[GHC] #12357: Increasing maximum constraint tuple size significantly blows up compiler allocations

[GHC]

#12357: Increasing maximum constraint tuple size significantly blows up compiler
allocations
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
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):

 It appears that much of the remaining difference is due to the
 construction of the superclass selector `OccName`s, which looks like this,
 {{{#!hs
 mkSuperDictSelOcc :: Int        -- ^ Index of superclass, e.g. 3
                   -> OccName    -- ^ Class, e.g. @Ord@
                   -> OccName    -- ^ Derived 'Occname', e.g. @$p3Ord@
 mkSuperDictSelOcc index cls_tc_occ
   = mk_deriv varName "$p" (show index ++ occNameString cls_tc_occ)
 }}}

 This is rather inefficient as we first decode the class's `OccName`, then
 reencode it. After fixing `concatFS` with Phab:D2384 and redefining this
 helper as
 {{{#!hs
 mkSuperDictSelOcc index cls_tc_occ
   = mkOccNameFS varName $ concatFS [fsLit "$p", fsLit $ show index,
 occNameFS cls_tc_occ]
 }}}
 I find that allocations return to 96MBytes, which is very close to the
 allocations prior to the superclass constraint patch. I suspect that this
 rework should be performed to all of the helpers in `OccName`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12357#comment:12>
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