[GHC] #7923: Optimization for takeMVar/putMVar when MVar left empty

[GHC]

#7923: Optimization for takeMVar/putMVar when MVar left empty
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    Reporter:  ezyang            |       Owner:  ezyang                 
        Type:  task              |      Status:  patch                  
    Priority:  normal            |   Milestone:                         
   Component:  Runtime System    |     Version:  7.7                    
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
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Comment(by igloo):

 Shouldn't this hunk:
 {{{
 -        unlockClosure(mvar, stg_MVAR_DIRTY_info);
 +        unlockClosure(mvar, info);
 +        // escape without dirty_MVar!
 }}}
 use `stg_MVAR_CLEAN_info` instead of `info`? And likewise in
 `stg_tryTakeMVarzh`.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7923#comment:2>
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