[MaulingMonkey]

> What does this have to do with speed?

You generally need to mask potentially-tagged pointers before dereferencing them. (Ab)using virtual memory or unconnected lines lets you skip the mask, eliminating the cost in any code that's otherwise unconcerned about the tags. This in turn may let you save a byte or register here and there (and thus save memory bandwith / potentially spill fewer registers, maybe saving some performance).

GCs may abuse pointer tagging for keeping track of what they scaned. Ruby's VALUE type is pointer sized, and will point to Ruby objects such as strings and symbols - but it can also directly represent e.g. a 31-bit integer value ('Fixnum') on 32-bit systems without needing to be dereferenced, and without needing to consume a separate type field.

[jacobush]

Also used in early revisions of the Amiga. (BCPL or Amiga Basic used pointer tags for something like garbage collect or frame pointers or something IIRC.) Mac had something called 32 bit clean which I imagined could have been pointer tagging.)

[krallja]

"32 bit clean" meant that the ROM didn't use pointer tagging; otherwise it was using the top 8 bits for its own purposes and wouldn't run on machines which used the full 32-bit address line.

http://lowendmac.com/2015/32-bit-addressing-on-older-macs/