[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):

 Well the bit array was 4x faster than IntSet in my unscientific test. And
 it would probably be a bit faster still when the option to test is known
 and can be inlined. But there could be little left to gain after the
 switch to IntSet.

 A pretty easy experiment to do, if we had an easy way to benchmark
 compiler performance...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2159#comment:22>
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