Pointer top 16 bits use

[Nicholas Wilson via Digitalmars-d]

In Dicebot's DConf talk he mentioned that it is possible to use 
the top 16 bits for tagging pointers and the associated risks.

Regarding the GC not seeing a pointer if the top 16 bits are 
used, whats to stop us from changing the GC to (or adding an 
option to) ignore those bits when checking?

I imagine that it would cause a lot more false pointers, but when 
we get precise GC that problem would go away.