[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 rwbarton):

 Simon: `MO_Add2` is translated to an `ADD` (which is expected to either
 set or clear the carry flag as appropriate) followed by an `ADC`. Printing
 the `ADD` as (in this case) `dec` rather than `add` loses the effect on
 the carry flag, so the subsequent `ADC` will just use whatever garbage
 happens to be in the carry flag.

 Like Simon, I'm not totally thrilled with the organization of the code
 here, but one simple fix would be to add an `ADD'` instruction which means
 the same as `ADD` except it indicates that the side effect of setting the
 carry flag is important. Then the pretty printer can produce `dec` for an
 `ADD` of `-1` but not for an `ADD'` of `-1`. GHC doesn't expose the carry
 flag to Haskell directly so we would only need to use `ADD'` in the
 translation of `MO_Add2` and other similar operations.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9013#comment:6>
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