ghc floats out constant despite -fno-cse
Consider this program using Data.Vector.unsafeThaw
and -fno-cse
:
{-# OPTIONS_GHC -fno-cse #-}
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as VM
main :: IO ()
main = do
foo 100000
foo 100000
let f = foo 100000 in f >> f
foo :: Int -> IO ()
foo n = do
indexVector <- V.unsafeThaw $ V.generate n id
x <- VM.read indexVector 5
VM.write indexVector 5 (x * x)
print x
-- In GHCI, we get:
--
-- > :set -fno-cse
--
-- > foo 100000
-- 5
-- > foo 100000
-- 5
--
-- > let f = foo 100000 in f >> f
-- 5
-- 25
I would expect that
> let f = foo 100000 in f >> f
5
5
Shouldn't -fno-cse
fix this?
(Note: This also happens with ghc
instead of ghci
.)