#6166: Performance regression in mwc-random since 7.0.x --------------------------------------------+------------------------------ Reporter: bos | Owner: Type: bug | Status: new Priority: high | Milestone: 7.6.2 Component: Compiler | Version: 7.4.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 Type of failure: Runtime performance bug | (amd64) Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by Khudyakov): Bug is still present in 7.6.3. I've made a reduced test case with stripped-down standard inlined. Note taht adding or removing return in main loop have no effect. Something interesting is going on with blocks. Replacing f with constant or removing cons all makes bug go away. Simplifying go function changes runtime drastically. {{{ {-# LANGUAGE BangPatterns #-} import qualified Data.Vector.Unboxed as I import Control.Monad.Primitive (PrimMonad, PrimState) import Data.Word import Data.Bits import Control.Monad import System.Random.MWC main :: IO () main = do g <- create replicateM_ (200*1000) $ standard g standard :: PrimMonad m => Gen (PrimState m) -> m Double {-# INLINE standard #-} standard gen = do u <- (subtract 1 . (*2)) `liftM` uniform gen ri <- uniform gen let i = fromIntegral ((ri :: Word32) .&. 127) bi = (I.!) blocks i return $! u * bi where blocks = I.cons r -- Removing cons $ I.unfoldrN 130 go $ T r f where go (T b g) = let !u = T h (exp (-0.5 * h * h)) h = sqrt (-2 * log (v / b + g)) in Just (h, u) {-# NOINLINE blocks #-} v = 9.91256303526217e-3 r = 3.442619855899 f = exp (-0.5 * r * r) -- Replacing with constant make bug go away! -- Unboxed 2-tuple data T = T {-# UNPACK #-} !Double {-# UNPACK #-} !Double }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6166#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler