reduction Clause

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

5.5.8 reduction Clause


Name: reduction	Properties: data-environment attribute, data-sharing attribute, privatization, reduction scoping, reduction participating

Arguments


Name	Type	Properties

list	list of variable list item type	default

Modiﬁers


Name	Modiﬁes	Type	Properties

reduction-identiﬁer	list	An OpenMP reduction identiﬁer	required, ultimate

reduction-modiﬁer	list	Keyword: default, inscan, task	default

Directives

do, for, loop, parallel, scope, sections, simd, taskloop, teams

Semantics

The reduction clause is a reduction scoping clause and a reduction participating clause, as described in Section 5.5.6 and Section 5.5.7. For each list item, a private copy is created for each implicit task or SIMD lane and is initialized with the initializer value of the reduction-identiﬁer. After the end of the region, the original list item is updated with the values of the private copies using the combiner associated with the reduction-identiﬁer.

If reduction-modiﬁer is not present or the default reduction-modiﬁer is present, the behavior is as follows. For parallel and worksharing constructs, one or more private copies of each list item are created for each implicit task, as if the private clause had been used. For the simd construct, one or more private copies of each list item are created for each SIMD lane, as if the private clause had been used. For the taskloop construct, private copies are created according to the rules of the reduction scoping clauses. For the teams construct, one or more private copies of each list item are created for the initial task of each team in the league, as if the private clause had been used. For the loop construct, private copies are created and used in the construct according to the description and restrictions in Section 5.3. At the end of a region that corresponds to a construct for which the reduction clause was speciﬁed, the original list item is updated by combining its original value with the ﬁnal value of each of the private copies, using the combiner of the speciﬁed reduction-identiﬁer.

If the inscan reduction-modiﬁer is present, a scan computation is performed over updates to the list item performed in each logical iteration of the loop associated with the worksharing-loop, worksharing-loop SIMD, or simd construct (see Section 5.6). The list items are privatized in the construct according to the description and restrictions in Section 5.3. At the end of the region, each original list item is assigned the value described in Section 5.6.

If the task reduction-modiﬁer is present for a parallel or worksharing construct, then each list item is privatized according to the description and restrictions in Section 5.3, and an unspeciﬁed number of additional private copies may be created to support task reductions. Any copies associated with the reduction are initialized before they are accessed by the tasks that participate in the reduction, which include all implicit tasks in the corresponding region and all participating explicit tasks that specify an in_reduction clause (see Section 5.5.10). After the end of the region, the original list item contains the result of the reduction.

Restrictions

Restrictions to the reduction clause are as follows:

All restrictions common to all reduction clauses, as listed in Section 5.5.5, apply to this clause.
A list item that appears in a reduction clause on a worksharing construct must be shared in the parallel region to which a corresponding worksharing region binds.
If an array section or array element appears as a list item in a reduction clause on a worksharing construct, all threads of the team must specify the same storage location.
Each list item speciﬁed with the inscan reduction-modiﬁer must appear as a list item in an inclusive or exclusive clause on a scan directive enclosed by the construct.
If the inscan reduction-modiﬁer is speciﬁed, a reduction clause without the inscan reduction-modiﬁer must not appear on the same construct.
A reduction clause with the task reduction-modiﬁer may only appear on a parallel construct, a worksharing construct or a combined or composite construct for which any of the aforementioned constructs is a constituent construct and neither simd nor loop are constituent constructs.
A reduction clause with the inscan reduction-modiﬁer may only appear on a worksharing-loop construct, a simd construct or a combined or composite construct for which any of the aforementioned constructs is a constituent construct and distribute is not a constituent construct.
The inscan reduction-modiﬁer must not be speciﬁed on a construct for which the ordered or schedule clause is speciﬁed.
A list item that appears in a reduction clause of the innermost enclosing worksharing or parallel construct must not be accessed in an explicit task generated by a construct for which an in_reduction clause over the same list item does not appear.
The task reduction-modiﬁer must not appear in a reduction clause if the nowait clause is speciﬁed on the same construct.

∙ If a list item in a reduction clause on a worksharing construct has a reference type then it must bind to the same object for all threads of the team. ∙ If a list item in a reduction clause on a worksharing construct is an array section or an array element then the base pointer must point to the same variable for all threads of the team. ∙ A variable of class type (or array thereof) that appears in a reduction clause with the inscan reduction-modiﬁer requires an accessible, unambiguous default constructor for the class type; the number of calls to it while performing the scan computation is unspeciﬁed. ∙ A variable of class type (or array thereof) that appears in a reduction clause with the inscan reduction-modiﬁer requires an accessible, unambiguous copy assignment operator for the class type; the number of calls to it while performing the scan computation is unspeciﬁed.

Cross References

List Item Privatization, see Section 5.3
do directive, see Section 11.5.2
for directive, see Section 11.5.1
loop directive, see Section 11.7
ordered clause, see Section 4.4.4
parallel directive, see Section 10.1
private clause, see Section 5.4.3
scan directive, see Section 5.6
schedule clause, see Section 11.5.3
scope directive, see Section 11.2
sections directive, see Section 11.3
simd directive, see Section 10.4
taskloop directive, see Section 12.6
teams directive, see Section 10.2

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