How speculative optimizations work in RaptorJIT about blog

[emmericp]

It's not immediately obvious why the second assembler listing works.
The trick is: Lua/RaptorJIT uses NaN tagging, i.e., pointers etc look like NaN values when interpreted as double. NaN values have ~50 bits free to encode arbitrary information in them so this works.

Thought that tidbit might be useful to someone not familar with the internals of Lua/RaptorJIT :)

[lukego]

Quite so :).

I'm using a bit of artistic license too. The JIT would actually check the type while loading the slot from memory and you wouldn't really have other values making it into xmm registers. Hopefully I can be forgiven and the points that I am trying to make still stand though :)