[guenthert]

That also implies that NIL cannot be represented as 0, which is a pity since testing the Z flag would be quick. I'd think someone ran the numbers and found the chosen encoding superior, but that would have been long ago (in the CMUCL code base).

[twoodfin]

I suspect with modern CPU pipelining and branch prediction that most of the (very interesting) debates about exactly how to tag values and pointers have become inconsequential above the noise level.

Would love to see recent work demonstrating this isn’t true!