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

[GHC]

#2159: Use a more efficient representation than [DynFlag]
---------------------------------------------+------------------------------
    Reporter:  igloo                         |       Owner:             
        Type:  compile-time performance bug  |      Status:  new        
    Priority:  normal                        |   Milestone:  6.10 branch
   Component:  Compiler                      |     Version:  6.8.2      
    Severity:  normal                        |    Keywords:             
  Difficulty:  Unknown                       |    Testcase:             
Architecture:  Unknown                       |          Os:  Unknown    
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 In `compiler/typecheck/TcRnMonad.lhs`, if I replace the existing
 definitions with
 {{{
 traceOptIf _ _ = return ()
 traceOptTcRn _ _ = return ()
 dumpTcRn _ = return ()
 }}}
 then we see an improvement in nofib's "Compile Times" sections:
 {{{
 -1 s.d.         -----   -8.5%
 +1 s.d.         -----   +1.8%
 Average         -----   -3.5%
 }}}
 By using some sort of bitmap representation, we ought to see a modest
 compile time improvement from all the checks that the trace functions
 make, as well as various other checks that are made (do we need to warn
 about missing sigs, are overlapping instances allowed, etc).

 The tricky bit is designing it so that it is hard to shoot yourself in the
 foot. `Lexer.x` already has a bitmap (`genericsBit`, `ffiBit` etc) but the
 mapping of feature to bit number is done by hand, which isn't ideal.
 Perhaps this is even important enough to deserve a language extension?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2159>
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