atomic_compare_exchange functions

atomic_compare_exchange functions.

bool atomic_compare_exchange_strong ( volatile A  *object,
  C  *expected,
  C  desired)
bool atomic_compare_exchange_strong_explicit ( volatile A  *object,
  C  *expected,
  C  desired,
  memory_order success,
  memory_order failure)
bool atomic_compare_exchange_strong_explicit ( volatile A  *object,
  C  *expected,
  C  desired,
  memory_order success,
  memory_order failure,
  memory_scope scope)
bool atomic_compare_exchange_weak ( volatile A  *object,
  C  *expected,
  C  desired)
bool atomic_compare_exchange_weak_explicit ( volatile A  *object,
  C  *expected,
  C  desired,
  memory_order success,
  memory_order failure)
bool atomic_compare_exchange_weak_explicit ( volatile A  *object,
  C  *expected,
  C  desired,
  memory_order success,
  memory_order failure,
  memory_scope scope)

Parameters

object
expected
desired
success
failure
The failure argument shall not be memory_order_release nor memory_order_acq_rel.
scope

Description

These functions can only be used with an object of any atomic integer type.

The failure argument shall be no stronger than the success argument. Atomically, compares the value pointed to by object for equality with that in expected, and if true, replaces the value pointed to by object with desired, and if false, updates the value in expected with the value pointed to by object. Further, if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure. These operations are atomic read-modify-write operations (as defined by section 5.1.2.4 of the C11 specification).

NOTE: The effect of the compare-and-exchange operations is:

if (*object == *expected)
    *object = desired;
else
    *expected = *object;

The weak compare-and-exchange operations may fail spuriously. That is, even when the contents of memory referred to by expected and object are equal, it may return zero and store back to expected the same memory contents that were originally there. This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g. load-locked store-conditional machines.

These generic functions return the result of the comparison.

In these operation definitions:

  • An A refers to one of the atomic types.
  • A C refers to its corresponding non-atomic type.
  • An M refers to the type of the other argument for arithmetic operations. For atomic integer types, M is C.
  • The functions not ending in explicit have the same semantics as the corresponding explicit function with memory_order_seq_cst for the memory_order argument.
  • The functions that do not have memory_scope argument have the same semantics as the corresponding functions with the memory_scope argument set to memory_scope_device.

NOTE: With fine-grained system SVM, sharing happens at the granularity of individual loads and stores anywhere in host memory. Memory consistency is always guaranteed at synchronization points, but to obtain finer control over consistency, the OpenCL atomics functions may be used to ensure that the updates to individual data values made by one unit of execution are visible to other execution units. In particular, when a host thread needs fine control over the consistency of memory that is shared with one or more OpenCL devices, it must use atomic and fence operations that are compatible with the C11 atomic operations.

We can't require C11 atomics since host programs can be implemented in other programming languages and versions of C or C++, but we do require that the host programs use atomics and that those atomics be compatible with those in C11.

Restrictions

All operations on atomic types must be performed using the built-in atomic functions. C11 and C++11 support operators on atomic types. OpenCL C does not support operators with atomic types. Using atomic types with operators should result in a compilation error.

The atomic_bool, atomic_char, atomic_uchar, atomic_short, atomic_ushort, atomic_intmax_t and atomic_uintmax_t types are not supported by OpenCL C.

OpenCL C requires that the built-in atomic functions on atomic types are lock-free.

The _Atomic type specifier and _Atomic type qualifier are not supported by OpenCL C.

The behavior of atomic operations where pointer arguments to the atomic functions refers to an atomic type in the private address space is undefined

Specification

OpenCL Specification

Also see

Atomic Functions

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