Avoid creating the fields_obj just to freeze the class

Author: byroot

variable.c

@@ -2031,13 +2031,17 @@ rb_vm_set_ivar_id(VALUE obj, ID id, VALUE val)
 bool
 rb_shape_set_shape_id(VALUE obj, shape_id_t shape_id)
 {
-    if (rb_obj_shape_id(obj) == shape_id) {
+    shape_id_t old_shape_id = rb_obj_shape_id(obj);
+    if (old_shape_id == shape_id) {
         return false;
     }
 
 #if SHAPE_IN_BASIC_FLAGS
     if (BUILTIN_TYPE(obj) == T_CLASS || BUILTIN_TYPE(obj) == T_MODULE) {
-        RBASIC_SET_SHAPE_ID(RCLASS_ENSURE_FIELDS_OBJ(obj), shape_id);
+        // Avoid creating the fields_obj just to freeze the class
+        if (!(shape_id == SPECIAL_CONST_SHAPE_ID && old_shape_id == ROOT_SHAPE_ID)) {
+            RBASIC_SET_SHAPE_ID(RCLASS_ENSURE_FIELDS_OBJ(obj), shape_id);
+        }
     }
     // FIXME: How to do multi-shape?
     RBASIC_SET_SHAPE_ID(obj, shape_id);