The Intel® C++ Compiler 12.0 available in Intel® Parallel Composer 2011 and Intel® C++ Composer XE now supports the atomic data types. These standard data types provide a standard way of implementing fine grained atomic locking in C and C++ which can be useful for many different parallel methodologies, from native threads to Intel® Threading Building Blocks to OpenMP* to Intel® Cilk™ Plus. This article covers the basic syntax and how it works to ensure threadsafe accesses of shared data.
- Intel® Parallel Composer 2011
- #include the <atomic> or <stdatomic.h> header files
The atomic_flag data type is the basis for all atomic data types. It provides two operations that can be done atomically - a test and set operation, and a clear operation. The flag is a bool that is set to 0 when clear() is used. For test_and_set, the current value of the bool is returned and then the flag is set to 1.
The syntax for this is:
bool y = x.test_and_set();
Other atomic data types
In short, void* (as atomic_address), bool (atomic_bool), and various char/integer types are supported. These can also be alternatively declared in C++ using atomic<T> where T is void*, bool, or the char/int type.
atomic_bool supports the following operations:
-- Atomically return the value of atomic_bool. Can also happen when using the "=" assignment operator or during conversions.
-- Atomically set the value of atomic_bool. Can also happen when using the "=" assignment operator or during conversions.
-- Atomically swap the values of the current atomic_bool with the boolean input. Return the old value of the atomic bool.
bool compare_exchange_weak(bool & expected, bool desired)
-- Atomically: compare the current value of atomic_bool with expected. If equal, set atomic_bool value to value of desired and return true. If not equal, set expected to desired value and return false. Intended for use in loops.
bool compare_exchange_strong(bool & expected, bool desired)
-- Atomically: compare the current value of atomic_bool with expected. If equal, set atomic_bool value to value of desired and return true. If not equal, set expected to desired value and return false. Intended for use outside of loops.
atomic_address supports the above atomic_bool operations for void* types, and also the following:
-- Atomically increment the atomic_address by the input number of bytes and store
-- Atomically decrement the atomic_address by the input number of bytes and store
Other atomic types:
The other types that are supported:
These data types support the atomic_bool and atomic_address functions above for their data type, plus the following:
-- Atomically increment by 1
-- Atomically decrement by 1
Operator &= (T)
-- Atomically bitwise-and with input and store
Operator |= (T)
-- Atomically bitwise-or with input and store
Operator ^= (T)
-- Atomically bitwise-xor with input and store
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