[GHC] #7883: enable GHC LLVM backend to use LLVM provided CAS / Atomicity primitives?

[GHC]

#7883: enable GHC LLVM backend to use LLVM provided CAS / Atomicity primitives?
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        Reporter:  carter            |            Owner:  carter
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      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 rrnewton):

 Some notes on which operations the "atomic-primops" library is currently
 exposing, and the other concurrent data structure libs (e.g. chaselev-
 deque) are using:

  * CAS on MutVars, MutableArray#, and MutableByteArray#
  * fetch and add on MutableByteArray#
  * barriers / memory fences

 Drafts of .cmm for these can be found inside the atomic-primops library.
 Note that *only* casMutVar# is currently shipped with GHC.

 Ultimately there's no reason that we shouldn't aim for a fairly "complete
 set".  For example, why not have fetch-and-sub and the other "atomicrmw"
 variants?  Relating these to the LLVM atomics and memory orderings, they
 become:

  * CAS variants = LLVM cmpxchg with SequentiallyConsistent ordering
  * fetch-and-X variants = LLVM atomicrmw with SequentiallyConsistent
  * store_load_barrier = LLVM fenceInst with SequentiallyConsistent
  * write_barrier and load_load_barrier = I *think* these are both covered
 by a FenceInst with AcquireRelease ordering...

 I'm not an LLVM expert, so please double check these yourself before
 trusting them.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7883#comment:11>
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