After concurrent-marking, G1 will walk through the discovered list (DL) and drop references whose referents are null or live, because those references should not be processed. If a referent is live, `make_referent_alive` is called so that everything reachable from that referent should be kept live as well.

However, `make_referent_alive` would always be a no-op because concurrent-marking is done, and mutators calling `Reference.get()` only enqueues a pointer, *not* updating the marking status. (Those enqueued pointers are processed later in the Remark phase.)

The attachment contains a contrived example to stress G1 precleaning, ~3 million weak refs on the DL.