[GHC] #12367: Commit adding instances to GHC.Generics regression compiler performance

[GHC]

#12367: Commit adding instances to GHC.Generics regression compiler performance
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 So it appears that something is indeed calling for the `V1` dictionary and
 is therefore forcing the load of the `GHC.Generics` interface file,
 {{{
 ...
 end stage interact with inerts }
 runStage top-level reactions {
   workitem   =  [W] $dFoldable_a2pp :: Foldable [] (CDictCan)
 doTopReact [W] $dFoldable_a2pp :: Foldable [] (CDictCan)
 matchClassInst pred = Foldable []
 Starting fork { Dict fun $fFoldable[]
 ...
 } ending fork Dict fun $fFoldable[]
 Starting fork { Dict fun $fFoldableV1
 Starting fork { Declaration for $fFoldableV1
 Loading decl for $fFoldableV1
 updating EPS_
 Need decl for V1
 Considering whether to load GHC.Generics {- SYSTEM -}
 Reading interface for GHC.Generics; reason: Need decl for V1
 ...
 }}}

 Now to work out what.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12367#comment:12>
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