id,summary,reporter,owner,description,type,status,priority,milestone,version,resolution,keywords,cc
20,Optimization strategy unclear (-Odph / -O2),choener,,"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
}}}
",defect,closed,major,0.7,0.6,wontfix,"documentation, optimization",alexey.skladnoy@…