[GHC] #12234: 'deriving Eq' on recursive datatype makes ghc eat a lot of CPU and RAM

[GHC]

#12234: 'deriving Eq' on recursive datatype makes ghc eat a lot of CPU and RAM
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        Reporter:  slyfox            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  deriving-perf
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by niteria):

 Shot in the dark: does solving coercibility for recursive newtypes involve
 finding fixpoints?

 I've run into one place in the code generator once where not returning the
 elements of a set in the Unique order would make it never find a fixpoint,
 because the insertion order would oscillate between 2 values.

 Otherwise, maybe using Unique order for SCC had some nice properties in
 this case.

 But under normal circumstances, I wouldn't expect my commit to cause that.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12234#comment:9>
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