11.4  workshare Construct

Clauses

Binding

Semantics

An implementation of the workshare construct must insert any synchronization that is required to maintain standard Fortran semantics. For example, the effects of one statement within the structured block must appear to occur before the execution of succeeding statements, and the evaluation of the right hand side of an assignment must appear to complete prior to the effects of assigning to the left hand side.

The statements in the workshare construct are divided into units of work as follows:

• For array expressions within each statement, including transformational array intrinsic functions that compute scalar values from arrays:

  • Evaluation of each element of the array expression, including any references to elemental functions, is a unit of work.

  • Evaluation of transformational array intrinsic functions may be freely subdivided into any number of units of work.

• For array assignment statements, assignment of each element is a unit of work.

• For scalar assignment statements, each assignment operation is a unit of work.

• For WHERE statements or constructs, evaluation of the mask expression and the masked assignments are each a unit of work.

• For FORALL statements or constructs, evaluation of the mask expression, expressions occurring in the specification of the iteration space, and the masked assignments are each a unit of work.

• For atomic constructs, critical constructs, and parallel constructs, the construct is a unit of work. A new thread team executes the statements contained in a parallel construct.

• If none of the rules above apply to a portion of a statement in the structured block, then that portion is a unit of work.

The transformational array intrinsic functions are MATMUL, DOT_PRODUCT, SUM, PRODUCT, MAXVAL, MINVAL, COUNT, ANY, ALL, SPREAD, PACK, UNPACK, RESHAPE, TRANSPOSE, EOSHIFT, CSHIFT, MINLOC, and MAXLOC.

How units of work are assigned to the threads that execute a workshare region is unspecified.

If an array expression in the block references the value, association status, or allocation status of private variables, the value of the expression is undefined, unless the same value would be computed by every thread.

If an array assignment, a scalar assignment, a masked array assignment, or a FORALL assignment assigns to a private variable in the block, the result is unspecified.

The workshare directive causes the sharing of work to occur only in the workshare construct, and not in the remainder of the workshare region.

Execution Model Events

The workshare-begin event occurs after an implicit task encounters a workshare construct but before the task starts to execute the structured block of the workshare region.

The workshare-end event occurs after an implicit task finishes execution of a workshare region but before it resumes execution of the enclosing context.

Tool Callbacks

A thread dispatches a registered ompt_callback_work callback with ompt_scope_begin as its endpoint argument and ompt_work_workshare as its work_type argument for each occurrence of a workshare-begin event in that thread. Similarly, a thread dispatches a registered ompt_callback_work callback with ompt_scope_end as its endpoint argument and ompt_work_workshare as its work_type argument for each occurrence of a workshare-end event in that thread. The callbacks occur in the context of the implicit task. The callbacks have type signature ompt_callback_work_t.

Restrictions

• The only OpenMP constructs that may be closely nested inside a workshare construct are the atomic, critical, and parallel constructs.

• Base language statements that are encountered inside a workshare construct but that are not enclosed within a parallel or atomic construct that is nested inside the workshare construct must consist of only the following:

  • array assignments;

  • scalar assignments;

  • FORALL statements;

  • FORALL constructs;

  • WHERE statements;

  • WHERE constructs; and

  • BLOCK constructs that are strictly structured blocks associated with OpenMP directives.

• All array assignments, scalar assignments, and masked array assignments that are encountered inside a workshare construct but are not nested inside a parallel construct that is nested inside the workshare construct must be intrinsic assignments.

• The construct must not contain any user-defined function calls unless either the function is pure and elemental or the function call is contained inside a parallel construct that is nested inside the workshare construct.

Cross References

• ompt_callback_work_t, see Section 19.5.2.5

• ompt_scope_endpoint_t, see Section 19.4.4.11

• ompt_work_t, see Section 19.4.4.16

• atomic directive, see Section 15.8.4

• critical directive, see Section 15.2

• nowait clause, see Section 15.6

• parallel directive, see Section 10.1