[GHC] #7258: Compiling DynFlags is jolly slow

[GHC]

#7258: Compiling DynFlags is jolly slow
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        Reporter:  simonpj           |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.1
      Resolution:                    |             Keywords:  deriving-perf
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 tdammers):

 Replying to [comment:37 bgamari]:
 > While chatting with tdammers about this I had a peek at the core; it
 seems that 5000 terms of the 7000 terms in the simplified core of `W2` are
 in `creadPrec_rdaO`. Moreover, much of this is repetition. I've proposed
 an approach for dealing with this in #14364.

 I've just done a bit more profiling, and it turns out that removing the
 derived `Read` instance from the `W3` example (`W2` extended to 500
 fields) cuts overall compilation time down from 25.5 seconds to 3.6. So it
 seems that the edge case isn't just "constructors with many fields", but
 rather, "derived `Read` instances for constructors with many fields".

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7258#comment:40>
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