[GHC] #11781: Improve common sub-expression

[GHC]

#11781: Improve common sub-expression
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2074
       Wiki Page:                    |
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Comment (by nomeata):

 >  Oh hang on. Maybe the program being compiled is changing size! Try
 -dshow-passes before and after.

 No, no diff there (of the program with `-v -ddump-cse`) besides
 insignificant changes in per-pass runtime and per-pass allocation. So it
 must have indeed made some code path that is overly exercised by this
 extreme program slower.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11781#comment:4>
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