This should be the most efficient implementation of atomic counters. You probably don't need the others! (Except for testing/debugging.)
- data AtomicCounter
- type CTicket = Int
- newCounter :: Int -> IO AtomicCounter
- readCounterForCAS :: AtomicCounter -> IO CTicket
- readCounter :: AtomicCounter -> IO Int
- peekCTicket :: CTicket -> Int
- writeCounter :: AtomicCounter -> Int -> IO ()
- casCounter :: AtomicCounter -> CTicket -> Int -> IO (Bool, CTicket)
- incrCounter :: Int -> AtomicCounter -> IO Int
- incrCounter_ :: Int -> AtomicCounter -> IO ()
You should not depend on this type. It varies between different implementations of atomic counters.
Opaque tickets cannot be constructed, but they can be destructed into values.
Make a non-atomic write to the counter. No memory-barrier.
Compare and swap for the counter ADT. Similar behavior to
casIORef, in particular, in both success and failure cases it
returns a ticket that you should use for the next attempt. (That is, in the
success case, it actually returns the new value that you provided as input, but in
Increment the counter by a given amount. Returns the value AFTER the increment (in contrast with the behavior of the underlying instruction on architectures like x86.)
Note that UNLIKE with boxed implementations of counters, where increment is based on CAS, this increment is O(1). Fetch-and-add does not require a retry loop like CAS.