unexpected thread blocking

Hello,

Actually I am not sure if this is really a bug, but anyway the following 
program
behaves in different manners between with and without -O.
Interestingly, ghci provides the same (expected) behavior as -O.

\begin{code}

import Control.Concurrent
import Control.Concurrent.MVar
import IO(hPutStrLn, stderr)

main = do
          mvar <-newEmptyMVar          hPutStrLn stderr "forking..."
          thread <-forkIO (undef `seq` (putMVar mvar hoge))
          isEmpty <-isEmptyMVar mvar
          hPutStrLn stderr ("isEmpty = " ++ show isEmpty)

undef = undef
hoge  = "hoge"

\end{code}

Results:
 $ ghc testConcurrent.lhs -o tto.exe
 $ tto.exe
 forking...
 isEmpt<<<DOES NOT COME BACK>>>

 $ ghc testConcurrent.lhs -o tto.exe -O
 $ tto.exe
 forking...
 isEmpty = True

The difference also applies for more complicated programs.
Since both ghc-5.04-2 on WinXP and ghc-5.04-1 on Solaris 8 yield the same 
result,
it is quite likely that the problem is platform independent.

Thank you very much.

- Susumu

% BTW, what I want to do is something like
%   undef = length hoge
%   hoge  = somethingLazy -- result of auto-generated module that may not 
terminate.
% or I may well say that I need takeMVar with a timeout parameter.
% I would appreciate also if someone knows a good workaround other than 
giving -O/--interact.