OpenMP Reduction Expressions

| OPENMP API Specification: "Version 5.2 -- GIT rev 95b2e3a44"

5.5.2 OpenMP Reduction Expressions

A reduction expression is an OpenMP stylized expression that is relevant to reduction clauses. It is either a combiner expression or an initializer expression.

Restrictions

Restrictions to reduction expressions are as follows:

If execution of a reduction expression results in the execution of an OpenMP construct or an OpenMP API call, the behavior is unspeciﬁed.

∙ If a reduction expression corresponds to a reduction identiﬁer that is used in a target region, a declare target directive must be speciﬁed for any function that can be accessed through the expression.

∙ Any subroutine or function used in a reduction expression must be an intrinsic function, or must have an accessible interface. ∙ Any user-deﬁned operator, deﬁned assignment or extended operator used in a reduction expression must have an accessible interface. ∙ If any subroutine, function, user-deﬁned operator, deﬁned assignment or extended operator is used in a reduction expression, it must be accessible to the subprogram in which the corresponding reduction clause is speciﬁed. ∙ Any subroutine used in a reduction expression must not have any alternate returns appear in the argument list. ∙ If the list item in the corresponding reduction clause is an array or array section, any procedure used in a reduction expression must either be elemental or have dummy arguments that are scalar. ∙ Any procedure called in the region of a reduction expression must be pure and may not reference any host-associated variables. ∙ If a reduction expression corresponds to a reduction identiﬁer that is used in a target region, a declare target directive must be speciﬁed for any function or subroutine that can be accessed through the expression.

5.5.2.1 OpenMP Combiner Expressions

A combiner expression speciﬁes how a reduction combines partial results into a single value.

A combiner expression is an assignment statement or a subroutine name followed by an argument list.

In the deﬁnition of a combiner expression, omp_in and omp_out correspond to two special variable identiﬁers that refer to storage of the type of the reduction list item to which the reduction applies. If the list item is an array or array section, the identiﬁers to which omp_in and omp_out correspond each refer to an array element. Each of the two special variable identiﬁers denotes one of the values to be combined before executing the combiner expression. The special omp_out identiﬁer refers to the storage that holds the resulting combined value after executing the combiner expression. The number of times that the combiner expression is executed and the order of these executions for any reduction clause are unspeciﬁed.

If the combiner expression is a subroutine name with an argument list, the combiner expression is evaluated by calling the subroutine with the speciﬁed argument list. If the combiner expression is an assignment statement, the combiner expression is evaluated by executing the assignment statement.

If a generic name is used in a combiner expression and the list item in the corresponding reduction clause is an array or array section, it is resolved to the speciﬁc procedure that is elemental or only has scalar dummy arguments.

Restrictions

Restrictions to combiner expressions are as follows:

The only variables allowed in a combiner expression are omp_in and omp_out.

∙ Any selectors in the designator of omp_in and omp_out must be component selectors.

5.5.2.2 OpenMP Initializer Expressions

An initializer expression determines the initializer for the private copies of reduction list items. If the initialization of the copies is not determined a priori, the syntax of an initializer expression is as follows:

 
omp_priv = initializer

 
omp_priv initializer

 
function-name(argument-list)

 
omp_priv = expression

 
subroutine-name(argument-list)

In the deﬁnition of an initializer expression, the omp_priv special variable identiﬁer refers to the storage to be initialized. The special variable identiﬁer omp_orig can be used in an initializer expression to refer to the storage of the original variable to be reduced. The number of times that an initializer expression is evaluated and the order of these evaluations are unspeciﬁed.

If an initializer expression is a function name with an argument list, it is evaluated by calling the function with the speciﬁed argument list. Otherwise, an initializer expression speciﬁes how omp_priv is declared and initialized.

If an initializer expression is a subroutine name with an argument list, the initializer-expr is evaluated by calling the subroutine with the speciﬁed argument list. If an initializer expression is an assignment statement, the initializer expression is evaluated by executing the assignment statement.

The a priori initialization of private copies that are created for reductions follows the rules for initialization of objects with static storage duration.

The a priori initialization of private copies that are created for reductions follows the rules for default-initialization.

The rules for a priori initialization of private copies that are created for reductions are as follows:

For complex, real, or integer types, the value 0 will be used.
For logical types, the value .false. will be used.
For derived types for which default initialization is speciﬁed, default initialization will be used.
Otherwise, the behavior is unspeciﬁed.

Restrictions

Restrictions to initializer expressions are as follows:

The only variables allowed in an initializer expression are omp_priv and omp_orig.
If an initializer expression modiﬁes the variable omp_orig, the behavior is unspeciﬁed.

∙ If an initializer expression is a function name with an argument list, one of the arguments must be the address of omp_priv.

∙ If an initializer expression is a function name with an argument list, one of the arguments must be omp_priv or the address of omp_priv.

∙ If an initializer expression is a subroutine name with an argument list, one of the arguments must be omp_priv.

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