[Haskell-cafe] Re: fast integer base-2 log function?

Jens Blanck writes:

{-# LANGUAGE MagicHash #-} import GHC.Exts import Data.Bits -- experiment with using a LUT here (hint: FFI + static arrays in C) ...

Sorry I don't have an answer, only more questions. Is {-# LANGUAGE MagicHash #-} documented somewhere? I've seen it referenced a few times now, but I can't find any details about it. Chad