[GHC] #2159: Use a more efficient representation than [DynFlag]

[GHC]

#2159: Use a more efficient representation than [DynFlag]
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        Reporter:  igloo             |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  6.8.2
      Resolution:  fixed             |             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 rwbarton):

 According to a statistical profiling run that I did some time ago GHC
 spends about 0.3% of its total time in the module
 `libraries/containers/Data/IntSet/Base.hs`. I don't know how much of that
 0.3% is from `DynFlag`-checking versus other users of `IntSet` (another
 one is `UnVarSet`, whatever that is).

 Unfortunately, reliably measuring a speedup that is expected to be at most
 ~0.25% is itself a somewhat nontrivial project.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2159#comment:24>
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