[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):

 Ah, interesting. The problem is not `mkIdDot2` (that has Arity 1
 allright), the problem is `unIdDot2`, which suddenly as artiy 2!
 {{{
 T7542.unIdDot2
   :: forall a_az8 b_az9. (a_az8 -> T7542.Id b_az9) -> a_az8 -> b_az9
 [LclIdX,
  Arity=2,
  Str=DmdType,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=2, Value=True,
          ConLike=True, WorkFree=True, Expandable=True,
          Guidance=IF_ARGS [20 0] 50 0}]
 T7542.unIdDot2 =
   \ (@ a_aKU)
     (@ b_aKV)
     (f_aze :: a_aKU -> T7542.Id b_aKV)
     (eta_B1 :: a_aKU) ->
     case f_aze of f_Xzl { __DEFAULT ->
     T7542.unId @ b_aKV (f_Xzl eta_B1)
     }
 }}}
 (This is already after `InitialPhase [Gentle]`). One would expect `MkId`
 and `unId` to behave the same...

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7542#comment:14>
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