While both fastmem and the BLR optimization depend on fault handling,
the BLR optimization doesn't depend on fastmem, and there are cases
where you might want the BLR optimization but not fastmem. For me
personally, it's useful when I try to use a debugger on Android and have
to disable fastmem so I don't get SIGSEGVs all the time, but it would be
especially useful for iOS users.