On Jul 31, 2007, at 10:20 AM, Simon Peyton-Jones wrote:
| However my point was more on a semantic point of view: If I write a function | in a recursive way, but actually do nothing else than a loop, I would like | a) that the compiler unrolls it to a loop and | b) that I can specify such a requirement, while violating it emits an error.
What does it mean to say "the compiler unrolls it to a loop". If GHC sees a tail recursive function, it certainly compiles it to a loop! (But that's not called "unrolling".)
I think what's meant here is translating something like this: {-# INLINE f #-} f x y z = ... f x' y' z' ... into this: {-# INLINE f #-} f x y z = f' x y z where f' x y z = ... f' x' y' z' ... That is, shoving (all of) the recursion in a level. Then inlining f results in a fresh loop, which presumably can be specialized or optimized in various ways. I did this in pH, mostly because it was less irritating than performing the same transformation by hand on key bits of the Prelude. Whether it's actually beneficial on a large scale probably depends on the effectiveness of transformations that can specialise f' to the site where it was inlined; invariant argument elimination comes to mind here, and I never did a good job with that. It does remove one irritant from performance tuning, though, by letting us write a simple top-level recursive function and have it inlined. -Jan
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