[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:  new             
Priority:  normal                   |      Component:  Runtime System  
 Version:  7.7                      |       Keywords:                  
      Os:  Unknown/Multiple         |   Architecture:  Unknown/Multiple
 Failure:  Runtime performance bug  |      Blockedby:                  
Blocking:                           |        Related:                  
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 Right now, we always add an MVar to the mutable list when we
 takeMVar/putMVar. However, this is unnecessary when the MVar is left
 empty. This patch arranges that we don't add the MVar to the mutable list
 in those cases. I've validated the patch but haven't checked performance
 changes yet.

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