[kvakvs]

Virtual machines use pointer tagging in least significant bits, as most of the data is 4x or 8x byte aligned, and you can assume that zeroing those bits will always give you a safe correct pointer value. And the tag bits can instruct the code that the value is not a pointer but contains more data in other bits, or that the value is a structure on heap with special treatment.

[khuey]

Depends on the virtual machine. LuaJIT and Spidermonkey (and probably others that I don't know about) use NaN-boxing which ends up storing the metadata in the higher bits.