[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 osa1):

 Right, fixing this in Cmm is also what I suggested in comment:24. I think
 two key questions are:

 - Can we optimise this? That is, can we know for certain that the argument
 of a `dataToTag#` points directly to a value (without going through
 blackholes or other indirections like IND_STATIC)?
 - Is this worth optimising?

 Unless the answer to both of those is yes, then I think it makes sense to
 do this in Cmm (when compiling the primop). Then we can remove a bunch of
 notes about `getTag` and hacks in CorePrep, and simplify `getTag` to
 `getTag = dataToTag#`. I think we can also update the primop as `can_fail
 = False`.

 It seems to me that Core is not the right place to answer (1). Even in STG
 I don't know if we can answer that question with certainty. I think it's
 doable in Cmm where we know about "lambda form" of ids (`LambdaFormInfo`)
 which tells us about how to enter an object. If "lambda form" of an
 argument is `LFCon` then we optimise, otherwise we enter.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:38>
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