[GHC] #13236: Improve floating for join points

[GHC]

#13236: Improve floating for join points
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by lukemaurer):

 Yes, that's similar to the example in `FloatOut` with nested loops---which
 came directly from list-fusion code (filter of filter of filter). Floating
 to make things trivial (as the Simplifier does) is less important than
 floating to share computation, but it can be significant. (And the
 Simplifier can't do the floating in this case because it doesn't realize
 that `j2` doesn't refer to `y`.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13236#comment:5>
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