UseCompressedOops was disabled for C++ interpreter by JDK-6909153. It eventually morphed to disabled UseCompressedOops for all Zero configurations when C++ interpreter was removed with JDK-8074457 and followups. But I see nothing that actually prevents Zero from using compressed oops: all local/stack operands are always unpacked (like they are in non-interpreted modes), all heap accesses are calling into utility methods that handle compressed oops, there are no raw accesses to oops anywhere. I believe this is a simple omission from untangling the C++ interpreter and Zero.