[GHC] #5536: IO performance regression in 7.2 vs 7.0

[GHC]

#5536: IO performance regression in 7.2 vs 7.0
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    Reporter:  simonmar          |        Owner:  batterseapower
        Type:  bug               |       Status:  new           
    Priority:  high              |    Milestone:  7.4.1         
   Component:  libraries/base    |      Version:  7.2.1         
    Keywords:                    |     Testcase:                
   Blockedby:                    |   Difficulty:                
          Os:  Unknown/Multiple  |     Blocking:                
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown  
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Comment(by batterseapower):

 I've pushed those changes as 60907a3. The fact that it runs in ~1.75s vs
 1.45s still indicates a 20% perf penalty though.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5536#comment:5>
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