While conceptually similar (and made me think of the same thing), this doesn't really look like the same thing. In the Mac case, it was (ab)using technically u

While conceptually similar (and made me think of the same thing), this doesn't really look like the same thing. In the Mac case, it was (ab)using technically unused addressed bits in unintended ways. In the ARM case, it is an explicit architectural feature. More like pointer tags in Lisp machines.