@ void rb_sweep_method_entry(void *vm);
 
 const rb_method_entry_t *rb_method_entry_clone(const rb_method_entry_t *me);
 const rb_callable_method_entry_t *rb_method_entry_complement_defined_class(const rb_method_entry_t *src_me, ID called_id, VALUE defined_class);
+void rb_method_entry_copy(rb_method_entry_t *dst, const rb_method_entry_t *src);
 
 void rb_scope_visibility_set(rb_method_visibility_t);
 
diff --git a/proc.c b/proc.c
index 0fd2126fff..667b86ac1b 100644
--- a/proc.c
+++ b/proc.c
@@ -1426,7 +1426,7 @@ bm_compact(void *ptr)
     UPDATE_REFERENCE(data->recv);
     UPDATE_REFERENCE(data->klass);
     UPDATE_REFERENCE(data->iclass);
-    UPDATE_TYPED_REFERENCE(const rb_method_entry_t *, data->me);
+    UPDATE_TYPED_REFERENCE(rb_method_entry_t *, data->me);
 }
 
 static size_t
@@ -1474,9 +1474,19 @@ mnew_missing(VALUE klass, VALUE obj, ID id, VALUE mclass)
 {
     struct METHOD *data;
     VALUE method = TypedData_Make_Struct(mclass, struct METHOD, &method_data_type, data);
+    rb_method_entry_t *me;
+    rb_method_definition_t *def;
+
     RB_OBJ_WRITE(method, &data->recv, obj);
     RB_OBJ_WRITE(method, &data->klass, klass);
-    RB_OBJ_WRITE(method, &data->me, rb_method_entry_for_missing(id, klass));
+
+    def = ZALLOC(rb_method_definition_t);
+    def->type = VM_METHOD_TYPE_MISSING;
+    def->original_id = id;
+
+    me = rb_method_entry_create(id, klass, METHOD_VISI_UNDEF, def);
+
+    RB_OBJ_WRITE(method, &data->me, me);
 
     OBJ_INFECT(method, klass);
 
@@ -1519,7 +1529,7 @@ mnew_internal(const rb_method_entry_t *me, VALUE klass, VALUE iclass,
 	if (me->defined_class) {
 	    VALUE klass = RCLASS_SUPER(RCLASS_ORIGIN(me->defined_class));
 	    id = me->def->original_id;
-            me = (const rb_method_entry_t *)rb_callable_method_entry_with_refinements(klass, id, &iclass);
+            me = (rb_method_entry_t *)rb_callable_method_entry_with_refinements(klass, id, &iclass);
 	}
 	else {
 	    VALUE klass = RCLASS_SUPER(me->owner);
@@ -1557,7 +1567,7 @@ mnew(VALUE klass, VALUE obj, ID id, VALUE mclass, int scope)
         me = rb_method_entry_with_refinements(klass, id, &iclass);
     }
     else {
-        me = (const rb_method_entry_t *)rb_callable_method_entry_with_refinements(klass, id, &iclass);
+        me = (rb_method_entry_t *)rb_callable_method_entry_with_refinements(klass, id, &iclass);
     }
     return mnew_from_me(me, klass, iclass, obj, id, mclass, scope);
 }
@@ -2947,7 +2957,7 @@ method_super_method(VALUE method)
     super_class = RCLASS_SUPER(RCLASS_ORIGIN(iclass));
     mid = data->me->called_id;
     if (!super_class) return Qnil;
-    me = (const rb_method_entry_t *)rb_callable_method_entry_with_refinements(super_class, mid, &iclass);
+    me = (rb_method_entry_t *)rb_callable_method_entry_with_refinements(super_class, mid, &iclass);
     if (!me) return Qnil;
     return mnew_internal(me, me->owner, iclass, data->recv, mid, rb_obj_class(method), FALSE, FALSE);
 }
diff --git a/vm.c b/vm.c
index 16a15c2151..e104688d71 100644
--- a/vm.c
+++ b/vm.c
@@ -298,6 +298,17 @@ rb_vm_cref_new_toplevel(void)
     return vm_cref_new_toplevel(GET_EC());
 }
 
+static void
+vm_cref_dump(const char *mesg, const rb_cref_t *cref)
+{
+    fprintf(stderr, "vm_cref_dump: %s (%p)\n", mesg, (void *)cref);
+
+    while (cref) {
+	fprintf(stderr, "= cref| klass: %s\n", RSTRING_PTR(rb_class_path(CREF_CLASS(cref))));
+	cref = CREF_NEXT(cref);
+    }
+}
+
 void
 rb_vm_block_ep_update(VALUE obj, const struct rb_block *dst, const VALUE *ep)
 {
@@ -1595,7 +1606,7 @@ static enum rb_id_table_iterator_result
 check_redefined_method(ID mid, VALUE value, void *data)
 {
     VALUE klass = (VALUE)data;
-    const rb_method_entry_t *me = (const rb_method_entry_t *)value;
+    const rb_method_entry_t *me = (rb_method_entry_t *)value;
     const rb_method_entry_t *newme = rb_method_entry(klass, mid);
 
     if (newme != me) rb_vm_check_redefinition_opt_method(me, me->owner);
diff --git a/vm_backtrace.c b/vm_backtrace.c
index 3a8ad9143d..44a4ac0784 100644
--- a/vm_backtrace.c
+++ b/vm_backtrace.c
@@ -1349,7 +1349,7 @@ frame2iseq(VALUE frame)
 	    return (const rb_iseq_t *)frame;
 	  case imemo_ment:
 	    {
-                const rb_callable_method_entry_t *cme = (const rb_callable_method_entry_t *)frame;
+		const rb_callable_method_entry_t *cme = (rb_callable_method_entry_t *)frame;
 		switch (cme->def->type) {
 		  case VM_METHOD_TYPE_ISEQ:
 		    return cme->def->body.iseq.iseqptr;
@@ -1405,7 +1405,7 @@ frame2klass(VALUE frame)
     if (frame == Qnil) return Qnil;
 
     if (RB_TYPE_P(frame, T_IMEMO)) {
-        const rb_callable_method_entry_t *cme = (const rb_callable_method_entry_t *)frame;
+	const rb_callable_method_entry_t *cme = (rb_callable_method_entry_t *)frame;
 
 	if (imemo_type(frame) == imemo_ment) {
 	    return cme->defined_class;
diff --git a/vm_core.h b/vm_core.h
index 192165b497..8eb155585e 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -491,7 +491,7 @@ rb_iseq_check(const rb_iseq_t *iseq)
 }
 
 static inline const rb_iseq_t *
-def_iseq_ptr(const rb_method_definition_t *def)
+def_iseq_ptr(rb_method_definition_t *def)
 {
 #if VM_CHECK_MODE > 0
     if (def->type != VM_METHOD_TYPE_ISEQ) rb_bug("def_iseq_ptr: not iseq (%d)", def->type);
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index ef5a6db1af..127c8c3f9e 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -19,7 +19,8 @@
 #include "ruby/config.h"
 #include "debug_counter.h"
 
-extern void rb_method_entry_spoof(const rb_method_entry_t *me);
+extern rb_method_definition_t *rb_method_definition_create(rb_method_type_t type, ID mid);
+extern void rb_method_definition_set(const rb_method_entry_t *me, rb_method_definition_t *def, void *opts);
 extern int rb_method_definition_eq(const rb_method_definition_t *d1, const rb_method_definition_t *d2);
 extern VALUE rb_make_no_method_exception(VALUE exc, VALUE format, VALUE obj,
                                          int argc, const VALUE *argv, int priv);
@@ -578,8 +579,8 @@ vm_getspecial(const rb_execution_context_t *ec, const VALUE *lep, rb_num_t key,
     return val;
 }
 
-PUREFUNC(static const rb_callable_method_entry_t *check_method_entry(VALUE obj, int can_be_svar));
-static const rb_callable_method_entry_t *
+PUREFUNC(static rb_callable_method_entry_t *check_method_entry(VALUE obj, int can_be_svar));
+static rb_callable_method_entry_t *
 check_method_entry(VALUE obj, int can_be_svar)
 {
     if (obj == Qfalse) return NULL;
@@ -590,7 +591,7 @@ check_method_entry(VALUE obj, int can_be_svar)
 
     switch (imemo_type(obj)) {
       case imemo_ment:
-        return (const rb_callable_method_entry_t *)obj;
+	return (rb_callable_method_entry_t *)obj;
       case imemo_cref:
 	return NULL;
       case imemo_svar:
@@ -609,7 +610,7 @@ MJIT_STATIC const rb_callable_method_entry_t *
 rb_vm_frame_method_entry(const rb_control_frame_t *cfp)
 {
     const VALUE *ep = cfp->ep;
-    const rb_callable_method_entry_t *me;
+    rb_callable_method_entry_t *me;
 
     while (!VM_ENV_LOCAL_P(ep)) {
 	if ((me = check_method_entry(ep[VM_ENV_DATA_INDEX_ME_CREF], FALSE)) != NULL) return me;
@@ -620,7 +621,7 @@ rb_vm_frame_method_entry(const rb_control_frame_t *cfp)
 }
 
 static rb_cref_t *
-method_entry_cref(const rb_callable_method_entry_t *me)
+method_entry_cref(rb_callable_method_entry_t *me)
 {
     switch (me->def->type) {
       case VM_METHOD_TYPE_ISEQ:
@@ -644,7 +645,7 @@ check_cref(VALUE obj, int can_be_svar)
 
     switch (imemo_type(obj)) {
       case imemo_ment:
-        return method_entry_cref((const rb_callable_method_entry_t *)obj);
+	return method_entry_cref((rb_callable_method_entry_t *)obj);
       case imemo_cref:
 	return (rb_cref_t *)obj;
       case imemo_svar:
@@ -1390,6 +1391,9 @@ calccall(const struct rb_call_info *ci, const struct rb_call_cache *cc, const rb
     else if (LIKELY(cc->me != me)) {
         return vm_call_general; /* normal cases */
     }
+    else if (UNLIKELY(cc->def != me->def)) {
+        return vm_call_general;  /* cc->me was refined elsewhere */
+    }
     /* "Calling a formerly-public method, which is now privatised, with an
      * explicit receiver" is the only situation we have to check here.  A
      * formerly-private method now publicised is an absolutely safe thing.
@@ -1412,6 +1416,7 @@ rb_vm_search_method_slowpath(const struct rb_call_info *ci, struct rb_call_cache
         GET_GLOBAL_METHOD_STATE(),
         RCLASS_SERIAL(klass),
         me,
+        me ? me->def : NULL,
         calccall(ci, cc, me),
     };
     VM_ASSERT(callable_method_entry_p(cc->me));
@@ -2573,31 +2578,32 @@ find_defined_class_by_owner(VALUE current_class, VALUE target_owner)
     return current_class; /* maybe module function */
 }
 
-static const void*
-aliased_callable_method_entry0(const rb_method_entry_t *me)
+static const rb_callable_method_entry_t *
+aliased_callable_method_entry(const rb_callable_method_entry_t *me)
 {
     const rb_method_entry_t *orig_me = me->def->body.alias.original_me;
     const rb_callable_method_entry_t *cme;
 
-    if (orig_me->defined_class != 0) {
-        VM_ASSERT(callable_class_p(orig_me->defined_class));
-        return orig_me;
-    }
-    else {
+    if (orig_me->defined_class == 0) {
 	VALUE defined_class = find_defined_class_by_owner(me->defined_class, orig_me->owner);
 	VM_ASSERT(RB_TYPE_P(orig_me->owner, T_MODULE));
 	cme = rb_method_entry_complement_defined_class(orig_me, me->called_id, defined_class);
-        const rb_method_entry_t *ret =
-            rb_method_entry_from_template((const rb_method_entry_t*)me, cme);
-        rb_method_entry_spoof(ret);
-        return ret;
+
+	if (me->def->alias_count + me->def->complemented_count == 0) {
+	    RB_OBJ_WRITE(me, &me->def->body.alias.original_me, cme);
+	}
+	else {
+	    rb_method_definition_t *def =
+		rb_method_definition_create(VM_METHOD_TYPE_ALIAS, me->def->original_id);
+	    rb_method_definition_set((rb_method_entry_t *)me, def, (void *)cme);
+	}
+    }
+    else {
+	cme = (const rb_callable_method_entry_t *)orig_me;
     }
-}
 
-static const rb_callable_method_entry_t*
-aliased_callable_method_entry(const rb_callable_method_entry_t *me)
-{
-    return aliased_callable_method_entry0((const void*)me);
+    VM_ASSERT(callable_method_entry_p(cme));
+    return cme;
 }
 
 static const rb_callable_method_entry_t *
diff --git a/vm_method.c b/vm_method.c
index 1508b9a481..554d209110 100644
--- a/vm_method.c
+++ b/vm_method.c
@@ -41,7 +41,7 @@ struct cache_entry {
     rb_serial_t method_state;
     rb_serial_t class_serial;
     ID mid;
-    const rb_method_entry_t* me;
+    rb_method_entry_t* me;
     VALUE defined_class;
 };
 
@@ -132,7 +132,9 @@ rb_add_method_cfunc(VALUE klass, ID mid, VALUE (*func)(ANYARGS), int argc, rb_me
 {
     if (argc < -2 || 15 < argc) rb_raise(rb_eArgError, "arity out of range: %d for -2..15", argc);
     if (func != rb_f_notimplement) {
-        rb_method_cfunc_t opt = { func, 0, argc, };
+	rb_method_cfunc_t opt;
+	opt.func = func;
+	opt.argc = argc;
 	rb_add_method(klass, mid, VM_METHOD_TYPE_CFUNC, &opt, visi);
     }
     else {
@@ -140,11 +142,9 @@ rb_add_method_cfunc(VALUE klass, ID mid, VALUE (*func)(ANYARGS), int argc, rb_me
     }
 }
 
-void
-rb_free_method_entry(const rb_method_entry_t *me)
+static void
+rb_method_definition_release(rb_method_definition_t *def, int complemented)
 {
-    rb_method_definition_t *def = me->def;
-
     if (def != NULL) {
 	const int alias_count = def->alias_count;
 	const int complemented_count = def->complemented_count;
@@ -158,7 +158,7 @@ rb_free_method_entry(const rb_method_entry_t *me)
 	    xfree(def);
 	}
 	else {
-            if (def->complemented_count > 0) def->complemented_count--;
+	    if (complemented) def->complemented_count--;
 	    else if (def->alias_count > 0) def->alias_count--;
 
 	    if (METHOD_DEBUG) fprintf(stderr, "-%p-%s:%d->%d,%d->%d (dec)\n", (void *)def, rb_id2name(def->original_id),
@@ -167,17 +167,23 @@ rb_free_method_entry(const rb_method_entry_t *me)
     }
 }
 
-static inline const rb_method_entry_t *search_method(VALUE klass, ID id, VALUE *defined_class_ptr);
+void
+rb_free_method_entry(const rb_method_entry_t *me)
+{
+    rb_method_definition_release(me->def, METHOD_ENTRY_COMPLEMENTED(me));
+}
+
+static inline rb_method_entry_t *search_method(VALUE klass, ID id, VALUE *defined_class_ptr);
 extern int rb_method_definition_eq(const rb_method_definition_t *d1, const rb_method_definition_t *d2);
 
-static inline const rb_method_entry_t *
+static inline rb_method_entry_t *
 lookup_method_table(VALUE klass, ID id)
 {
     st_data_t body;
     struct rb_id_table *m_tbl = RCLASS_M_TBL(klass);
 
     if (rb_id_table_lookup(m_tbl, id, &body)) {
-        return (const rb_method_entry_t *) body;
+	return (rb_method_entry_t *) body;
     }
     else {
 	return 0;
@@ -212,9 +218,97 @@ static VALUE
 }
 
 static void
+setup_method_cfunc_struct(rb_method_cfunc_t *cfunc, VALUE (*func)(), int argc)
+{
+    cfunc->func = func;
+    cfunc->argc = argc;
+    cfunc->invoker = call_cfunc_invoker_func(argc);
+}
+
+MJIT_FUNC_EXPORTED void
+rb_method_definition_set(const rb_method_entry_t *me, rb_method_definition_t *def, void *opts)
+{
+    *(rb_method_definition_t **)&me->def = def;
+
+    if (opts != NULL) {
+	switch (def->type) {
+	  case VM_METHOD_TYPE_ISEQ:
+	    {
+		rb_method_iseq_t *iseq_body = (rb_method_iseq_t *)opts;
+		rb_cref_t *method_cref, *cref = iseq_body->cref;
+
+		/* setup iseq first (before invoking GC) */
+		RB_OBJ_WRITE(me, &def->body.iseq.iseqptr, iseq_body->iseqptr);
+
+		if (0) vm_cref_dump("rb_method_definition_create", cref);
+
+		if (cref) {
+		    method_cref = cref;
+		}
+		else {
+		    method_cref = vm_cref_new_toplevel(GET_EC()); /* TODO: can we reuse? */
+		}
+
+		RB_OBJ_WRITE(me, &def->body.iseq.cref, method_cref);
+		return;
+	    }
+	  case VM_METHOD_TYPE_CFUNC:
+	    {
+		rb_method_cfunc_t *cfunc = (rb_method_cfunc_t *)opts;
+		setup_method_cfunc_struct(UNALIGNED_MEMBER_PTR(def, body.cfunc), cfunc->func, cfunc->argc);
+		return;
+	    }
+	  case VM_METHOD_TYPE_ATTRSET:
+	  case VM_METHOD_TYPE_IVAR:
+	    {
+		const rb_execution_context_t *ec = GET_EC();
+		rb_control_frame_t *cfp;
+		int line;
+
+		def->body.attr.id = (ID)(VALUE)opts;
+
+		cfp = rb_vm_get_ruby_level_next_cfp(ec, ec->cfp);
+
+		if (cfp && (line = rb_vm_get_sourceline(cfp))) {
+		    VALUE location = rb_ary_new3(2, rb_iseq_path(cfp->iseq), INT2FIX(line));
+		    RB_OBJ_WRITE(me, &def->body.attr.location, rb_ary_freeze(location));
+		}
+		else {
+		    VM_ASSERT(def->body.attr.location == 0);
+		}
+		return;
+	    }
+	  case VM_METHOD_TYPE_BMETHOD:
+            RB_OBJ_WRITE(me, &def->body.bmethod.proc, (VALUE)opts);
+	    return;
+	  case VM_METHOD_TYPE_NOTIMPLEMENTED:
+	    setup_method_cfunc_struct(UNALIGNED_MEMBER_PTR(def, body.cfunc), rb_f_notimplement, -1);
+	    return;
+	  case VM_METHOD_TYPE_OPTIMIZED:
+	    def->body.optimize_type = (enum method_optimized_type)opts;
+	    return;
+	  case VM_METHOD_TYPE_REFINED:
+	    {
+		const rb_method_refined_t *refined = (rb_method_refined_t *)opts;
+		RB_OBJ_WRITE(me, &def->body.refined.orig_me, refined->orig_me);
+		RB_OBJ_WRITE(me, &def->body.refined.owner, refined->owner);
+		return;
+	    }
+	  case VM_METHOD_TYPE_ALIAS:
+	    RB_OBJ_WRITE(me, &def->body.alias.original_me, (rb_method_entry_t *)opts);
+	    return;
+	  case VM_METHOD_TYPE_ZSUPER:
+	  case VM_METHOD_TYPE_UNDEF:
+	  case VM_METHOD_TYPE_MISSING:
+	    return;
+	}
+    }
+}
+
+static void
 method_definition_reset(const rb_method_entry_t *me)
 {
-    const rb_method_definition_t *def = me->def;
+    rb_method_definition_t *def = me->def;
 
     switch(def->type) {
       case VM_METHOD_TYPE_ISEQ:
@@ -247,218 +341,14 @@ method_definition_reset(const rb_method_entry_t *me)
     }
 }
 
-static rb_cref_t*
-the_top_cref(void)
+MJIT_FUNC_EXPORTED rb_method_definition_t *
+rb_method_definition_create(rb_method_type_t type, ID mid)
 {
-    static rb_cref_t *top = NULL;
-    if (!top) {
-        top = vm_cref_new_toplevel(GET_EC());
-        rb_gc_register_mark_object((VALUE)top); // cref is an IMEMO.
-    }
-    return top;
-}
-
-static rb_method_iseq_t
-the_method_iseq(const rb_method_iseq_t *p)
-{
-    return (rb_method_iseq_t) {
-        .iseqptr = p->iseqptr,
-        .cref    = p->cref ? p->cref : the_top_cref(),
-    };
-}
-
-static rb_method_cfunc_t
-the_method_cfunc(const rb_method_cfunc_t *p)
-{
-    return (rb_method_cfunc_t) {
-        .func    = p->func,
-        .invoker = call_cfunc_invoker_func(p->argc),
-        .argc    = p->argc,
-    };
-}
-
-static VALUE
-the_location(void)
-{
-    const rb_execution_context_t *ec = GET_EC();
-    const rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(ec, ec->cfp);
-    int line;
-
-    if (!cfp) {
-        return Qfalse;
-    }
-    else if (! (line = rb_vm_get_sourceline(cfp))) {
-        return Qfalse;
-    }
-    else {
-        VALUE loc = rb_ary_new3(2, rb_iseq_path(cfp->iseq), INT2FIX(line));
-        rb_ary_freeze(loc);
-        return loc;
-    }
-}
-
-static rb_method_attr_t
-the_method_attr(const void *p)
-{
-    return (rb_method_attr_t) {
-        .id       = (ID)(VALUE)p,
-        .location = the_location(),
-    };
-}
-
-static rb_method_bmethod_t
-the_method_bmethod(const void *p)
-{
-    return (rb_method_bmethod_t) {
-        .proc  = (VALUE)p,
-        .hooks = NULL,
-    };
-}
-
-static rb_method_cfunc_t
-the_method_notimplemented(void)
-{
-    return (rb_method_cfunc_t) {
-        .func    = rb_f_notimplement,
-        .invoker = call_cfunc_m1,
-        .argc    = -1,
-    };
-}
-
-static enum method_optimized_type
-the_method_optimized(const void *p)
-{
-    return (enum method_optimized_type)p;
-}
-
-static rb_method_refined_t
-the_method_refined(const rb_method_refined_t *p)
-{
-    if (!p) {
-        return (rb_method_refined_t) { 0, };
-    }
-    else {
-        return (rb_method_refined_t) {
-            .orig_me = p->orig_me,
-            .owner   = p->owner,
-        };
-    }
-}
-
-static rb_method_alias_t
-the_method_alias(const rb_method_entry_t *p)
-{
-    return (rb_method_alias_t) {
-        .original_me = p,
-    };
-}
-
-static rb_method_definition_t
-rb_method_definition_new(rb_method_type_t type, ID mid, const void *opts)
-{
-    switch (type) {
-      case VM_METHOD_TYPE_ISEQ:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .iseq    = the_method_iseq(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_CFUNC:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .cfunc   = the_method_cfunc(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_ATTRSET:
-      case VM_METHOD_TYPE_IVAR:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .attr    = the_method_attr(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_BMETHOD:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .bmethod = the_method_bmethod(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_NOTIMPLEMENTED:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .cfunc   = the_method_notimplemented(),
-            }
-        };
-
-      case VM_METHOD_TYPE_OPTIMIZED:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .optimize_type = the_method_optimized(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_REFINED:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .refined = the_method_refined(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_ALIAS:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-            .body        = {
-                .alias   = the_method_alias(opts),
-            }
-        };
-
-      case VM_METHOD_TYPE_ZSUPER:
-      case VM_METHOD_TYPE_UNDEF:
-      case VM_METHOD_TYPE_MISSING:
-        return (rb_method_definition_t) {
-            .type        = type,
-            .original_id = mid,
-        };
-    }
-
-    UNREACHABLE_RETURN((rb_method_definition_t){0});
-}
-
-MJIT_FUNC_EXPORTED void
-rb_method_entry_spoof(const rb_method_entry_t *me)
-{
-    VALUE v = (VALUE)me;
-    VALUE o = me->owner;
-    rb_id_table_insert(RCLASS_M_TBL(o), me->called_id, v);
-    RB_OBJ_WRITTEN(o, Qundef, v);
-    rb_clear_method_cache_by_class(o);
-}
-
-static const rb_method_definition_t *
-rb_method_definition_create(rb_method_type_t type, ID mid, const void *opts)
-{
-    rb_method_definition_t template = rb_method_definition_new(type, mid, opts);
-    void *ptr = ALLOC(rb_method_definition_t);
-    memcpy(ptr, &template, sizeof template);
-    return ptr;
+    rb_method_definition_t *def;
+    def = ZALLOC(rb_method_definition_t);
+    def->type = type;
+    def->original_id = mid;
+    return def;
 }
 
 static rb_method_definition_t *
@@ -477,15 +367,10 @@ method_definition_addref_complement(rb_method_definition_t *def)
     return def;
 }
 
-static const rb_method_entry_t *
-rb_method_entry_alloc(VALUE flags, ID called_id, VALUE owner, VALUE defined_class, const rb_method_definition_t *def)
-{
-    VALUE v = rb_imemo_new(imemo_ment, (VALUE)def, (VALUE)called_id, owner, defined_class);
-    RBASIC(v)->flags =
-        (RBASIC(v)->flags & ~(IMEMO_FL_USER0|IMEMO_FL_USER1|IMEMO_FL_USER2)) |
-                   (flags &  (IMEMO_FL_USER0|IMEMO_FL_USER1|IMEMO_FL_USER2));
-    const rb_method_entry_t *me = (void *)v;
-    method_definition_reset(me);
+static rb_method_entry_t *
+rb_method_entry_alloc(ID called_id, VALUE owner, VALUE defined_class, const rb_method_definition_t *def)
+{
+    rb_method_entry_t *me = (rb_method_entry_t *)rb_imemo_new(imemo_ment, (VALUE)def, (VALUE)called_id, owner, defined_class);
     return me;
 }
 
@@ -503,58 +388,33 @@ filter_defined_class(VALUE klass)
     rb_bug("filter_defined_class: %s", rb_obj_info(klass));
 }
 
-static inline VALUE
-method_entry_flags(rb_method_visibility_t visi)
-{
-    return (visi << (IMEMO_FL_USHIFT+0)) |
-            ((!ruby_running) << (IMEMO_FL_USHIFT+2));
-}
-
-MJIT_FUNC_EXPORTED const rb_method_entry_t *
-rb_method_entry_from_template(
-    const rb_method_entry_t *me,
-    const void *body)
-{
-    return rb_method_entry_alloc(
-        me->flags,
-        me->called_id,
-        me->owner,
-        me->defined_class,
-        rb_method_definition_create(
-            me->def->type,
-            me->def->original_id,
-            body));
-}
-
-const rb_method_entry_t *
-rb_method_entry_for_missing(ID mid, VALUE klass)
+rb_method_entry_t *
+rb_method_entry_create(ID called_id, VALUE klass, rb_method_visibility_t visi, const rb_method_definition_t *def)
 {
-    return rb_method_entry_alloc(
-        method_entry_flags(METHOD_VISI_UNDEF),
-        mid,
-        klass,
-        filter_defined_class(klass),
-        rb_method_definition_create(
-            VM_METHOD_TYPE_MISSING,
-            mid,
-            NULL));
+    rb_method_entry_t *me = rb_method_entry_alloc(called_id, klass, filter_defined_class(klass), def);
+    METHOD_ENTRY_FLAGS_SET(me, visi, ruby_running ? FALSE : TRUE);
+    if (def != NULL) method_definition_reset(me);
+    return me;
 }
 
 const rb_method_entry_t *
 rb_method_entry_clone(const rb_method_entry_t *src_me)
 {
-    return rb_method_entry_alloc(
-        src_me->flags,
-        src_me->called_id,
-        src_me->owner,
-        src_me->defined_class,
-        method_definition_addref(src_me->def));
+    rb_method_entry_t *me = rb_method_entry_alloc(src_me->called_id, src_me->owner, src_me->defined_class,
+						  method_definition_addref(src_me->def));
+    METHOD_ENTRY_FLAGS_COPY(me, src_me);
+    return me;
 }
 
 MJIT_FUNC_EXPORTED const rb_callable_method_entry_t *
 rb_method_entry_complement_defined_class(const rb_method_entry_t *src_me, ID called_id, VALUE defined_class)
 {
-    const rb_method_definition_t *def = src_me->def;
+    rb_method_definition_t *def = src_me->def;
+    rb_method_entry_t *me;
+    struct {
+	const struct rb_method_entry_struct *orig_me;
+	VALUE owner;
+    } refined = {0};
 
     if (!src_me->defined_class &&
 	def->type == VM_METHOD_TYPE_REFINED &&
@@ -562,69 +422,73 @@ rb_method_entry_complement_defined_class(const rb_method_entry_t *src_me, ID cal
 	const rb_method_entry_t *orig_me =
 	    rb_method_entry_clone(def->body.refined.orig_me);
 	RB_OBJ_WRITE((VALUE)orig_me, &orig_me->defined_class, defined_class);
-        def = rb_method_definition_create(
-            VM_METHOD_TYPE_REFINED,
-            called_id,
-            &(rb_method_refined_t) {
-                .orig_me = orig_me,
-                .owner   = orig_me->owner});
-
+	refined.orig_me = orig_me;
+	refined.owner = orig_me->owner;
+	def = NULL;
     }
     else {
-        def = method_definition_addref_complement((rb_method_definition_t *)def);
+	def = method_definition_addref_complement(def);
+    }
+    me = rb_method_entry_alloc(called_id, src_me->owner, defined_class, def);
+    METHOD_ENTRY_FLAGS_COPY(me, src_me);
+    METHOD_ENTRY_COMPLEMENTED_SET(me);
+    if (!def) {
+	def = rb_method_definition_create(VM_METHOD_TYPE_REFINED, called_id);
+	rb_method_definition_set(me, def, &refined);
     }
-    const rb_method_entry_t *me =
-        rb_method_entry_alloc(
-            src_me->flags,
-            called_id,
-            src_me->owner,
-            defined_class,
-            def);
 
     VM_ASSERT(RB_TYPE_P(me->owner, T_MODULE));
 
-    return (const rb_callable_method_entry_t *)me;
+    return (rb_callable_method_entry_t *)me;
 }
 
-static const rb_method_entry_t*
-make_method_entry_refined(VALUE owner, const rb_method_entry_t *me)
+void
+rb_method_entry_copy(rb_method_entry_t *dst, const rb_method_entry_t *src)
+{
+    *(rb_method_definition_t **)&dst->def = method_definition_addref(src->def);
+    method_definition_reset(dst);
+    dst->called_id = src->called_id;
+    RB_OBJ_WRITE((VALUE)dst, &dst->owner, src->owner);
+    RB_OBJ_WRITE((VALUE)dst, &dst->defined_class, src->defined_class);
+    METHOD_ENTRY_FLAGS_COPY(dst, src);
+}
+
+static void
+make_method_entry_refined(VALUE owner, rb_method_entry_t *me)
 {
     if (me->def->type == VM_METHOD_TYPE_REFINED) {
-        return me;
+	return;
     }
     else {
+	struct {
+	    struct rb_method_entry_struct *orig_me;
+	    VALUE owner;
+	} refined;
+	rb_method_definition_t *def;
+
 	rb_vm_check_redefinition_opt_method(me, me->owner);
-        /* :FIXME: can rb_method_entry_from_template be tweaked to also be
-         * applicable here? */
-        return rb_method_entry_alloc(
-            method_entry_flags(METHOD_VISI_PUBLIC),
-            me->called_id,
-            me->owner,
-            me->defined_class,
-            rb_method_definition_create(
-                VM_METHOD_TYPE_REFINED,
-                me->called_id,
-                &(rb_method_refined_t) {
-                    .owner   = owner,
-                    .orig_me =
-                        rb_method_entry_alloc(
-                            me->flags,
-                            me->called_id,
-                            me->owner,
-                            me->defined_class ?
-                            me->defined_class : owner,
-                            method_definition_addref(me->def))}));
+
+	refined.orig_me =
+	    rb_method_entry_alloc(me->called_id, me->owner,
+				  me->defined_class ?
+				  me->defined_class : owner,
+				  method_definition_addref(me->def));
+	METHOD_ENTRY_FLAGS_COPY(refined.orig_me, me);
+	refined.owner = owner;
+
+	def = rb_method_definition_create(VM_METHOD_TYPE_REFINED, me->called_id);
+	rb_method_definition_set(me, def, (void *)&refined);
+	METHOD_ENTRY_VISI_SET(me, METHOD_VISI_PUBLIC);
     }
 }
 
 void
 rb_add_refined_method_entry(VALUE refined_class, ID mid)
 {
-    const rb_method_entry_t *me = lookup_method_table(refined_class, mid);
+    rb_method_entry_t *me = lookup_method_table(refined_class, mid);
 
     if (me) {
-        me = make_method_entry_refined(refined_class, me);
-        rb_method_entry_spoof(me);
+	make_method_entry_refined(refined_class, me);
 	rb_clear_method_cache_by_class(refined_class);
     }
     else {
@@ -654,10 +518,11 @@ check_override_opt_method(VALUE klass, VALUE arg)
  * If def is given (!= NULL), then just use it and ignore original_id and otps.
  * If not given, then make a new def with original_id and opts.
  */
-static const rb_method_entry_t *
+static rb_method_entry_t *
 rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibility_t visi,
-                     rb_method_type_t type, const rb_method_definition_t *def, ID original_id, void *opts)
+		     rb_method_type_t type, rb_method_definition_t *def, ID original_id, void *opts)
 {
+    rb_method_entry_t *me;
     struct rb_id_table *mtbl;
     st_data_t data;
     int make_refined = 0;
@@ -685,7 +550,7 @@ rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibil
 	rb_add_refined_method_entry(refined_class, mid);
     }
     if (type == VM_METHOD_TYPE_REFINED) {
-        const rb_method_entry_t *old_me = lookup_method_table(RCLASS_ORIGIN(klass), mid);
+	rb_method_entry_t *old_me = lookup_method_table(RCLASS_ORIGIN(klass), mid);
 	if (old_me) rb_vm_check_redefinition_opt_method(old_me, klass);
     }
     else {
@@ -695,8 +560,8 @@ rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibil
 
     /* check re-definition */
     if (rb_id_table_lookup(mtbl, mid, &data)) {
-        const rb_method_entry_t *old_me = (const rb_method_entry_t *)data;
-        const rb_method_definition_t *old_def = old_me->def;
+	rb_method_entry_t *old_me = (rb_method_entry_t *)data;
+	rb_method_definition_t *old_def = old_me->def;
 
 	if (rb_method_definition_eq(old_def, def)) return old_me;
 	rb_vm_check_redefinition_opt_method(old_me, klass);
@@ -733,16 +598,9 @@ rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibil
     }
 
     /* create method entry */
-    if (!def) {
-        def = rb_method_definition_create(type, original_id, opts);
-    }
-    const rb_method_entry_t *me =
-        rb_method_entry_alloc(
-            method_entry_flags(visi),
-            mid,
-            defined_class,
-            filter_defined_class(defined_class),
-            def);
+    me = rb_method_entry_create(mid, defined_class, visi, NULL);
+    if (def == NULL) def = rb_method_definition_create(type, original_id);
+    rb_method_definition_set(me, def, opts);
 
     rb_clear_method_cache_by_class(klass);
 
@@ -764,7 +622,7 @@ rb_method_entry_make(VALUE klass, ID mid, VALUE defined_class, rb_method_visibil
     }
 
     if (make_refined) {
-        me = make_method_entry_refined(klass, me);
+	make_method_entry_refined(klass, me);
     }
 
     rb_id_table_insert(mtbl, mid, (VALUE)me);
@@ -812,21 +670,27 @@ rb_add_method(VALUE klass, ID mid, rb_method_type_t type, void *opts, rb_method_
 MJIT_FUNC_EXPORTED void
 rb_add_method_iseq(VALUE klass, ID mid, const rb_iseq_t *iseq, rb_cref_t *cref, rb_method_visibility_t visi)
 {
-    rb_method_iseq_t iseq_body = { iseq, cref };
+    struct { /* should be same fields with rb_method_iseq_struct */
+        const rb_iseq_t *iseqptr;
+        rb_cref_t *cref;
+    } iseq_body;
+
+    iseq_body.iseqptr = iseq;
+    iseq_body.cref = cref;
     rb_add_method(klass, mid, VM_METHOD_TYPE_ISEQ, &iseq_body, visi);
 }
 
-static const rb_method_entry_t *
+static rb_method_entry_t *
 method_entry_set(VALUE klass, ID mid, const rb_method_entry_t *me,
 		 rb_method_visibility_t visi, VALUE defined_class)
 {
-    const rb_method_entry_t *newme = rb_method_entry_make(klass, mid, defined_class, visi,
+    rb_method_entry_t *newme = rb_method_entry_make(klass, mid, defined_class, visi,
 						    me->def->type, method_definition_addref(me->def), 0, NULL);
     method_added(klass, mid);
     return newme;
 }
 
-const rb_method_entry_t *
+rb_method_entry_t *
 rb_method_entry_set(VALUE klass, ID mid, const rb_method_entry_t *me, rb_method_visibility_t visi)
 {
     return method_entry_set(klass, mid, me, visi, klass);
@@ -860,10 +724,10 @@ rb_get_alloc_func(VALUE klass)
     return 0;
 }
 
-static inline const rb_method_entry_t*
+static inline rb_method_entry_t*
 search_method(VALUE klass, ID id, VALUE *defined_class_ptr)
 {
-    const rb_method_entry_t *me;
+    rb_method_entry_t *me;
 
     for (; klass; klass = RCLASS_SUPER(klass)) {
 	RB_DEBUG_COUNTER_INC(mc_search_super);
@@ -887,12 +751,12 @@ rb_method_entry_at(VALUE klass, ID id)
  * if you need method entry with method cache (normal case), use
  * rb_method_entry() simply.
  */
-static const rb_method_entry_t *
+static rb_method_entry_t *
 method_entry_get_without_cache(VALUE klass, ID id,
 			       VALUE *defined_class_ptr)
 {
     VALUE defined_class;
-    const rb_method_entry_t *me = search_method(klass, id, &defined_class);
+    rb_method_entry_t *me = search_method(klass, id, &defined_class);
 
     if (ruby_running) {
 	if (OPT_GLOBAL_METHOD_CACHE) {
@@ -924,11 +788,11 @@ method_entry_get_without_cache(VALUE klass, ID id,
 }
 
 static void
-verify_method_cache(VALUE klass, ID id, VALUE defined_class, const rb_method_entry_t *me)
+verify_method_cache(VALUE klass, ID id, VALUE defined_class, rb_method_entry_t *me)
 {
     if (!VM_DEBUG_VERIFY_METHOD_CACHE) return;
     VALUE actual_defined_class;
-    const rb_method_entry_t *actual_me =
+    rb_method_entry_t *actual_me =
       method_entry_get_without_cache(klass, id, &actual_defined_class);
 
     if (me != actual_me || defined_class != actual_defined_class) {
@@ -936,7 +800,7 @@ verify_method_cache(VALUE klass, ID id, VALUE defined_class, const rb_method_ent
     }
 }
 
-static const rb_method_entry_t *
+static rb_method_entry_t *
 method_entry_get(VALUE klass, ID id, VALUE *defined_class_ptr)
 {
     struct cache_entry *ent;
@@ -977,7 +841,7 @@ prepare_callable_method_entry(VALUE defined_class, ID id, const rb_method_entry_
 
 	if (mtbl && rb_id_table_lookup(mtbl, id, (VALUE *)&me)) {
 	    RB_DEBUG_COUNTER_INC(mc_cme_complement_hit);
-            cme = (const rb_callable_method_entry_t *)me;
+	    cme = (rb_callable_method_entry_t *)me;
 	    VM_ASSERT(callable_method_entry_p(cme));
 	}
 	else {
@@ -1001,7 +865,7 @@ MJIT_FUNC_EXPORTED const rb_callable_method_entry_t *
 rb_callable_method_entry(VALUE klass, ID id)
 {
     VALUE defined_class;
-    const rb_method_entry_t *me = method_entry_get(klass, id, &defined_class);
+    rb_method_entry_t *me = method_entry_get(klass, id, &defined_class);
     return prepare_callable_method_entry(defined_class, id, me);
 }
 
@@ -1115,7 +979,7 @@ static void
 remove_method(VALUE klass, ID mid)
 {
     VALUE data;
-    const rb_method_entry_t *me = NULL;
+    rb_method_entry_t *me = 0;
     VALUE self = klass;
 
     klass = RCLASS_ORIGIN(klass);
@@ -1125,7 +989,7 @@ remove_method(VALUE klass, ID mid)
     }
 
     if (!rb_id_table_lookup(RCLASS_M_TBL(klass), mid, &data) ||
-        !(me = (const rb_method_entry_t *)data) ||
+	!(me = (rb_method_entry_t *)data) ||
 	(!me->def || me->def->type == VM_METHOD_TYPE_UNDEF) ||
         UNDEFINED_REFINED_METHOD_P(me->def)) {
 	rb_name_err_raise("method `%1$s' not defined in %2$s",
@@ -1183,18 +1047,10 @@ rb_mod_remove_method(int argc, VALUE *argv, VALUE mod)
     return mod;
 }
 
-static inline void
-METHOD_ENTRY_VISI_SET(rb_method_entry_t *me, rb_method_visibility_t visi)
-{
-    VM_ASSERT((int)visi >= 0 && visi <= 3);
-    VALUE flags = (me->flags & ~(IMEMO_FL_USER0 | IMEMO_FL_USER1)) | (visi << (IMEMO_FL_USHIFT+0));
-    memcpy((void *)&me->flags, &flags, sizeof flags);
-}
-
 static void
 rb_export_method(VALUE klass, ID name, rb_method_visibility_t visi)
 {
-    const rb_method_entry_t *me;
+    rb_method_entry_t *me;
     VALUE defined_class;
     VALUE origin_class = RCLASS_ORIGIN(klass);
 
@@ -1212,7 +1068,7 @@ rb_export_method(VALUE klass, ID name, rb_method_visibility_t visi)
 	rb_vm_check_redefinition_opt_method(me, klass);
 
 	if (klass == defined_class || origin_class == defined_class) {
-            METHOD_ENTRY_VISI_SET((rb_method_entry_t *)me, visi);
+	    METHOD_ENTRY_VISI_SET(me, visi);
 
 	    if (me->def->type == VM_METHOD_TYPE_REFINED && me->def->body.refined.orig_me) {
 		METHOD_ENTRY_VISI_SET((rb_method_entry_t *)me->def->body.refined.orig_me, visi);
@@ -1740,7 +1596,7 @@ rb_alias(VALUE klass, ID alias_name, ID original_name)
 	method_added(target_klass, alias_name);
     }
     else {
-        const rb_method_entry_t *alias_me;
+	rb_method_entry_t *alias_me;
 
 	alias_me = method_entry_set(target_klass, alias_name, orig_me, visi, orig_me->owner);
 	RB_OBJ_WRITE(alias_me, &alias_me->owner, target_klass);
@@ -1931,7 +1787,7 @@ rb_mod_ruby2_keywords(int argc, VALUE *argv, VALUE module)
     for (i = 0; i < argc; i++) {
         VALUE v = argv[i];
         ID name = rb_check_id(&v);
-        const rb_method_entry_t *me;
+        rb_method_entry_t *me;
         VALUE defined_class;
 
         if (!name) {