[GHC] #13566: Bigger core size in ghc8 compared to ghc7

[GHC]

#13566: Bigger core size in ghc8 compared to ghc7
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        Reporter:  pacak             |                Owner:  (none)
            Type:  bug               |               Status:  infoneeded
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      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 pacak):

 > I'm a bit lost here. Am I to understand from comment:7 that this was an
 issue in GHC 8.0 that is fixed in 8.2.1-rc1?

 It can probably be closed for now. The problem is this sample is very
 fragile - for example if I remove AdditiveGroup instance which is not used
 at all or even specialization pragma - it suddenly works as expected in
 ghc 8.0. There are more code using generics in our project. If I find a
 way to reproduce it for ghc 8.2 - I can reopen it again with new data.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13566#comment:9>
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