A review of https://github.com/openjdk/jdk/pull/20395 makes the suggestion:

My initial reaction was that we are breaking away from the invariant that "evac means evac: cset is not empty, there are forwarded objects", and generational hijacks evacuation for its promotional needs. In some sense, this is still valid: promotion young->old can be logically treated as "evacuation". Was this the intent?

I suggest introducing a local var: bool is_relocating_objects = !heap->collection_set()->is_empty(), an use it in this code.