GHC is bad at CSE. Of course, in general CSE might not be a good idea, but
with strict computations it is. So someone needs to add a CSE pass.
On Sat, Mar 29, 2008 at 2:23 AM, Don Stewart <[EMAIL PROTECTED]> wrote:
> conal:
> > I'd like to know if it's possible to get GHC to perform some simple
> CSE
> > for function-level programming. Here's a simple example:
> >
> > liftA2 (*) sin sin :: Double -> Double
> >
> > which inlines and simplifies to
> >
> > \ t -> sin t * sin t
> >
> > A more realistic, equivalent, example:
> >
> > let b = sin <$> id in liftA2 (*) b b
> >
> > Can GHC be nudged into computing 'sin t' once rather than twice?
> >
>
> So GHC does do some light CSE, but not in this case, as far as I could
> see.
>
> I had a go with a rewrite rule that I felt should have matched, but it
> didn't seem to want to fire. Possibly the dictionaries were getting in
> the way.
>
>
> import System.Environment
> import Prelude hiding (sin)
> import qualified Prelude
>
> sin :: Double -> Double
> sin x = Prelude.sin x
> {-# NOINLINE sin #-}
>
> times :: Double -> Double -> Double
> times x y = x * y
> {-# NOINLINE times #-}
>
> {-# RULES
>
> "sin/cse" forall x.
> times (sin x) (sin x) = case Prelude.sin x of y -> y * y
>
> #-}
>
>
> main = do
> [x] <- getArgs
> let n = read x
> print $ sin n `times` sin n
>
>
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