[GHC] #13424: readMutVar# is marked can_fail

[GHC]

#13424: readMutVar# is marked can_fail
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 `readMutVar#` can only fail if its argument is not a valid pointer. As far
 as I know, we never check whether a pointer is valid; we ensure it's valid
 by construction. So I think we should set `can_fail = False`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13424>
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