[GHC] #13916: Optimizations create run time seg faults

[GHC]

#13916: Optimizations create run time seg faults
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        Reporter:  newthin           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
  (CodeGen)                          |
      Resolution:                    |             Keywords:  optimization
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 The code you point to implements `MVar`s. The example code doesn't seem to
 handle any of those directly. The only `MVar`s I see in the vicinity are
 in `async`, and there don't seem to be any arrays around there to index
 improperly. Specifically, `forConcurrently_` (used in `main.hs`) is
 ultimately defined in terms of `concurrently`, which uses an `MVar`. But
 that doesn't bring any potentially-dangerous vector indexes close enough
 to make those look like good candidates for the problem.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13916#comment:4>
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