17.1  Nesting of Regions

This section describes a set of restrictions on the nesting of regions. The restrictions on nesting are as follows:

• A worksharing region may not be closely nested inside a worksharing, task, taskloop, critical, ordered, atomic, or masked region.
• A barrier region may not be closely nested inside a worksharing, task, taskloop, critical, ordered, atomic, or masked region.
• A masked region may not be closely nested inside a worksharing, atomic, task, or taskloop region.
• An ordered region that corresponds to an ordered construct without any clause or with the threads or depend clause may not be closely nested inside a critical, ordered, loop, atomic, task, or taskloop region.
• An ordered region that corresponds to an ordered construct without the simd clause specified must be closely nested inside a worksharing-loop region.
• An ordered region that corresponds to an ordered construct with the simd clause specified must be closely nested inside a simd or worksharing-loop SIMD region.
• An ordered region that corresponds to an ordered construct with both the simd and threads clauses must be closely nested inside a worksharing-loop SIMD region or closely nested inside a worksharing-loop and simd region.
• A critical region may not be nested (closely or otherwise) inside a critical region with the same name. This restriction is not sufficient to prevent deadlock.
• OpenMP constructs may not be encountered during execution of an atomic region.
• The only OpenMP constructs that can be encountered during execution of a simd (or worksharing-loop SIMD) region are the atomic construct, the loop construct without a defined binding region, the simd construct and the ordered construct with the simd clause.
• If a target update, target data, target enter data, or target exit data construct is encountered during execution of a target region, the behavior is unspecified.
• If a target construct is encountered during execution of a target region and a device clause in which the ancestor device-modifier appears is not present on the construct, the behavior is unspecified.
• A teams region must be strictly nested either within the implicit parallel region that surrounds the whole OpenMP program or within a target region. If a teams construct is nested within a target construct, that target construct must contain no statements, declarations or directives outside of the teams construct.
• distribute regions, including any distribute regions arising from composite constructs, parallel regions, including any parallel regions arising from combined constructs, loop regions, omp_get_num_teams() regions, and omp_get_team_num() regions are the only OpenMP regions that may be strictly nested inside the teams region.
• A loop region that binds to a teams region must be strictly nested inside a teams region.
• A distribute region must be strictly nested inside a teams region.
• If cancel-directive-name is taskgroup, the cancel construct must be closely nested inside a task construct and the cancel region must be closely nested inside a taskgroup region. Otherwise, the cancel construct must be closely nested inside an OpenMP construct for which directive-name is cancel-directive-name.
• A cancellation point construct for which cancel-directive-name is taskgroup must be closely nested inside a task construct, and the cancellation point region must be closely nested inside a taskgroup region. Otherwise, a cancellation point construct must be closely nested inside an OpenMP construct for which directive-name is cancel-directive-name.
• The only constructs that may be encountered inside a region that corresponds to a construct with an order clause that specifies concurrent are the loop, parallel and simd constructs, and combined constructs for which directive-name-A is parallel.
• A region that corresponds to a construct with an order clause that specifies concurrent may not contain calls to the OpenMP Runtime API or to procedures that contain OpenMP directives.