[GHC] #12070: SMP primitives broken on power(pc)

[GHC]

#12070: SMP primitives broken on power(pc)
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  trommler
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Runtime System    |              Version:  8.0.1-rc4
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:  powerpc
 Type of failure:  None/Unknown      |            Test Case:
                                     |  concurrent/prog001/concprog001
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2225
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by trommler):

 * status:  new => patch
 * differential:   => Phab:D2225


Comment:

 Validated on powerpc64 and AMD64.

 Please, someone check that it does not break on ARM. The description for
 the built_in used in `xchg` suggests that the value stored in memory is
 implementation dependent.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12070#comment:5>
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