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

[GHC]

#13656: Consider implementing atomicModifyMutVar# in Haskell
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        Reporter:  dfeuer            |                Owner:  dfeuer
            Type:  task              |               Status:  closed
        Priority:  low               |            Milestone:  8.4.1
       Component:  Runtime System    |              Version:  8.2.1-rc2
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by dfeuer):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 fryguybob first explained that the implementation above will allocate like
 mad. I countered with

 {{{#!hs
 atomicModifyMutVar# :: s ~ RealWorld => MutVar# s a -> (a -> (a,b)) ->
 State# s -> (# State# s, b #)
 atomicModifyMutVar# mv f s =
   case readMutVar# mv s of { (# s', old #) ->
   case newMutVar# old s' of { (# s'', tempy #) ->
     let thunk = case runRW# $ \ss -> readMutVar# tempy ss of
                   (# _, now #) -> f now
     in go# tempy mv (fst thunk) (snd thunk) s''}}

 go# :: s ~ RealWorld => MutVar# s a -> MutVar# s a -> a -> b -> State# s
 -> (# State# s, b #)
 go# tempy mv thunka thunkb s =
   case readMutVar# mv s of { (# s', old #) ->
   case writeMutVar# tempy old s of { s'' ->
   case casMutVar# mv old thunka s'' of { (# s''', unchanged, new #) ->
   case unchanged of
     0# -> (# s''', thunkb #)
     _ -> go# tempy mv thunka thunkb s''' }}}
 }}}

 fryguybob then explained that the cmm implementation is able to avoid
 write barrier inefficiencies that we can't in Haskell.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13656#comment:2>
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