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### 6.51 Built-in Functions for Memory Model Aware Atomic Operations

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`’ (see __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 GCC 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.

`__ATOMIC_RELAXED`

  No barriers or synchronization.

`__ATOMIC_CONSUME`

  Data dependency only for both barrier and synchronization with another thread.

`__ATOMIC_ACQUIRE`

  Barrier to hoisting of code and synchronizes with release (or stronger) semantic stores from another thread.

`__ATOMIC_RELEASE`

  Barrier to sinking of code and synchronizes with acquire (or stronger) semantic loads from another thread.

`__ATOMIC_ACQ_REL`

  Barrier in both directions and synchronizes with acquire loads and release stores in another thread.

`__ATOMIC_SEQ_CST`

  Barrier in both directions and synchronizes with acquire loads and release stores in all threads.

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.

Built-in Function: type __atomic_load_n (type *ptr, int memmodel)

  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`.

Built-in Function: void __atomic_load (type *ptr, type *ret, int memmodel)

  This is the generic version of an atomic load. It returns the contents of `*ptr` in `*ret`.

Built-in Function: void __atomic_store_n (type *ptr, type val, int memmodel)

  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`.

Built-in Function: void __atomic_store (type *ptr, type *val, int memmodel)

  This is the generic version of an atomic store. It stores the value of `*val` into `*ptr`.

Built-in Function: type __atomic_exchange_n (type *ptr, type val, int memmodel)

  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`.

Built-in Function: void __atomic_exchange (type *ptr, type *val, type *ret, int memmodel)

  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`.

Built-in Function: bool __atomic_compare_exchange_n (type *ptr, type *expected, type desired, bool weak, int success_memmodel, int failure_memmodel)

  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.

Built-in Function: bool __atomic_compare_exchange (type *ptr, type *expected, type *desired, bool weak, int success_memmodel, int failure_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.

Built-in Function: type __atomic_add_fetch (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_sub_fetch (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_and_fetch (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_xor_fetch (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_or_fetch (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_nand_fetch (type *ptr, type val, int memmodel)

  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.

Built-in Function: type __atomic_fetch_add (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_fetch_sub (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_fetch_and (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_fetch_xor (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_fetch_or (type *ptr, type val, int memmodel)
Built-in Function: type __atomic_fetch_nand (type *ptr, type val, int memmodel)

  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.

Built-in Function: bool __atomic_test_and_set (void *ptr, int memmodel)

  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.

Built-in Function: void __atomic_clear (bool *ptr, int memmodel)

  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`.

Built-in Function: void __atomic_thread_fence (int memmodel)

  This built-in function acts as a synchronization fence between threads based on the specified memory model.

  All memory orders are valid.

Built-in Function: void __atomic_signal_fence (int memmodel)

  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.

Built-in Function: bool __atomic_always_lock_free (size_t size, void *ptr)

  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))

Built-in Function: bool __atomic_is_lock_free (size_t size, void *ptr)

  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.

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