[vector] #20: Optimization strategy unclear (-Odph / -O2)

[vector]

#20: Optimization strategy unclear (-Odph / -O2)
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Reporter:  choener  |       Owner:                             
    Type:  defect   |      Status:  new                        
Priority:  minor    |   Milestone:  0.7                        
 Version:  0.6      |    Keywords:  documentation, optimization
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 The vector tutorial states that -Odph should be used. On ghc 6.12.1 this
 leads to abysmal performance with some functions. Simple program attached.
 n=20000 has 60s (dph) vs. 5s (O2).

 "g n" is for comparison only and requires 12s (O2) if you are interested.
 -fno-method-sharing is required for good vector performance (otherwise:
 17s).

 {{{
 {-# OPTIONS_GHC -fno-method-sharing #-}
 module Main where

 import qualified Data.Vector.Unboxed as V
 import System.Environment (getArgs)

 f :: Int -> Int
 f n = V.sum $ V.concatMap (\k -> V.enumFromN 1 k) $ V.enumFromN 1 n

 g :: Int -> Int
 g n = sum $ concatMap (\k -> enumFromTo 1 k) $ enumFromTo 1 n

 main = do
   (a:_) <- getArgs
   let n = read a :: Int
   print $ f n
 }}}

-- 
Ticket URL: <http://trac.haskell.org/vector/ticket/20>
vector <http://trac.haskell.org/vector>
Package vector