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

15 Jul 2015

      #10639: Optimization changes concurrent program's behaviour
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        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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