Hello Simon, Friday, May 30, 2008, 5:30:25 PM, you wrote: may be i don't understand something. isn't it better to do automatic SAT and inline results for every recursive function marked as INLINE? it's how i want to work - just mark with INLINE speed-critical funcs. manual checking that they are recursive and doing appropriate transformation is too hard for me :) and, btw, how about adding warnings about functions marked as INLINE which was not actually inlined due to some reasons - may be very helpful for optimizing programs without going into studying Core output
Others have explained this nicely. But there's a real tension here. The fast version comes from a combination of (a) the static argument transformation, so you get the first version above, and (b) bodily inlining the entire function, so that at *each call site* you get a locally-recursive function where 'f' is known. That's ok for small functions, but not so good for big ones. Furthermore, the code duplication is only worthwhile if the specialisation is truly useful. For example, would it be better to write append like this (++) xs ys = letrec app [] = ys app (x:xs) = x : app xs in app xs and inline that at every call of (++)? Probably not.
So that is why GHC does not automate this transformation. If you know that's what you want, write a local recursion, and use an INLINE pragma.
If someone felt like summarising this thread on the Haskell performance-advice wiki that would be great. http://haskell.org/haskellwiki/Performance
Meanwhile, I'll clarify in the user manual that recursive functions are not inlined.
Simon
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