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| OPENMP API Specification: Version 5.1 November 2020

1.4.1  Structure of the OpenMP Memory Model

The OpenMP API provides a relaxed-consistency, shared-memory model. All OpenMP threads have access to a place to store and to retrieve variables, called the memory. A given storage location in the memory may be associated with one or more devices, such that only threads on associated devices have access to it. In addition, each thread is allowed to have its own temporary view of the memory. The temporary view of memory for each thread is not a required part of the OpenMP memory model, but can represent any kind of intervening structure, such as machine registers, cache, or other local storage, between the thread and the memory. The temporary view of memory allows the thread to cache variables and thereby to avoid going to memory for every reference to a variable. Each thread also has access to another type of memory that must not be accessed by other threads, called threadprivate memory.

A directive that accepts data-sharing attribute clauses determines two kinds of access to variables used in the directive’s associated structured block: shared and private. Each variable referenced in the structured block has an original variable, which is the variable by the same name that exists in the program immediately outside the construct. Each reference to a shared variable in the structured block becomes a reference to the original variable. For each private variable referenced in the structured block, a new version of the original variable (of the same type and size) is created in memory for each task or SIMD lane that contains code associated with the directive. Creation of the new version does not alter the value of the original variable. However, the impact of attempts to access the original variable from within the region corresponding to the directive is unspecified; see Section 2.21.4.3 for additional details. References to a private variable in the structured block refer to the private version of the original variable for the current task or SIMD lane. The relationship between the value of the original variable and the initial or final value of the private version depends on the exact clause that specifies it. Details of this issue, as well as other issues with privatization, are provided in Section 2.21.

The minimum size at which a memory update may also read and write back adjacent variables that are part of another variable (as array or structure elements) is implementation defined but is no larger than the base language requires.

A single access to a variable may be implemented with multiple load or store instructions and, thus, is not guaranteed to be atomic with respect to other accesses to the same variable. Accesses to variables smaller than the implementation defined minimum size or to C or C++ bit-fields may be implemented by reading, modifying, and rewriting a larger unit of memory, and may thus interfere with updates of variables or fields in the same unit of memory.

Two memory operations are considered unordered if the order in which they must complete, as seen by their affected threads, is not specified by the memory consistency guarantees listed in Section 1.4.6. If multiple threads write to the same memory unit (defined consistently with the above access considerations) then a data race occurs if the writes are unordered. Similarly, if at least one thread reads from a memory unit and at least one thread writes to that same memory unit then a data race occurs if the read and write are unordered. If a data race occurs then the result of the program is unspecified.

A private variable in a task region that subsequently generates an inner nested parallel region is permitted to be made shared for implicit tasks in the inner parallel region. A private variable in a task region can also be shared by an explicit task region generated during its execution. However, the programmer must use synchronization that ensures that the lifetime of the variable does not end before completion of the explicit task region sharing it. Any other access by one task to the private variables of another task results in unspecified behavior.

A storage location in memory that is associated with a given device has a device address that may be dereferenced by a thread executing on that device, but it may not be generally accessible from other devices. A different device may obtain a device pointer that refers to this device address. The manner in which a program can obtain the referenced device address from a device pointer, outside of mechanisms specified by OpenMP, is implementation defined.