Huh, that’s clever! Do you have to choose that at compile or launch time, or does a program start like that and then “grow” when it uses more than 32GB of heap?

In Java you have to set max heap somehow - either ergonomics or just -Xmx command line option. Max heap is given (many a reason, and it sets before running the main method), so if you pick under the 32GB it'd auto use compressed pointers (optimize for size - optimize for speed). That option (compressed pointers) can be switched off, of course, via a command line option as well.