[GHC] #13656: Consider implementing atomicModifyMutVar# in Haskell

[GHC]

#13656: Consider implementing atomicModifyMutVar# in Haskell
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  low            |         Milestone:  8.4.1
          Component:  Runtime        |           Version:  8.2.1-rc2
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I believe we could implement `atomicModifyMutVar#` in Haskell, rather than
 making it a primop. This seems like a simpler/cleaner approach (among
 other things, it gets a perfectly legitimate type signature!), but we'd
 have to see how it affects performance. I think something like this should
 work:

 {{{#!hs
 atomicModifyMutVar# :: MutVar# s a -> (a -> (a,b)) -> State# s -> (#
 State# s, b #)
 atomicModifyMutVar# mv f s =
   case readMutVar# mv s of { (# s', old #) ->
   let f_old = f old in
   case casMutVar# mv old (fst $ f_old) s of { (# s', unchanged, new #) ->
   case unchanged of
     0# -> (# s', snd f_old #)
     _ -> atomicModifyMutVar# mv f s' }}
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13656>
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