5.5.5  Properties Common to All Reduction Clauses

The clause-specification of a reduction clause has a clause-argument-specification that specifies an OpenMP variable list argument and has a required reduction-identifier modifier that specifies the reduction identifier to use for the reduction. The reduction identifier must match a previously declared reduction identifier of the same name and type for each of the list items. This match is done by means of a name lookup in the base language.

The list items that appear in a reduction clause may include array sections.

If a list item is an array or array section, it will be treated as if a reduction clause would be applied to each separate element of the array section.

If a list item is an array section, the elements of any copy of the array section will be stored contiguously.

Any copies of a list item associated with the reduction are initialized with the initializer value of the reduction identifier. Any copies are combined using the combiner associated with the reduction identifier.

Execution Model Events

Tool Callbacks

A thread dispatches a registered ompt_callback_reduction with ompt_sync_region_reduction in its kind argument and ompt_scope_begin as its endpoint argument for each occurrence of a reduction-begin event in that thread. Similarly, a thread dispatches a registered ompt_callback_reduction with ompt_sync_region_reduction in its kind argument and ompt_scope_end as its endpoint argument for each occurrence of a reduction-end event in that thread. These callbacks occur in the context of the task that performs the reduction and has the type signature ompt_callback_sync_region_t.

Restrictions

• Any array element must be specified at most once in all list items on a directive.
• For a reduction identifier declared in a declare reduction directive, the directive must appear before its use in a reduction clause.
• If a list item is an array section, it must specify contiguous storage, it cannot be a zero-length array section and its base expression must be a base language identifier.

• If a list item is an array section or an array element, accesses to the elements of the array outside the specified array section or array element result in unspecified behavior.

  ∙ The type of a list item that appears in a reduction clause must be valid for the reduction identifier. For a max or min reduction in C, the type of the list item must be an allowed arithmetic data type: char, int, float, double, or _Bool, possibly modified with long, short, signed, or unsigned. For a max or min reduction in C++, the type of the list item must be an allowed arithmetic data type: char, wchar_t, int, float, double, or bool, possibly modified with long, short, signed, or unsigned. ∙ A list item that appears in a reduction clause must not be const-qualified. ∙ The reduction identifier for any list item must be unambiguous and accessible.
  ∙ The type, type parameters and rank of a list item that appears in a reduction clause must be valid for the combiner expression and the initializer expression. ∙ A list item that appears in a reduction clause must be definable. ∙ A procedure pointer must not appear in a reduction clause. ∙ A pointer with the INTENT(IN) attribute must not appear in a reduction clause. ∙ An original list item with the POINTER attribute or any pointer component of an original list item that is referenced in a combiner expression must be associated at entry to the construct that contains the reduction clause. Additionally, the list item or the pointer component of the list item must not be deallocated, allocated, or pointer assigned within the region. ∙ An original list item with the ALLOCATABLE attribute or any allocatable component of an original list item that corresponds to a special variable identifier in the combiner expression or the initializer expression must be in the allocated state at entry to the construct that contains the reduction clause. Additionally, the list item or the allocatable component of the list item must be neither deallocated nor allocated, explicitly or implicitly, within the region. ∙ If the reduction identifier is defined in a declarereduction directive, the declarereduction directive must be in the same subprogram, or accessible by host or use association. ∙ If the reduction identifier is a user-defined operator, the same explicit interface for that operator must be accessible at the location of the declare reduction directive that defines the reduction identifier. ∙ If the reduction identifier is defined in a declare reduction directive, any procedure referenced in the initializer clause or the combiner expression must be an intrinsic function, or must have an explicit interface where the same explicit interface is accessible as at the declare reduction directive.

Cross References

• ompt_callback_sync_region_t, see Section 19.5.2.13
• ompt_scope_endpoint_t, see Section 19.4.4.11
• ompt_sync_region_t, see Section 19.4.4.14