#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): I've been able to remove all stuff from mwc-random. Here is test case. Again it's slow version. {{{ {-# LANGUAGE BangPatterns #-} import qualified Data.Vector.Unboxed as I import Data.Vector.Unboxed ((!)) import Control.Monad main :: IO () main = replicateM_ (100*1000) (return $! standard) standard :: Double {-# INLINE standard #-} standard = blocks ! 0 where blocks :: I.Vector Double blocks = I.cons r $ 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,r,f :: Double v = 9.91256303526217e-3 r = 3.442619855899 -- f = 2.669629083880923e-3 -- FAST f = exp (-0.5 * r * r) -- SLOW -- Unboxed 2-tuple data T = T {-# UNPACK #-} !Double {-# UNPACK #-} !Double }}} Couple of observations 1. Replacing `f` with constant restores run time to normal. AFAIR GHC cannot constant fold `exp` and similar functions. So it may matter 2. Floating `block` to top level or removing `I.cons` restores run time too. 3. Simplifying `go` function changes run time. Removing `sqrt` or `log` reduce rim time. It looks like `blocks` is reevaluated every time `standard` is evaluated. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6166#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler