[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
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 Ah, I see what you're saying. The 0 tag is handled by the `case` wrapper
 (or C-- equivalent). So for small types, we don't need any extra test; we
 can just use the tag bits directly. I suppose also that my attempt to use
 tag bits for unknown types is more trouble than it's worth; we should just
 assume that those have too many constructors.

 As for the type-driven rewriting, I think we basically have two options:

 1. Do it in PrelRules. I think this requires something like the
 `dataToTagWith#` primop I mentioned earlier.

 2. Do it sometime later (tidy core, core prep, or lowering to STG). This
 lets us stick with `dataToTag#` throughout core2core, which is nice, but I
 don't know enough to know if any of those make sense or where/how to slot
 in the rule.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:42>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler