With JDK-8305895, we added a field _prototype_header in Klass, which encodes that Klass* when compact headers are turned on. The purpose was to simplify the allocation paths, where we can initialize the new object's header with that prototype.
The prototype can be removed and we can instead encode the Klass* on the allocation paths. This might actually be faster, because we save a load from memory, and we already have the Klass* in a register and only need to do some cheap shifting and masking on it.