This is used in some dynamic language runtimes as a way to represent a compact (64bit) tagged union. It's known as nan-boxing, and a couple years ago most of t

This is used in some dynamic language runtimes as a way to represent a compact (64bit) tagged union. It's known as nan-boxing, and a couple years ago most of the JS vms did it (in fairly different ways) but I'm not sure they still do, as my understanding is that it was only useful for the pre-optimized codepath.