[GHC] #15493: Elide empty dictionaries

[GHC]

#15493: Elide empty dictionaries
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        Reporter:  mnoonan           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 In Core, an empty dictionary turns into a dead variable, which is
 optimised away (with `-O` anyway).

 But it'll still take up a field in `FPack1`.  Reason: it might be bottom.

 I'd be surprised if you found any appreciable performance impact.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15493#comment:4>
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