This module shouldn't need to exist. It exists because GHC's implementation of floor is currently (6.10.3) very slow. See the following tickets:

http://hackage.haskell.org/trac/ghc/ticket/1434

http://hackage.haskell.org/trac/ghc/ticket/2271

This is a special version of the regular floor function. It works by directly calling the low-level internal GHC primitives, and thus is as fast as you'd expect for such a trivial operation.

(The standard floor function does something crazy like converting a Double to a numerator/denominator Integer pair and then computing the integer part of the quotient as an Integer, then truncating that to a Word8. Which, obviously, is ludicrously slow.)

Hopefully one day the need for this low-level hackery will disappear.