atomic-primops-0.6.1.1: A safe approach to CAS and other atomic ops in Haskell.

Safe HaskellNone
LanguageHaskell2010

Data.Atomics

Contents

Description

Provides atomic memory operations on IORefs and Mutable Arrays.

Pointer equality need not be maintained by a Haskell compiler. For example, Int values will frequently be boxed and unboxed, changing the pointer identity of the thunk. To deal with this, the compare-and-swap (CAS) approach used in this module is uses a sealed representation of pointers into the Haskell heap (Tickets). Currently, the user cannot coin new tickets, rather a Ticket provides evidence of a past observation, and grants permission to make a future change.

Synopsis

Types for atomic operations

data Ticket a Source

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).

Instances

Eq (Ticket a) 
Show (Ticket a) 

peekTicket :: Ticket a -> a Source

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

Atomic operations on IORefs

readForCAS :: IORef a -> IO (Ticket a) Source

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.

casIORef Source

Arguments

:: IORef a

The IORef containing a value current

-> Ticket a

A ticket for the old value

-> a

The new value to replace current if old == current

-> IO (Bool, Ticket a)

Success flag, plus ticket for the NEXT operation.

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.

casIORef2 Source

Arguments

:: IORef a 
-> Ticket a

A ticket for the old value

-> Ticket a

A ticket for the new value

-> IO (Bool, Ticket a) 

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

atomicModifyIORefCAS Source

Arguments

:: IORef a

Mutable location to modify

-> (a -> (a, b))

Computation runs one or more times (speculation)

-> IO b 

A drop-in replacement for atomicModifyIORefCAS 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.

atomicModifyIORefCAS_ :: IORef t -> (t -> t) -> IO () Source

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

Atomic operations on mutable arrays

casArrayElem :: MutableArray RealWorld a -> Int -> Ticket a -> a -> IO (Bool, Ticket a) Source

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.

casArrayElem2 :: MutableArray RealWorld a -> Int -> Ticket a -> Ticket a -> IO (Bool, Ticket a) Source

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

readArrayElem :: forall a. MutableArray RealWorld a -> Int -> IO (Ticket a) Source

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

Atomic operations on byte arrays

casByteArrayInt :: MutableByteArray RealWorld -> Int -> Int -> Int -> IO Int Source

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).

fetchAddByteArrayInt :: MutableByteArray RealWorld -> Int -> Int -> IO Int Source

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.

Atomic operations on raw MutVars

A lower-level version of the IORef interface.

casMutVar :: MutVar# RealWorld a -> Ticket a -> a -> IO (Bool, Ticket a) Source

MutVar counterpart of casIORef.

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

casMutVar2 :: MutVar# RealWorld a -> Ticket a -> Ticket a -> IO (Bool, Ticket a) Source

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

Memory barriers

storeLoadBarrier :: IO () Source

Memory barrier implemented by the GHC rts (see SMP.h).

loadLoadBarrier :: IO () Source

Memory barrier implemented by the GHC rts (see SMP.h).

writeBarrier :: IO () Source

Memory barrier implemented by the GHC rts (see SMP.h).