[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:  osa1
            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:  #14677, #15155    |  Differential Rev(s):  Phab:D5196,
       Wiki Page:                    |  Phab:D5201
-------------------------------------+-------------------------------------

Comment (by osa1):

 > That is indeed a mystery. Is the code you are showing the output of the
 simplifier?

 The code I showed is the input of `FloatIn` when a file is compiled with
 `-O`.
 The full command to reproduce (after building stage 1):

 {{{
 "inplace/bin/ghc-stage1" -hisuf hi -osuf  o -hcsuf hc -static  -O0 -H64m
 -Wall      -this-unit-id ghc-heap-8.7 -hide-all-packages -i -ilibraries
 /ghc-heap/. -ilibraries/ghc-heap/dist-install/build -Ilibraries/ghc-heap
 /dist-install/build -ilibraries/ghc-heap/dist-install/build/./autogen
 -Ilibraries/ghc-heap/dist-install/build/./autogen -Ilibraries/ghc-heap/.
 -optP-include -optPlibraries/ghc-heap/dist-
 install/build/./autogen/cabal_macros.h -package-id base-4.12.0.0 -package-
 id ghc-prim-0.5.3 -package-id rts -Wall -XHaskell2010 -XNoImplicitPrelude
 -O  -no-user-package-db -rtsopts  -Wno-deprecated-flags
 -Wnoncanonical-monad-instances  -odir libraries/ghc-heap/dist-
 install/build -hidir libraries/ghc-heap/dist-install/build -stubdir
 libraries/ghc-heap/dist-install/build   -dynamic-too -c libraries/ghc-
 heap/./GHC/Exts/Heap/ClosureTypes.hs -o libraries/ghc-heap/dist-
 install/build/GHC/Exts/Heap/ClosureTypes.o -dyno libraries/ghc-heap/dist-
 install/build/GHC/Exts/Heap/ClosureTypes.dyn_o
 }}}

 > Can you give instructions to reproduce (maybe make a wip/ branch)?

 Committed code to `wip/T15696`. Note that -ddump-simpl-iterations is not
 printing Core right before `FloatIn` so you need some extra prints to be
 able to trace this. I'm currently using
 [https://gist.githubusercontent.com/osa1/9755f7d7238a3757cfee0b37a183a6d8/raw/7d28a30632cf9d39f85fa4630c040fed80c52036/gistfile1.txt
 this] patch for that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:72>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler