[GHC] #9013: plusWord2# is buggy

[GHC]

#9013: plusWord2# is buggy
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        Reporter:  pumpkin           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by pumpkin):

 Reid Barton tracked down the culprit on IRC:
 https://github.com/ghc/ghc/blob/master/compiler/nativeGen/X86/Ppr.hs#L553

 So it looks like:
 1. it's not specific to `plusWord2#`
 2. it only happens on compile-time literals (so is admittedly pretty rare)
 3. it's not always sound to switch an `add` to a `dec` because the latter
 doesn't affect the carry flag and the former does

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9013#comment:3>
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