The following built-in functions approximately match the requirements for C++11 concurrency and memory models. They are all identified by being prefixed with __atomic and most are overloaded so that they work with multiple types.
These functions are intended to replace the legacy __sync builtins. The main difference is that the memory model to be used is a parameter to the functions. New code should always use the __atomic builtins rather than the __sync builtins.
Note that the __atomic builtins assume that programs will conform to the C++11 model for concurrency. In particular, they assume that programs are free of data races. See the C++11 standard for detailed definitions.
The __atomic builtins can be used with any integral scalar or pointer type that is 1, 2, 4, or 8 bytes in length. 16-byte integral types are also allowed if __int128 (__int128) is supported by the architecture.
The four non-arithmetic functions (load, store, exchange, and compare_exchange) all have a generic version as well. This generic version works on any data type. If the data type size maps to one of the integral sizes that may have lock free support, the generic version uses the lock free built-in function. Otherwise an external call is left to be resolved at run time. This external call is the same format with the addition of a size_t parameter inserted as the first parameter indicating the size of the object being pointed to. All objects must be the same size.
There are 6 different memory models that can be specified. These map to the C++11 memory models with the same names, see the C++11 standard or the http://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSyncGCC wiki on atomic synchronization for detailed definitions. Individual targets may also support additional memory models for use on specific architectures. Refer to the target documentation for details of these.
The memory models integrate both barriers to code motion as well as synchronization requirements with other threads. They are listed here in approximately ascending order of strength.
Note that the scope of a C++11 memory model depends on whether or not the function being called is a fence (such as __atomic_thread_fence). In a fence, all memory accesses are subject to the restrictions of the memory model. When the function is an operation on a location, the restrictions apply only to those memory accesses that could affect or that could depend on the location.
Target architectures are encouraged to provide their own patterns for each of these built-in functions. If no target is provided, the original non-memory model set of __sync atomic built-in functions are used, along with any required synchronization fences surrounding it in order to achieve the proper behavior. Execution in this case is subject to the same restrictions as those built-in functions.
If there is no pattern or mechanism to provide a lock free instruction sequence, a call is made to an external routine with the same parameters to be resolved at run time.
When implementing patterns for these built-in functions, the memory model parameter can be ignored as long as the pattern implements the most restrictive __ATOMIC_SEQ_CST model. Any of the other memory models execute correctly with this memory model but they may not execute as efficiently as they could with a more appropriate implementation of the relaxed requirements.
Note that the C++11 standard allows for the memory model parameter to be determined at run time rather than at compile time. These built-in functions map any run-time value to __ATOMIC_SEQ_CST rather than invoke a runtime library call or inline a switch statement. This is standard compliant, safe, and the simplest approach for now.
The memory model parameter is a signed int, but only the lower 16 bits are reserved for the memory model. The remainder of the signed int is reserved for target use and should be 0. Use of the predefined atomic values ensures proper usage.
This built-in function implements an atomic load operation. It returns the contents of *``ptr``.
The valid memory model variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE, and __ATOMIC_CONSUME.
This is the generic version of an atomic load. It returns the contents of *``ptr`` in *``ret``.
This built-in function implements an atomic store operation. It writes ``val`` into *``ptr``.
The valid memory model variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, and __ATOMIC_RELEASE.
This is the generic version of an atomic store. It stores the value of *``val`` into *``ptr``.
This built-in function implements an atomic exchange operation. It writes val into *``ptr``, and returns the previous contents of *``ptr``.
The valid memory model variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, __ATOMIC_ACQUIRE, __ATOMIC_RELEASE, and __ATOMIC_ACQ_REL.
This is the generic version of an atomic exchange. It stores the contents of *``val`` into *``ptr``. The original value of *``ptr`` is copied into *``ret``.
This built-in function implements an atomic compare and exchange operation. This compares the contents of *``ptr`` with the contents of *``expected``. If equal, the operation is a read-modify-write which writes desired into *``ptr``. If they are not equal, the operation is a read and the current contents of *``ptr`` is written into *``expected``. weak is true for weak compare_exchange, and false for the strong variation. Many targets only offer the strong variation and ignore the parameter. When in doubt, use the strong variation.
True is returned if desired is written into *``ptr`` and the operation is considered to conform to the memory model specified by success_memmodel. There are no restrictions on what memory model can be used here.
False is returned otherwise, and the operation is considered to conform to failure_memmodel. This memory model cannot be __ATOMIC_RELEASE nor __ATOMIC_ACQ_REL. It also cannot be a stronger model than that specified by success_memmodel.
This built-in function implements the generic version of __atomic_compare_exchange. The function is virtually identical to __atomic_compare_exchange_n, except the desired value is also a pointer.
These built-in functions perform the operation suggested by the name, and return the result of the operation. That is,
{ *ptr ``op``= val; return *ptr; }
All memory models are valid.
These built-in functions perform the operation suggested by the name, and return the value that had previously been in *``ptr``. That is,
{ tmp = *ptr; *ptr ``op``= val; return tmp; }
All memory models are valid.
This built-in function performs an atomic test-and-set operation on the byte at *``ptr``. The byte is set to some implementation defined nonzero set value and the return value is true if and only if the previous contents were set. It should be only used for operands of type bool or char. For other types only part of the value may be set.
All memory models are valid.
This built-in function performs an atomic clear operation on *``ptr``. After the operation, *``ptr`` contains 0. It should be only used for operands of type bool or char and in conjunction with __atomic_test_and_set. For other types it may only clear partially. If the type is not bool prefer using __atomic_store.
The valid memory model variants are __ATOMIC_RELAXED, __ATOMIC_SEQ_CST, and __ATOMIC_RELEASE.
This built-in function acts as a synchronization fence between threads based on the specified memory model.
All memory orders are valid.
This built-in function acts as a synchronization fence between a thread and signal handlers based in the same thread.
All memory orders are valid.
This built-in function returns true if objects of size bytes always generate lock free atomic instructions for the target architecture. size must resolve to a compile-time constant and the result also resolves to a compile-time constant.
ptr is an optional pointer to the object that may be used to determine alignment. A value of 0 indicates typical alignment should be used. The compiler may also ignore this parameter.
if (_atomic_always_lock_free (sizeof (long long), 0))
This built-in function returns true if objects of size bytes always generate lock free atomic instructions for the target architecture. If it is not known to be lock free a call is made to a runtime routine named __atomic_is_lock_free.
ptr is an optional pointer to the object that may be used to determine alignment. A value of 0 indicates typical alignment should be used. The compiler may also ignore this parameter.