[GHC] #9291: Don't reconstruct sum types if the type subtly changes

[GHC]

#9291: Don't reconstruct sum types if the type subtly changes
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        Reporter:  schyler           |                Owner:  nomeata
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2871
       Wiki Page:                    |
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Comment (by Joachim Breitner <mail@…>):

 In [changeset:"19d5c7312bf0ad9ae764168132aecf3696d5410b/ghc" 19d5c73/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="19d5c7312bf0ad9ae764168132aecf3696d5410b"
 Add a CSE pass to Stg (#9291)

 This CSE pass only targets data constructor applications. This is
 probably the best we can do, as function calls and primitive operations
 might have side-effects.

 Introduces the flag -fstg-cse, enabled by default with -O for now. It
 might also be a good candiate for -O2.

 Differential Revision: https://phabricator.haskell.org/D2871
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:42>
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