[Haskell-cafe] In-place modification
Hugh Perkins
hughperkins at gmail.com
Fri Aug 10 01:35:48 EDT 2007
On 7/15/07, Donald Bruce Stewart <dons at cse.unsw.edu.au> wrote:
>
> Oh, and I forgot you count up by two now. Here's the Haskell
> transliteration (again).
>
>
> {-# OPTIONS -O2 -optc-O -fbang-patterns #-}
>
> import Control.Monad.ST
> import Data.Array.ST
> import Data.Array.Base
> import System
> import Control.Monad
> import Data.Bits
>
> main = print (pureSieve 10000000)
>
> pureSieve :: Int -> Int
> pureSieve n = runST( sieve n )
>
> sieve n = do
> a <- newArray (3,n) True :: ST s (STUArray s Int Bool)
> let cutoff = truncate (sqrt (fromIntegral n)) + 1
> go a n cutoff 3 1
>
> go !a !m cutoff !n !c
> | n >= m = return c
> | otherwise = do
> e <- unsafeRead a n
> if e then
> if n < cutoff
> then let loop !j
> | j < m = do
> x <- unsafeRead a j
> when x $ unsafeWrite a j False
> loop (j+n)
>
> | otherwise = go a m cutoff (n+2) (c+1)
>
> in loop ( if n < 46340 then n * n else n `shiftL`
> 1)
> else go a m cutoff (n+2) (c+1)
>
> else go a m cutoff (n+2) c
>
>
> Marginally faster:
>
> $ time ./primes
> 664579
> ./primes 0.34s user 0.00s system 89% cpu 0.385 total
>
> Very cache-dependent though, so widely varying runtimes could be
> expected.
>
> -- Don
>
Hi Donald, quick question. So, one of the things that is very interesting
about Haskell is it's potential for automatic threading, ie you write a
trivial algorithm that looks like it runs in a single thread, and the
runtime splits it across multiple cores automatically.
It's fairly safe to say that maps, foldrs, foldls, and their derivatives are
safe to parallelize? (For example, hand-waving argument, a foldr of (/) on
[1,5,7,435,46,2] can be split into a foldr on [1,5,7] and a foldr on
[435,46,2], then their results combined).
To what extent is the technology you are using in your algorithm
parallizable? (I actually cant tell, it's a genuine question). In the case
that it is parallelizable, to what extent is it trivial for a runtime to
know this? (Again, I dont have enough information to tell)
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