[GHC] #10639: Optimization changes concurrent program's behaviour

[GHC]

#10639: Optimization changes concurrent program's behaviour
-------------------------------------+-------------------------------------
        Reporter:  gizmo.mk0         |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Windows           |            Architecture:  x86_64
 Type of failure:  Incorrect result  |  (amd64)
  at runtime                         |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by gizmo.mk0:

Old description:

> Here is a program that spawns a thread from the main thread, which tries
> to constantly write out a message to the console.
>

> {{{#!hs
> module Main where
>
> import Control.Concurrent (forkIO)
>
> main :: IO ()
> main = do
>   _ <- forkIO $ runForever $ putStrLn "Hey"
>   runForever $ return ()
>
> runForever :: IO () -> IO ()
> runForever action = action >> runForever action
> }}}
>
> If you compile it with 'ghc main', it works correctly - it prints out the
> message continuously, and you can terminate it by pressing Ctrl-C.
> However, if you compile it with 'ghc -O main' (or -O2, or -O3...), it
> doesn't print out anything, and the only way to exit is to kill the
> process from Task Manager.
>
> This was reproducable with GHC 7.10.1, on a Windows 7 x64 machine, with
> an AMD A4-5300 APU.
>
> ''(Disclaimer: this is my first bugreport, and I'm not sure what else can
> I do to investigate this issue.)''

New description:

 Here is a program that spawns a thread from the main thread, which tries
 to constantly write out a message to the console.


 {{{#!hs
 module Main where

 import Control.Concurrent (forkIO)

 main :: IO ()
 main = do
   _ <- forkIO $ runForever $ putStrLn "Hey"
   runForever $ return ()

 runForever :: IO () -> IO ()
 runForever action = action >> runForever action
 }}}

 If you compile it with 'ghc main', it works correctly - it prints out the
 message continuously, and you can terminate it by pressing Ctrl-C.
 However, if you compile it with 'ghc -O main' (or -O2, or -O3...), it
 doesn't print out anything, and the only way to exit is to kill the
 process from Task Manager.

 This was reproducable with GHC 7.10.1, on a Windows 7 x64 machine, with an
 AMD A4-5300 APU.

 ''EDIT: As it turns out, using "yield" instead of "return ()" solves the
 problem. It seems I misunderstood how forkIO works. However, I'm not sure
 if the current working is intentional or not, so I think I should leave
 this ticket open - just to be on the safe side.''

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10639#comment:1>
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