[GHC] #8972: Investigate adding fast compare-and-swap Int type/primops

[GHC]

#8972: Investigate adding fast compare-and-swap Int type/primops
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       Reporter:  tibbe             |             Owner:  tibbe
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.9
       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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 I've received reports that using `IORef Int` and `atomicModifyIORef` to
 implement an atomic counter in the ekg package has become a bottleneck for
 some of its users. These users update the counter thousands of times per
 second, using multiple threads.

 I will investigate whether adding a dedicated atomic `Int` reference type
 will offer significant speed improvements. Such a type can also be used to
 implement cheaper locks (by using bits in the int to represent different
 lock states, such as reader/write locks.)

 Lets call this new type `AtomicIntRef` for now. This new type needs to
 support at least these functions:

 {{{#!haskell
 add :: AtomicIntRef -> Int -> IO Int
 set :: AtomicIntRef -> Int -> IO ()
 get :: AtomicIntRef -> IO Int
 }}}

 `add` would be implemented using the `lock` and `xaddq` instructions.
 `set` and `get` are just simple loads and stores on x86, as these are
 atomic.

 We might also want to consider having other functions, such as a `cas`.
 Furthermore, there are subtleties with memory barriers that might motivate
 having barrier/barrier-less versions of some functions.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8972>
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