[GHC] #9646: Simplifer non-determinism leading to 8 fold difference in run time performance

[GHC]

#9646: Simplifer non-determinism leading to 8 fold difference in run time
performance
-------------------------------------+-------------------------------------
              Reporter:  erikd       |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:  Linux       |     Architecture:  x86_64 (amd64)
       Type of failure:  Runtime     |       Difficulty:  Unknown
  performance bug                    |       Blocked By:
             Test Case:              |  Related Tickets:
              Blocking:              |
Differential Revisions:              |
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Comment (by erikd):

 ​The repo at https://github.com/erikd/ghc-perfbug-test has been stripped
 back as far as possible and no longer needs either criterion or hspec.

 The code has been tested with ghc-7.8.3 and ghc-7.9.20140929 and both
 display the same problem.

 Cloning the repo, cd-ing into it and running `make` will build the code
 and then print out:

 {{{
     Fast version etas : 0
     Slow version etas : 18
 }}}

 which is simply the count of the occurence of the identifier `eta` in the
 twp -ddump-simpl outputs generated from the same file.

 When this bug is fixed, both values should be zero.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9646#comment:12>
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