[GHC] #15519: Minor code refactoring leads to drastic performance degradation

[GHC]

#15519: Minor code refactoring leads to drastic performance degradation
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        Reporter:  danilo2           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  SpecConstr
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 To summarise: once we have done #15578, we'll get good perf for
 {{{
 testGrammar1 = <blah>
 {-# INLINE testGrammar1 #-}

 test3 = oneShot (runTokenParser testGrammar1)
 }}}
 * The `INLINE` ensures that `testGrammar1` is inlined even if it is used
 in many places.

 * The `oneShot` is, I'm afraid, still necessary.  (You can import it from
 `GHC.Exts`.)  It says that GHC doesn't need to worry about sharing the
 work of `testGrammar1` between, say, two calls `(test3 src1)` and `(test3
 src2)`.

 You should get reliably good perf with these changes.  OK?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15519#comment:24>
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