[GHC] #7542: GHC doesn't optimize (strict) composition with id

[GHC]

#7542: GHC doesn't optimize (strict) composition with id
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        Reporter:  shachaf                  |            Owner:  simonpj
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  Compiler                 |          Version:  7.6.1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by nomeata):

 Tracing this back I found the `Note [Dealing with bottom]` in `CoreArity`,
 where the arity of case branches is used to calculate the arity of a case.
 Quote:

 > We ignore this "problem" (unless -fpedantic-bottoms is on), because
 being scrupulous would lose an important transformation for many programs.
 (See Trac #5587 for an example.)

 So the next step would be answering: Can we distinguish between cases
 where this dodgy optimization is useful and required, and cases where it
 hurts?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7542#comment:15>
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