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
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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