atomic-primops-0.6.0.6: A safe approach to CAS and other atomic ops in Haskell.

Safe HaskellNone
LanguageHaskell2010

Data.Atomics.Counter.Unboxed

Description

This should be the most efficient implementation of atomic counters. You probably don't need the others! (Except for testing/debugging.)

Synopsis

Documentation

data AtomicCounter

The type of mutable atomic counters.

type CTicket = Int

You should not depend on this type. It varies between different implementations of atomic counters.

newCounter :: Int -> IO AtomicCounter

Create a new counter initialized to the given value.

readCounterForCAS :: AtomicCounter -> IO CTicket

Just like the Data.Atomics CAS interface, this routine returns an opaque ticket that can be used in CAS operations. Except for the difference in return type, the semantics of this are the same as readCounter.

readCounter :: AtomicCounter -> IO Int

Equivalent to readCounterForCAS followed by peekCTicket.

peekCTicket :: CTicket -> Int

Opaque tickets cannot be constructed, but they can be destructed into values.

writeCounter :: AtomicCounter -> Int -> IO ()

Make a non-atomic write to the counter. No memory-barrier.

casCounter :: AtomicCounter -> CTicket -> Int -> IO (Bool, CTicket)

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 ticket form.)

incrCounter :: Int -> AtomicCounter -> IO Int

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.

incrCounter_ :: Int -> AtomicCounter -> IO ()

An alternate version for when you don't care about the old value.