Re: [GHC] #15696: Derived Ord instance for enumerations with more than 8 elements seems to be incorrect

11 Dec 2018

      #15696: Derived Ord instance for enumerations with more than 8 elements seems to be
incorrect
-------------------------------------+-------------------------------------
        Reporter:  mrkkrp            |                Owner:  osa1
            Type:  bug               |               Status:  closed
        Priority:  highest           |            Milestone:  8.6.2
       Component:  Compiler          |              Version:  8.6.1
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Incorrect result  |            Test Case:
  at runtime                         |  codeGen/should_run/T15696_1,
                                     |  T15696_2, T15696_3
      Blocked By:                    |             Blocking:
 Related Tickets:  #14677, #15155    |  Differential Rev(s):  Phab:D5196,
       Wiki Page:                    |  Phab:D5201, Phab:D5226
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"d549c081f19925dd0e4c70d45bded0497c649d49/ghc" d549c08/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d549c081f19925dd0e4c70d45bded0497c649d49"
 dmdAnal: Move handling of datacon strictness to mkWWstr_one

 Previously datacon strictness was accounted for when we demand analysed a
 case
 analysis. However, this results in pessimistic demands in some cases. For
 instance, consider the program (from T10482)

     data family Bar a
     data instance Bar (a, b) = BarPair !(Bar a) !(Bar b)
     newtype instance Bar Int = Bar Int

     foo :: Bar ((Int, Int), Int) -> Int -> Int
     foo f k =
       case f of
         BarPair x y -> case burble of
                           True -> case x of
                                     BarPair p q -> ...
                           False -> ...

 We really should be able to assume that `p` is already evaluated since it
 came
 from a strict field of BarPair.

 However, as written the demand analyser can not conclude this since we may
 end
 up in the False branch of the case on `burble` (which places no demand on
 `x`).
 By accounting for the data con strictness later, applied to the demand of
 the
 RHS, we get the strict demand signature we want.

 See Note [Add demands for strict constructors] for a more comprehensive
 discussion.

 Test Plan: Validate

 Reviewers: simonpj, osa1, goldfire

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15696

 Differential Revision: https://phabricator.haskell.org/D5226
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:91>
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