weird performance drop with -O2 on x86

Here's my program:

import Control.Concurrent
import Data.IORef

maker :: IORef Int -> IO ()
maker v = loop
    where
    loop = do
        x <- readIORef v
        writeIORef v $! x + 1
        forkIO (return ())
        loop

main :: IO ()
main = do
    v <- newIORef 0
    t <- forkIO (maker v)
    threadDelay 1000000
    killThread t
    x <- readIORef v
    print x

It's supposed to print the number of threads created in one second. With ghc -O2, I get around 61104; similarly for -O1. However, with no optimization I get results around 612274, i.e. approximately ten times more threads in the same time. What's going on here?

More data points:

1. 6.1 behaves similarly but the numbers are a bit higher (~10% more iterations).

<dons> be sure to mention that results appear normal on amd64.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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