On Thursday 29 May 2008, Tyson Whitehead wrote:
I thought this was interesting. Is it to be expected? Am I right in interpreting this to mean it was just too much for the strictness analyzer. I believe the first ultimately produces significantly superior code, so should one always write their recursive functions such that the constant (functional?) parameters are first captured in a closure?
In that vein, would it be useful if the compiler automatically transformed the second into the first?
I've had similar experiences. I've been working on sorting the mutable arrays in the new uvector library, and on at least one occasion, changing from code like: foo f x y = ... foo f x' y' to code like: foo f = loop where loop x y = ... loop x' y' resulted in 50% faster code (that is, 10 seconds -> 5 seconds). It doesn't always make such a dramatic difference, but it seems significant enough to have gotten me in the habit of writing such code by default (for the sorting, at least, where I'm trying to squeeze out as much performance as possible). Unfortunately, I think the resulting code is somewhat ugly, as I occasionally end up with code like: foo a b = loop1 c where loop1 c d = loop2 e where loop2 e = ... which may or may not actually be faster with more or less nested loops, but trying each possibility is a bit of a pain. If the compiler could automatically derive the more straightforward definition into the fast one, it'd be quite a boon, but I haven't thought very hard about whether there are any pitfalls to doing so. -- Dan