[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:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rwbarton):

 In fact it's even more involved than I originally thought. `MO_Add2`
 invokes the code generator for `MO_Add` to do the first addition. `MO_Add`
 sees that the second argument is constant so it can produce an `LEA`
 instruction (which wouldn't set the carry flag). The pretty-printer
 notices that the source and destination registers of the `LEA` instruction
 are the same, so it rewrites it to an `ADD` (which would set the carry
 flag); and then it notices that the addend is -1, so it finally emits
 `dec` (which doesn't set the carry flag).

 All of what the code generator does for `MO_Add` is fine, since the
 semantics of `MO_Add` don't include any effect (or lack thereof) on the
 condition register. The error is in the code generation for `MO_Add2`: it
 should not use `MO_Add`, for this reason.

 I will put up a patch for review shortly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9013#comment:8>
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