[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:                    |
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Comment (by dfeuer):

 Ah... I am not entirely wrong, but I am partly wrong, because types with
 many constructors get tagged 1 when evaluated. I think we should at least
 be able to add a rule that recognizes when `dataToTag#` is used with a
 type with only a few constructors: see the logic in the `cgAlts gc_plan
 bndr (AlgAlt tycon) alts` case in `compiler/codeGen/StgCmmExpr.hs`. That
 way we only have to call `getConstrTag` when

 1. The type is unknown,
 2. The type has too many constructors, or
 3. The tag bits are 0.

 I imagine you can do this quite easily by copying a bit of the
 `splitTyConApp_maybe` logic from `tagToEnumRule` to `dataToTagRule` in
 `PrelRules.hs` and adding a `DataToTagUsingTagOp` to supplement
 `DataToTagOp`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:26>
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