#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): Another simplification {{{ {-# LANGUAGE BangPatterns #-} import qualified Data.Vector.Unboxed as I import Data.Vector.Unboxed ((!)) import Control.Monad main :: IO () main = replicateM_ (200*1000) (return $! standard) standard :: Double -- Removing or replacing with NOINLINE returns perfomance to normal {-# INLINE standard #-} standard = blocks ! 0 where blocks :: I.Vector Double blocks = I.cons 0.123 $ I.unfoldrN 130 go (T f) where go q@(T a) = Just (log (exp a), q) {-# NOINLINE blocks #-} r,f :: Double r = 3.442619855899 -- replacing f with constant return perfomance to normal -- f = 2.669629083880923e-3 f = exp (-0.5 * r * r) -- replacing data with newtype returns performance to normal data T = T {-# UNPACK #-} !Double }}} Problem is visible at the core level. Code is compiled down to the something similar to following pseudocode: {{{ loop i = if i /= 0 then evaluate (blocks ! 0) >> loop (i-1) else return () }}} blocks array is inlied despite being marked as NOINLINE and is evaluated on each iteration so performance is abysmal. When small chages to the program are made it's not inlined and evaluated only once. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6166#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler