Work-Sharing Constructs
DO / for Directive

• The DO / for directive specifies that the iterations of the loop immediately following it must be executed in parallel by the team. This assumes a parallel region has already been initiated, otherwise it executes in serial on a single processor.

Format:

Fortran	!$OMP DO [clause ...] SCHEDULE (type [,chunk]) ORDERED PRIVATE (list) FIRSTPRIVATE (list) LASTPRIVATE (list) SHARED (list) REDUCTION (operator | intrinsic : list) COLLAPSE (n) do_loop !$OMP END DO [ NOWAIT ]
C/C++	#pragma omp for [clause ...] newline schedule (type [,chunk]) ordered private (list) firstprivate (list) lastprivate (list) shared (list) reduction (operator: list) collapse (n) nowait for_loop

Clauses:

• SCHEDULE: Describes how iterations of the loop are divided among the threads in the team. The default schedule is implementation dependent.

  STATIC

  Loop iterations are divided into pieces of size chunk and then statically assigned to threads. If chunk is not specified, the iterations are evenly (if possible) divided contiguously among the threads.

  DYNAMIC

  Loop iterations are divided into pieces of size chunk, and dynamically scheduled among the threads; when a thread finishes one chunk, it is dynamically assigned another. The default chunk size is 1.

  GUIDED

  For a chunk size of 1, the size of each chunk is proportional to the number of unassigned iterations divided by the number of threads, decreasing to 1. For a chunk size with value k (greater than 1), the size of each chunk is determined in the same way with the restriction that the chunks do not contain fewer than k iterations (except for the last chunk to be assigned, which may have fewer than k iterations). The default chunk size is 1.

  RUNTIME

  The scheduling decision is deferred until runtime by the environment variable OMP_SCHEDULE. It is illegal to specify a chunk size for this clause.

  AUTO

  The scheduling decision is delegated to the compiler and/or runtime system.

• NO WAIT / nowait: If specified, then threads do not synchronize at the end of the parallel loop.

• ORDERED: Specifies that the iterations of the loop must be executed as they would be in a serial program.

• COLLAPSE: Specifies how many loops in a nested loop should be collapsed into one large iteration space and divided according to the schedule clause. The sequential execution of the iterations in all associated loops determines the order of the iterations in the collapsed iteration space.

• Other clauses are described in detail later, in the Data Scope Attribute Clauses section.

Restrictions:

• The DO loop can not be a DO WHILE loop, or a loop without loop control. Also, the loop iteration variable must be an integer and the loop control parameters must be the same for all threads.

• Program correctness must not depend upon which thread executes a particular iteration.

• It is illegal to branch out of a loop associated with a DO/for directive.

• The chunk size must be specified as a loop invarient integer expression, as there is no synchronization during its evaluation by different threads.

• ORDERED, COLLAPSE and SCHEDULE clauses may appear once each.

• See the OpenMP specification document for additional restrictions.