[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:  new
        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):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Omer, might you look at this.  With `-ddump-stg` I see
 {{{
 Main.cmpT :: forall a1 a2. a1 -> a2 -> GHC.Types.Ordering
 [GblId, Arity=2, Caf=NoCafRefs, Str=<S,U><S,U>, Unf=OtherCon []] =
     [] \r [a2_s3tf b_s3tg]
         case
             case a2_s3tf of sat_s3th [Occ=Once] {
               __DEFAULT -> dataToTag# [sat_s3th];
             }
         of
         a'_s3ti
         { __DEFAULT ->
               case
                   case b_s3tg of sat_s3tj [Occ=Once] {
                     __DEFAULT -> dataToTag# [sat_s3tj];
                   }
               of
               b'_s3tk
               { __DEFAULT ->
                     case <# [a'_s3ti b'_s3tk] of {
                       __DEFAULT ->
                           case ==# [a'_s3ti b'_s3tk] of {
                             __DEFAULT -> GHC.Types.GT [];
                             1# -> GHC.Types.EQ [];
                           };
                       1# -> GHC.Types.LT [];
                     };
               };
         };
 }}}
 which looks right.  In another variant (I made `dataToTag#` lazy) I saw
 {{{
 Main.cmpT :: forall a1 a2. a1 -> a2 -> GHC.Types.Ordering
 [GblId,
  Arity=2,
  Caf=NoCafRefs,
  Str=<S,1*U><S,1*U>,
  Unf=OtherCon []] =
     [] \r [a2_s3tf b_s3tg]
         case a2_s3tf of x1_s3th [Occ=Once] {
           __DEFAULT ->
               case dataToTag# [x1_s3th] of a'_s3ti {
                 __DEFAULT ->
                     case b_s3tg of x2_s3tj [Occ=Once] {
                       __DEFAULT ->
                           case dataToTag# [x2_s3tj] of b'_s3tk {
                             __DEFAULT ->
                                 case <# [a'_s3ti b'_s3tk] of {
                                   __DEFAULT ->
                                       case ==# [a'_s3ti b'_s3tk] of {
                                         __DEFAULT -> GHC.Types.GT [];
                                         1# -> GHC.Types.EQ [];
                                       };
                                   1# -> GHC.Types.LT [];
                                 };
                           };
                     };
               };
         };

 }}}
 But both stubbornly return `LT` instead of `EQ`.  This must be a code-gen
 or RTS issue.  I have not looked at the Cmm.  Might you do so?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler