This is a really dirty hack to do and it only works if your addresses are always aligned to a 2^n byte boundary. That said, there are plenty of cases where this

This is a really dirty hack to do and it only works if your addresses are always aligned to a 2^n byte boundary. That said, there are plenty of cases where this dirty hack is perfect for the job. Atomic lock free data structures is one example. Tagging the pointers can be used notify that a node is going to be deleted in order to avoid the ABA problem (as mentioned in the article). Tagging pointers is absolutely essential for more complex lock free data structures, like doubly linked lists. This can also be used in a garbage collector to add a few bits of metadata. In a GC you can (and probably should) decide to use memory objects with proper alignment, which leaves you with a few bits to store data in. A perfect solution to implement something like Lisp cons cells.