In `PSPromotionManager::copy_unmarked_to_survivor_space`, when a thread loses the CAS race, it will try to deallocate the space:

```
if (new_obj_is_tenured) {
  if (!_old_lab.unallocate_object(...)) {
    fill_with_obj(...);
  }
} else {
  if (!_young_lab.unallocate_object(...)) {
    fill_with_obj(...);
  }
}
```

The "filler object" logic is irrelevant to the current context and makes more sense inside `unallocate_object`. For example, `PLAB::undo_allocation` contains the filler object logic.