[GHC] #15717: Performance regression in for_ alternatives from GHC 8.2.2 to newer GHCs

[GHC]

#15717: Performance regression in for_ alternatives from GHC 8.2.2 to newer GHCs
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        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Doubling sounds bad.

 There are 9 tests in the repro case.  've lost track of what is what.  If
 someone was able to distil out a simple before-and-after on one test, that
 would be helpful.  Better still, get some insight into what is happening.
 Thanks!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15717#comment:3>
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