[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):

 I've put up a few attempts at reducing some of the costs associated with
 `FastString` operations and in particular `isBuiltInOcc_maybe`. The most
 promising so far is Phab:D2385, which reworks `isBuiltInOcc` to avoid
 unpacking the name being checked. It does this by moving the
 implementation away from `String` pattern matching and towards a
 `FastString` lookup in a `UniqFM`. Initial indications suggest this
 reduces allocations by around 5% when compiling
 `nofib/real/fulsom/Shape.hs`.

 Another thing I noticed while tracking this down is the behavior of
 `concatFS`, which unpacks the the `FastString`s being concatenated and
 then builds a new `FastString` from the resulting list. It seems like
 there might be slightly better constant factors to be had by simply
 concatenating the `ByteString`s themselves. I have tried reworking this in
 Phab:D2384, although I don't yet have solid numbers characterizing its
 effect.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12357#comment:10>
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