# Fun with GHC's optimiser

**Simon Peyton-Jones
**
simonpj@microsoft.com

*Tue, 19 Dec 2000 00:46:32 -0800*

Manuel
|* I have found a way of rephrasing the definition so that it
*|* is properly optimised by GHC. However, I think, it should
*|* be possible to do this automatically and it is maybe not
*|* unlike the optimisation done by simplCore/LiberateCase.
*
I agree with your example. But I think the way to handle
it is this. Suppose we have
f x y = ....(f ex (C a b))...
where C is a constructor, and where y is scrutinised by a
case expression somewhere in f's body. Then, in this recursive
call to f, we know what y will be, and we could save the case in
the recursive call. So make a specialised version of f, thus:
fs x a b = let y = C a b in (...original body of f...)
Now add a RULE to f, saying
f x (C a b) ===> fs x a b
This is very much the way type specialisation works. Now every
call to f that has a (C a b) argument will benefit. Including
the recursive call to f in fs's RHS, so fs will become the self-recursive
tight loop you want, leaving f as a sort of 'impedence matcher' for
callers elsewhere.
I'll try to find time to implement this.
Simon