[GHC] #15364: Replace the atomicModifyMutVar# primop

[GHC]

#15364: Replace the atomicModifyMutVar# primop
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Runtime System    |              Version:  8.5
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4884
       Wiki Page:                    |
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Comment (by dfeuer):

 We now have the first two primops. We'll need to add `atomicSwapMutVar#`
 in a separate commit. We don't yet support `xchg` in CMM, so we'll have to
 add it. I noticed, however, that we do have at least some support for it
 in the LLVM backend, at least (it's a case of read-modify-write).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15364#comment:3>
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