[GHC] #15696: Derived Ord instance for enumerations with more than 8 elements seems to be incorrect

[GHC]

#15696: Derived Ord instance for enumerations with more than 8 elements seems to be
incorrect
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        Reporter:  mrkkrp            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.6.2
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5196,
       Wiki Page:                    |  Phab:D5201
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Comment (by osa1):

 Here's my (hopefully final) proposal:

 (1) is best done in Core. I'll remove the relevant code from Phab:D5201.
 For cases that should be optimised but currently aren't, we can open a new
 ticket and track the progress there.

 (2) is best avoided. It opens a can of worms, and it's what caused this
 ticket.

 (3) Should be done in Cmm. Phab:D5201 already does this, but we need to do
 some refactoring to be able to reliably check whether the argument type is
 a large or small family (to decide whether to check tag bits of read the
 info table).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:45>
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