Dear devs, The following program shows worse measures when compiled with -O with both ghc-7.10.2 and ghc-7.8.3. Am I hitting a known bug? Or may someone point a mistake in the code? It also shows worse measures when using anything other than -N1. Thanks in advance, Facundo -- /opt/ghc-7.8.3/bin/ghc --make -threaded -fforce-recomp test.hs -- time ./test: 10 seconds -- -- /opt/ghc-7.8.3/bin/ghc --make -threaded -O -fforce-recomp test.hs -- time ./test: 48 seconds -- -- /opt/ghc-7.10.2/bin/ghc --make -threaded -fforce-recomp test.hs -- time ./test: 23 seconds -- -- /opt/ghc-7.10.2/bin/ghc --make -threaded -O -fforce-recomp test.hs -- time ./test: 57 seconds -- import Control.Concurrent import Control.Monad import Data.List main :: IO () main = do let xs = [ 1 .. 400000000] mv <- newEmptyMVar replicateM_ 4 $ forkIO $ putMVar mv $! foldl' (+) 0 xs nums <- replicateM 4 $ takeMVar mv print (nums :: [Integer])