[GHC] #10148: Optimization causes repeated computation

[GHC]

#10148: Optimization causes repeated computation
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        Reporter:  akio              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1-rc1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by nomeata):

 I tried to further minimize the code. Note that `undefined i` is required;
 if does not exhibit the bug with just `undefined`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10148#comment:5>
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