id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2280	randomR too slow	guest	rrnewton	"randomR is considerably slower than implementing it manually. Maybe I have not re-implemented it precisely, maybe it is just not inlined.

{{{
module Main (main) where

import System.Random (RandomGen(..), randomR, )
import qualified Data.ByteString as B

newtype KnuthRandomGen = KnuthRandomGen Int

{-# INLINE knuthFactor #-}
knuthFactor :: Int
knuthFactor = 40692

{-# INLINE knuthModulus #-}
knuthModulus :: Int
knuthModulus = 2^(31::Int)-249

-- we have to split the 32 bit integer in order to avoid overflow on multiplication
knuthSplit :: Int
knuthSplit = succ $ div knuthModulus knuthFactor

knuthSplitRem :: Int
knuthSplitRem = knuthSplit * knuthFactor - knuthModulus

instance RandomGen KnuthRandomGen where
   {-# INLINE next #-}
   next (KnuthRandomGen s) =
      -- efficient computation of @mod (s*knuthFactor) knuthModulus@ without Integer
      let (sHigh, sLow) = divMod s knuthSplit
      in  (s, KnuthRandomGen $ flip mod knuthModulus $
                   knuthSplitRem*sHigh + knuthFactor*sLow)
   {-# INLINE split #-}
   split (KnuthRandomGen s) =
      (KnuthRandomGen (s*s), KnuthRandomGen (13+s))
   {-# INLINE genRange #-}
   genRange _ = (1, pred knuthModulus)


main :: IO ()
main =
   do 
      -- for comparison: that's very fast
      putStrLn ""constant""
      B.writeFile ""random.out"" $ fst $
          B.unfoldrN 10000000
             (\g0@(KnuthRandomGen s) -> Just (fromIntegral s, g0))
             (KnuthRandomGen 1)

      -- 3 seconds on my machine
      putStrLn ""immediate""
      B.writeFile ""random.out"" $ fst $
          B.unfoldrN 10000000
             (\g0 -> let (w,g1) = next g0
                     in  Just (fromIntegral (mod w 256), g1))
             (KnuthRandomGen 1)

      -- 10 seconds on my machine
      putStrLn ""randomR""
      B.writeFile ""random.out"" $ fst $
          B.unfoldrN 10000000
             (\g0 -> Just (let (w,g1) = randomR (0,255) g0
                           in  (fromIntegral (w::Int), g1)))
             (KnuthRandomGen 1)


{-
ghc -o dist/build/randomtest -O -Wall -package bytestring-0.9.0.5 -ddump-simpl-iterations speedtest/RandomTest.hs
-}
{-
bytestring-0.9.0.1 as shipped with GHC-6.8.2 does not inline Data.ByteString.unfoldrN
-}

}}}

Is this related to Ticket 427?
"	bug	new	normal	_|_	libraries/random	6.8.2		randomR	ghc@…	Linux	x86	Runtime performance bug	Unknown				#427