optimization and rewrite rules questions

[Max Bolingbroke]

2009/2/24 Claus Reinke <claus.reinke at talk21.com>:
> In the recently burried haskell-cafe thread "speed: ghc vs gcc",
> Bulat pointed out some of the optimizations that GHC doesn't
> do, such as loop unrolling. I suggested a way of experimenting with loop
> unrolling, using template haskell to bypass GHC's blindspot (it usually
> doesn't unfold recursive definitions
> http://www.haskell.org/pipermail/glasgow-haskell-users/2007-July/012936.html
> ,
> but if we unfold a loop combinator at compile time, GHC's
> normal optimizations can take over from there):
>
> http://www.haskell.org/pipermail/haskell-cafe/2009-February/056241.html

Just a note - there is a solution that doesn't require Template
Haskell which I use in my own code. Here is a sketch:

fact = fix4 fact_worker

{-# INLINE fact_worker #-}
fact_worker recurse n
  | n <= 0 = 1
  | otherwise = n * recurse (n - 1)

{-# INLINE fix4 #-}
fix4 f = f1
  where
    f1 = f f2
    f2 = f f3
    f3 = f f4
    f4 = f f1

There is probably a way to generalise this to arbitrary levels of
unrolling by using instances of a typeclass on type level numerals.

Cheers,
Max