#3586: Slow array code
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Reporter: simonpj | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.4
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Comment (by dons):
The problem in the STUArray version is newArray_, which uses the generic
newArray, which fills from a list. It does not fuse.
Rewriting the STUArray version to initialize directly and we get identical
results to Clean.
{{{
{-# LANGUAGE BangPatterns #-}
{-# OPTIONS -fvia-C -optc-O3 -fexcess-precision -optc-msse3 #-}
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Base
main = print $ runST
(do a <- unsafeNewArray_ (1,2000000)
let init i | i >= 2000000 = return ()
| otherwise = unsafeWrite a i 137 >> init
(i+1)
init 0
go a 2000000 0.0 )
go :: STUArray s Int Double -> Int -> Double -> ST s Double
go !a i !acc
| i < 1 = return acc
| otherwise = do
b <- unsafeRead a i
unsafeWrite a i (b+3.0)
c <- unsafeRead a i
go a (i-1) (c+acc)
$ ghc -O2 --make B.hs -fvia-C -optc-O3 -optc-msse4
[1 of 1] Compiling Main ( B.hs, B.o )
Linking B ...
$ time ./B
2.79999863e8
./B 0.01s user 0.02s system 108% cpu 0.031 total
}}}
Recommendation: custom newArray_ methods in the Data.Array.Base module
that fill directly with a tail recursive loop.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3586#comment:5>
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