Data.Atomics

When performing compare-and-swaps, the ticket encapsulates proof that a thread observed a specific previous value of a mutable variable. It is provided in lieu of the "old" value to compare-and-swap.

Design note: Tickets exist to hide objects from the GHC compiler, which can normally perform many optimizations that change pointer equality. A Ticket, on the other hand, is a first-class object that can be handled by the user, but will not have its pointer identity changed by compiler optimizations (but will of course, change addresses during garbage collection).

A ticket contains or can get the usable Haskell value. This function does just that.

Ordinary processor load instruction (non-atomic, not implying any memory barriers).

The difference between this function and readIORef, is that it returns a ticket, for use in future compare-and-swap operations.

Performs a machine-level compare and swap (CAS) operation on an IORef. Returns a tuple containing a Bool which is True when a swap is performed, along with the most current value from the IORef. Note that this differs from the more common CAS behavior, which is to return the old value before the CAS occured.

The reason for the difference is the ticket API. This function always returns the ticket that you should use in your next CAS attempt. In case of success, this ticket corresponds to the new value which you yourself installed in the IORef, whereas in the case of failure it represents the preexisting value currently in the IORef.

Note "compare" here means pointer equality in the sense of reallyUnsafePtrEquality#. However, the ticket API absolves the user of this module from needing to worry about the pointer equality of their values, which in general requires reasoning about the details of the Haskell implementation (GHC).

By convention this function is strict in the "new" value argument. This isn't absolutely necesary, but we think it's a bad habit to use unevaluated thunks in this context.

This variant takes two tickets, i.e. the new value is a ticket rather than an arbitrary, lifted, Haskell value.

A drop-in replacement for atomicModifyIORef that optimistically attempts to compute the new value and CAS it into place without introducing new thunks or locking anything. Note that this is more STRICT than its standard counterpart and will only place evaluated (WHNF) values in the IORef.

The upside is that sometimes we see a performance benefit. The downside is that this version is speculative -- when it retries, it must reexecute the compution.

A simpler version that modifies the state but does not return anything.

Compare-and-swap. Follows the same rules as casIORef, returning the ticket for then next operation.

By convention this is WHNF strict in the "new" value provided.

This variant takes two tickets: the new value is a ticket rather than an arbitrary, lifted, Haskell value.

Ordinary processor load instruction (non-atomic, not implying any memory barriers).

Compare and swap on word-sized chunks of a byte-array. For indexing purposes the bytearray is treated as an array of words (Ints). Note that UNLIKE casIORef and casArrayTicketed, this does not need to operate on tickets.

Further, this version always returns the old value, that was read from the array during the CAS operation. That is, it follows the normal protocol for CAS operations (and matches the underlying instruction on most architectures).

Implies a full memory barrier.

Atomically add to a word of memory within a MutableByteArray, returning the value *before* the operation. Implies a full memory barrier.

Atomically subtract to a word of memory within a MutableByteArray, returning the value *before* the operation. Implies a full memory barrier.

Atomically bitwise AND to a word of memory within a MutableByteArray, returning the value *before* the operation. Implies a full memory barrier.

Atomically bitwise NAND to a word of memory within a MutableByteArray, returning the value *before* the operation. Implies a full memory barrier.

Atomically bitwise OR to a word of memory within a MutableByteArray, returning the value *before* the operation. Implies a full memory barrier.

Atomically bitwise XOR to a word of memory within a MutableByteArray, returning the value *before* the operation. Implies a full memory barrier.

Like readForCAS, but for MutVar#.

MutVar counterpart of casIORef.

By convention this is WHNF strict in the "new" value provided.

This variant takes two tickets, i.e. the new value is a ticket rather than an arbitrary, lifted, Haskell value.

A memory barrier that prevents future loads occurring before preceding stores.

A memory barrier that prevents future loads occurring before earlier loads.

A memory barrier that prevents future stores occurring before preceding stores.

Atomically add to a word of memory within a MutableByteArray.

This function returns the NEW value of the location after the increment. Thus, it is a bit misnamed, and in other contexts might be called "add-and-fetch", such as in GCC's __sync_add_and_fetch.