Strange performance effects with unsafePerformIO

[Björn Peemöller]

Hello,

we have a strange performance behaviour when we use unsafePerformIO, at
least with GHC 6.12.3 and 7.0.1.

Please consider the example program following at the end of this post.
Running the original code the execution time is about 26 seconds, while
uncommenting one (or both) of the comments shrinks it to about 0.01
seconds on our machine.

Is there an explanation for this effect?

Regards,
Bjoern

-- ---------------

module Main where

import System.IO.Unsafe

traverse []     = return ()
-- traverse (_:xs) = traverse xs
traverse (_:xs) = traverse xs >> return ()

makeList 0 = []
-- makeList n = () : (makeList (n - 1))
makeList n = () : (unsafePerformIO . return) (makeList (n - 1))

main = traverse $ makeList (10^5)