[GHC] #15975: semantics of concurrent program depends on -O level, -f[no-]omit-yields

[GHC]

#15975: semantics of concurrent program depends on -O level, -f[no-]omit-yields
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           Reporter:  j.waldmann     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  Research needed
          Component:  Runtime        |           Version:  8.6.2
  System                             |
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Incorrect result
  (amd64)                            |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I am surprised by the behaviour of the program below
 (the interesting property is whether it will output "foo").

 Behaviours (plural) actually: it seems to depend
 on optimisation level, on omit-yields,
 and on very small changes in the source code:

 It does print (mostly), when compiled with -O0.
 It does not, when compiled with -O2.
 With -O2 -fno-omit-yields, it will print.
 With -O0 -fno-omit-yields, and when I remove the two newTVar
 in the beginning, it will mostly not print.

 How come?

 These differences already occur with the
 last two lines replaced by "forever $ return ()",
 so the STM stuff may be inessential here.
 But that's the context where I came across the problem.


 See also discussion at https://mail.haskell.org/pipermail/haskell-
 cafe/2018-November/130274.html


 {{{
 import Control.Concurrent.STM
 import Control.Concurrent ( forkIO )
 import Control.Monad ( forever )
 import System.IO

 main = do

   atomically $ newTVar "bar"
   atomically $ newTVar False

   forkIO $ putStrLn "foo"

   x <- atomically $ newTVar False
   forever $ atomically $ writeTVar x True
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15975>
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