Worse performance with -O

The running time of the following program worsens when compiled with -O, and worsens more when compiled with ghc-7.10.2.

-- /opt/ghc-7.8.3/bin/ghc --make -threaded -fforce-recomp test.hs
-- time ./test: 3 seconds
--
-- /opt/ghc-7.8.3/bin/ghc --make -threaded -O -fforce-recomp test.hs
-- time ./test: 11 seconds
--
-- /opt/ghc-7.10.2/bin/ghc --make -threaded -fforce-recomp test.hs
-- time ./test: 5 seconds
--
-- /opt/ghc-7.10.2/bin/ghc --make -threaded -O -fforce-recomp test.hs
-- time ./test: 13 seconds
--

import Control.Concurrent
import Control.Monad
import Data.List

main :: IO ()
main = do
  let x = foldl' (+) 0 [1 .. 100000000]
  mv <- newEmptyMVar
  replicateM_ 4 $ forkIO $ putMVar mv $! x
  nums <- replicateM 4 $ takeMVar mv
  print (nums :: [Integer])

The following variant which doesn't share x improves with -O for ghc-7.10.2, but ghc-7.8.3 still produces a faster program.

-- /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: 11 seconds
--
-- /opt/ghc-7.10.2/bin/ghc --make -threaded -fforce-recomp test.hs
-- time ./test: 18 seconds
--
-- /opt/ghc-7.10.2/bin/ghc --make -threaded -O -fforce-recomp test.hs
-- time ./test: 15 seconds
--

import Control.Concurrent
import Control.Monad
import Data.IORef
import Data.List

main :: IO ()
main = do
  mv <- newEmptyMVar
  ref <- newIORef 0
  replicateM_ 4 $ forkIO $ do
    i <- readIORef ref
    putMVar mv $! foldl' (+) i [1 .. 100000000]
  nums <- replicateM 4 $ takeMVar mv
  print (nums :: [Integer])

Some related discussion here.

Trac metadata

Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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