[GHC] #8345: A more efficient atomicModifyIORef'

[GHC]

#8345: A more efficient atomicModifyIORef'
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              Reporter:  parcs       |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:              |          Version:  7.6.3
  libraries/base                     |         Keywords:  IORef
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Moderate (less
  Unknown/Multiple                   |  than a day)
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:  #5926
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 The big difference I see between the original and hvr's version is that
 the original installs a thunk in the `IORef` and then immediately forces
 it, whereas hvr's version forces it, and then installs it in the `IORef`.
 I don't think this difference can be observed, but I'm no expert. I think
 parcs's modification is just a milder step on the way to hvr's, which
 looks likely to be the simplest/best, but that would take benchmarking.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8345#comment:7>
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