[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 looks like much of the trouble is that we are unpacking `OccNames` in
 `isBuiltInOcc_maybe`, which gets called in `LoadIface.loadDecl` by
 `IfaceEnv.lookupOrig`. This results in a great deal of allocations by
 `utf8DecodeString`, which eagerly decodes the entire string buffer into a
 `[Char]`. This eager behavior seems a bit silly. Unfortunately, I made a
 few quick attempts at making this decoding lazy but sadly it typically
 hurt allocations more than it helped due to thunk allocations

 It also seems a bit silly that we inspect the name itself to determine
 whether it is built-in. Afterall, there is a finite universe of built-in
 `OccName`s and we already know the `FastString` hash of the `OccName` we
 are testing, so it seems like this really should just be a `UniqFM`
 lookup.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12357#comment:9>
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