Data-Sharing Attribute Rules

| OPENMP API Specification: Version 5.1 November 2020

2.21.1 Data-Sharing Attribute Rules

This section describes how the data-sharing attributes of variables referenced in data environments are determined. The following two cases are described separately:

Section 2.21.1.1 describes the data-sharing attribute rules for variables referenced in a construct.
Section 2.21.1.2 describes the data-sharing attribute rules for variables referenced in a region, but outside any construct.

2.21.1.1 Variables Referenced in a Construct

The data-sharing attributes of variables that are referenced in a construct can be predetermined, explicitly determined, or implicitly determined, according to the rules outlined in this section.

Specifying a variable in a data-sharing attribute clause, except for the private clause, or copyprivate clause of an enclosed construct, causes an implicit reference to the variable in the enclosing construct. Specifying a variable in a map clause of an enclosed construct may cause an implicit reference to the variable in the enclosing construct. Such implicit references are also subject to the data-sharing attribute rules outlined in this section.

Certain variables and objects have predetermined data-sharing attributes with respect to the construct in which they are referenced. The first matching rule from the following list of predetermined data-sharing attribute rules applies for variables and objects that are referenced in a construct.

Variables that appear in threadprivate directives or variables with the _Thread_local (in C) or thread_local (in C++) storage-class specifier are threadprivate.

∙ Variables with automatic storage duration that are declared in a scope inside the construct are private.

∙ Variables of non-reference type with automatic storage duration that are declared in a scope inside the construct are private.
Objects with dynamic storage duration are shared.
The loop iteration variable in any associated loop of a for, parallel for, taskloop, or distribute construct is private.
The loop iteration variable in the associated loop of a simd construct with just one associated loop is linear with a linear-step that is the increment of the associated loop.
The loop iteration variables in the associated loops of a simd construct with multiple associated loops are lastprivate.
The loop iteration variable in any associated loop of a loop construct is lastprivate.
The implicitly declared variables of a range-based for loop are private.
Variables with static storage duration that are declared in a scope inside the construct are shared.
If a list item in a map clause on the target construct has a base pointer, and the base pointer is a scalar variable that does not appear in a map clause on the construct, the base pointer is firstprivate.
If a list item in a reduction or in_reduction clause on a construct has a base pointer then the base pointer is private.
Static data members are shared.
The __func__ variable and similar function-local predefined variables are shared.

Variables declared within a BLOCK construct inside a construct that do not have the SAVE attribute are private.
Variables and common blocks that appear in threadprivate directives are threadprivate.
The loop iteration variable in any associated do-loop of a do, parallel do, taskloop, or distribute construct is private.
The loop iteration variable in the associated do-loop of a simd construct with just one associated do-loop is linear with a linear-step that is the increment of the associated do-loop.
The loop iteration variables in the associated do-loops of a simd construct with multiple associated do-loops are lastprivate.
The loop iteration variable in any associated do-loop of a loop construct is lastprivate.
Loop iteration variables inside parallel or task generating constructs are private in the innermost such construct that encloses the loop.
Implied-do, FORALL and DO CONCURRENT indices are private.
Cray pointees have the same data-sharing attribute as the storage with which their Cray pointers are associated. Cray pointer support has been deprecated.
Assumed-size arrays are shared.
Named constants are shared.
An associate name that may appear in a variable definition context is shared if its association occurs outside of the construct and otherwise it has the same data-sharing attribute as the selector with which it is associated.

Variables with predetermined data-sharing attributes may not be listed in data-sharing attribute clauses, except for the cases listed below. For these exceptions only, listing a predetermined variable in a data-sharing attribute clause is allowed and overrides the variable’s predetermined data-sharing attributes.

The loop iteration variable in any associated loop of a for, taskloop, distribute, or loop construct may be listed in a private or lastprivate clause.
If a simd construct has just one associated loop then its loop iteration variable may be listed in a private, lastprivate, or linear clause with a linear-step that is the increment of the associated loop.
If a simd construct has more than one associated loop then their loop iteration variables may be listed in a private or lastprivate clause.
Variables with const-qualified type with no mutable members may be listed in a firstprivate clause, even if they are static data members.
The __func__ variable and similar function-local predefined variables may be listed in a shared or firstprivate clause.

The loop iteration variable in any associated do-loop of a do, taskloop, distribute, or loop construct may be listed in a private or lastprivate clause.
The loop iteration variable in the associated do-loop of a simd construct with just one associated do-loop may be listed in a private, lastprivate, or linear clause with a linear-step that is the increment of the associated loop.
The loop iteration variables in the associated do-loops of a simd construct with multiple associated do-loops may be listed in a private or lastprivate clause.
Loop iteration variables of loops that are not associated with any OpenMP directive may be listed in data-sharing attribute clauses on the surrounding teams, parallel or task generating construct, and on enclosed constructs, subject to other restrictions.
Assumed-size arrays may be listed in a shared clause.
Named constants may be listed in a firstprivate clause.

Additional restrictions on the variables that may appear in individual clauses are described with each clause in Section 2.21.4.

Variables with explicitly determined data-sharing attributes are those that are referenced in a given construct and are listed in a data-sharing attribute clause on the construct.

Variables with implicitly determined data-sharing attributes are those that are referenced in a given construct, do not have predetermined data-sharing attributes, and are not listed in a data-sharing attribute clause on the construct.

Rules for variables with implicitly determined data-sharing attributes are as follows:

In a parallel, teams, or task generating construct, the data-sharing attributes of these variables are determined by the default clause, if present (see Section 2.21.4.1).
In a parallel construct, if no default clause is present, these variables are shared.
For constructs other than task generating constructs, if no default clause is present, these variables reference the variables with the same names that exist in the enclosing context.
In a target construct, variables that are not mapped after applying data-mapping attribute rules (see Section 2.21.7) are firstprivate.

In an orphaned task generating construct, if no default clause is present, formal arguments passed by reference are firstprivate.

In an orphaned task generating construct, if no default clause is present, dummy arguments are firstprivate.

In a task generating construct, if no default clause is present, a variable for which the data-sharing attribute is not determined by the rules above and that in the enclosing context is determined to be shared by all implicit tasks bound to the current team is shared.
In a task generating construct, if no default clause is present, a variable for which the data-sharing attribute is not determined by the rules above is firstprivate.

Additional restrictions on the variables for which data-sharing attributes cannot be implicitly determined in a task generating construct are described in Section 2.21.4.4.

2.21.1.2 Variables Referenced in a Region but not in a Construct

The data-sharing attributes of variables that are referenced in a region, but not in a construct, are determined as follows:

Variables with static storage duration that are declared in called routines in the region are shared.
File-scope or namespace-scope variables referenced in called routines in the region are shared unless they appear in a threadprivate directive.
Objects with dynamic storage duration are shared.
Static data members are shared unless they appear in a threadprivate directive.
In C++, formal arguments of called routines in the region that are passed by reference have the same data-sharing attributes as the associated actual arguments.
Other variables declared in called routines in the region are private.

Local variables declared in called routines in the region and that have the save attribute, or that are data initialized, are shared unless they appear in a threadprivate directive.
Variables belonging to common blocks, or accessed by host or use association, and referenced in called routines in the region are shared unless they appear in a threadprivate directive.
Dummy arguments of called routines in the region that have the VALUE attribute are private.
Dummy arguments of called routines in the region that do not have the VALUE attribute are private if the associated actual argument is not shared.
Dummy arguments of called routines in the region that do not have the VALUE attribute are shared if the actual argument is shared and it is a scalar variable, structure, an array that is not a pointer or assumed-shape array, or a simply contiguous array section. Otherwise, the data-sharing attribute of the dummy argument is implementation-defined if the associated actual argument is shared.
Cray pointees have the same data-sharing attribute as the storage with which their Cray pointers are associated. Cray pointer support has been deprecated.
Implied-do indices, DO CONCURRENT indices, FORALL indices, and other local variables declared in called routines in the region are private.

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