[masklinn]

Apple's been using that for more than a decade: https://www.mikeash.com/pyblog/friday-qa-2012-07-27-lets-bui... http://www.sealiesoftware.com/blog/archive/2013/09/24/objc_e...

OCaml's also been using tagging to mark unboxed type pretty much from the start, nearly 30 years ago: https://dev.realworldocaml.org/runtime-memory-layout.html#:~....

[duskwuff]

And it's not like Apple isn't aware of the dangers, either; they used top-byte tagging on the 68000, and it took them a lot of time and effort to migrate away from that to support 16+ MB systems.

[gpderetta]

IIRC the issue was that 68000 would transparently mask the top bits, so code could get away with avoiding the explicit masking, which of course breaks when you move to a larger address space.

More modern cpus enforce specific bit patterns for the MSBs, so broken code would be caught immediately.