While this tagged pointer construct is pretty common but makes some people wretch, realize that the entire web is built upon this same concept, only bigger. The

While this tagged pointer construct is pretty common but makes some people wretch, realize that the entire web is built upon this same concept, only bigger. The nan/nun-boxing that is used by various dynamic engines (e.g. All JS engines) relies on the fact that pointers on x86-64 hardware only use 48-bits of memory address space. They use the remaining 16 bits in the double for flags pertaining to what type of object it is. This has a nasty side effect of preventing these engines from running on any 64-bit architecture that does not have this memory address space restriction (e.g. Sparc, POWER). One either has to force the environment to only allocate space out of the lower 48-bits or come up with some other kind of encoding scheme to indicate if the address is a "far" address and should be added to the 48-bit base.