Implement dup on unbound method

In Rails `_read_attribute` is defined on the top level
ActiveRecord::Base rather than the subclasses. This results in object
shapes not being able to use their cache. By implementing `dup` on
unbound methods we can give each subclass a uniq iseq and unique inline
cache, increasing our shape cache hits.

Note: experimental change that must be paired with changes to Active
Record to see results.

method.h

@@ -239,6 +239,7 @@ VALUE rb_method_entry_location(const rb_method_entry_t *me);
 void rb_free_method_entry(const rb_method_entry_t *me);
 
 const rb_method_entry_t *rb_method_entry_clone(const rb_method_entry_t *me);
+const rb_method_entry_t *rb_method_entry_dup(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);
 

proc.c

@@ -2429,6 +2429,23 @@ method_clone(VALUE self)
     return clone;
 }
 
+static VALUE
+method_dup(VALUE self)
+{
+    VALUE dup;
+    struct METHOD *orig, *data;
+
+    TypedData_Get_Struct(self, struct METHOD, &method_data_type, orig);
+    dup = TypedData_Make_Struct(CLASS_OF(self), struct METHOD, &method_data_type, data);
+    CLONESETUP(dup, self);
+    RB_OBJ_WRITE(dup, &data->recv, orig->recv);
+    RB_OBJ_WRITE(dup, &data->klass, orig->klass);
+    RB_OBJ_WRITE(dup, &data->iclass, orig->iclass);
+    RB_OBJ_WRITE(dup, &data->owner, orig->owner);
+    RB_OBJ_WRITE(dup, &data->me, rb_method_entry_dup(orig->me));
+    return dup;
+}
+
 /*  Document-method: Method#===
  *
  *  call-seq:
@@ -4354,6 +4371,7 @@ Init_Proc(void)
     rb_define_method(rb_cUnboundMethod, "eql?", unbound_method_eq, 1);
     rb_define_method(rb_cUnboundMethod, "hash", method_hash, 0);
     rb_define_method(rb_cUnboundMethod, "clone", method_clone, 0);
+    rb_define_method(rb_cUnboundMethod, "dup", method_dup, 0);
     rb_define_method(rb_cUnboundMethod, "arity", method_arity_m, 0);
     rb_define_method(rb_cUnboundMethod, "inspect", method_inspect, 0);
     rb_define_method(rb_cUnboundMethod, "to_s", method_inspect, 0);

vm_method.c

@@ -702,6 +702,32 @@ rb_method_entry_clone(const rb_method_entry_t *src_me)
     return me;
 }
 
+const rb_method_entry_t *
+rb_method_entry_dup(const rb_method_entry_t *src_me)
+{
+    // Create an empty method definition.
+    rb_method_definition_t *def = rb_method_definition_create(src_me->def->type, src_me->def->original_id);
+
+    const rb_iseq_t *iseq = src_me->def->body.iseq.iseqptr;
+
+    // Duplicate the instruction sequence
+    VALUE str = rb_iseq_ibf_dump(iseq, Qnil);
+    const rb_iseq_t *new_iseq = rb_iseq_ibf_load(str);
+
+    // Assign the copied iseq to the new method definition
+    def->body.iseq.iseqptr = new_iseq;
+
+    // Create a new method entry
+    rb_method_entry_t *me = rb_method_entry_alloc(src_me->called_id, src_me->owner, src_me->defined_class, def);
+
+    if (METHOD_ENTRY_COMPLEMENTED(src_me)) {
+        method_definition_addref_complement(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)
 {