bool can_expand_young_list() {
   uint young_list_length = _g1->young_list()->length();
   uint young_list_max_length = _young_list_max_length;
   return young_list_length < young_list_max_length;
 }
 bool is_young_list_full() {
   uint young_list_length = _g1->young_list()->length();
   uint young_list_target_length = _young_list_target_length;
   return young_list_length >= young_list_target_length;
 }