[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:  new
        Priority:  highest           |            Milestone:  8.6.1
       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):
       Wiki Page:                    |
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Comment (by osa1):

 Ah, so it turns out we have a special case in CorePrep (which runs after
 simplifications) about `dataToTag#`, and we generate a case expression
 around its argument after all the simplifications. It's explained in `Note
 [dataToTag magic]` in CorePrep, and I can see in STG that it works as
 expected:

 {{{
 lvl_r6ru = \u [] f_r6rt ();

 lvl1_r6rv = CCS_DONT_CARE :! [lvl_r6ru []];

 main2 =
     \u []
         case
             case lvl_r6ru of sat_s6xD { __DEFAULT -> dataToTag#
 [sat_s6xD]; }
         of
         a#_s6xE
         { __DEFAULT ->
               case
                   case lvl_r6ru of sat_s6xF { __DEFAULT -> dataToTag#
 [sat_s6xF]; }
               of
               ...
 }}}

 So perhaps this is not because we get tag of a thunk. I don't know why not
 inlining `getTag` fixes this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:4>
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