#2280: randomR too slow
-----------------------------------------+----------------------------------
Reporter: guest | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Component: libraries/random
Version: 6.8.2 | Severity: normal
Keywords: randomR | Testcase:
Architecture: x86 | Os: Linux
-----------------------------------------+----------------------------------
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?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2280>
GHC <http://www.haskell.org/ghc/>
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