[GHC] #15019: Fix performance regressions from #14737

[GHC]

#15019: Fix performance regressions from #14737
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        Reporter:  tdammers          |                Owner:  tdammers
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14737            |  Differential Rev(s):  phab:D4568
       Wiki Page:                    |
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Description changed by simonpj:

Old description:

> #14737, while drastically improving performance on some edge cases,
> introduces performance regressions in 3 test cases:
>
> perf/compiler/parsing001.run
> perf/compiler/T9020.run
> perf/compiler/T12707.run

New description:

 #14737, while drastically improving performance on some edge cases,
 introduces performance regressions in 3 test cases:

 * perf/compiler/parsing001.run
 * perf/compiler/T9020.run
 * perf/compiler/T12707.run

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15019#comment:3>
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