simple CSE?

[Lennart Augustsson]

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 <dons at galois.com> 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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