Intel® Fortran Compiler Classic and Intel® Fortran Compiler Developer Guide and Reference

ID 767251
Date 11/07/2023
Public

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Rules for Nesting and Binding

This section describes the dynamic nesting and binding rules for OpenMP* Fortran API directives.

Binding Rules

The following rules apply to dynamic binding:

  • The DO, SECTIONS, SINGLE, MASKED, MASTER, and BARRIER directives bind to the dynamically enclosing PARALLEL directive, if one exists.

  • The ORDERED directive binds to the dynamically enclosing DO directive.

  • The ATOMIC directive enforces exclusive access with respect to ATOMIC directives in all threads, not just the current team.

  • The CRITICAL directive enforces exclusive access with respect to CRITICAL directives in all threads, not just the current team.

  • A directive can never bind to any directive outside the closest enclosing PARALLEL directive.

Nesting Rules

The following rules apply to dynamic nesting:

  • A PARALLEL directive dynamically inside another PARALLEL directive logically establishes a new team, which is composed of only the current thread unless nested parallelism is enabled.

  • DO, SECTIONS, and SINGLE directives that bind to the same PARALLEL directive are not allowed to be nested one inside the other.

  • DO, SECTIONS, and SINGLE directives are not permitted in the dynamic extent of CRITICAL, MASKED, and MASTER directives.

  • BARRIER directives are not permitted in the dynamic extent of DO, SECTIONS, SINGLE, MASKED, MASTER, and CRITICAL directives.

  • MASKED and MASTER directives are not permitted in the dynamic extent of DO, SECTIONS, and SINGLE directives.

  • ORDERED sections are not allowed in the dynamic extent of CRITICAL sections.

  • Any directive set that is legal when executed dynamically inside a PARALLEL region is also legal when executed outside a parallel region. When executed dynamically outside a user-specified parallel region, the directive is executed with respect to a team composed of only the primary thread.

Examples

The following example shows nested PARALLEL regions:

  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP DO
          DO I =1, N
  !$OMP PARALLEL SHARED(I,N)
  !$OMP DO
          DO J =1, N
            CALL WORK(I,J)
          END DO
  !$OMP END PARALLEL
        END DO
  !$OMP END PARALLEL

Note that the inner and outer DO directives bind to different PARALLEL regions.

The following shows a variation of the preceding example:

  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP DO
          DO I =1, N
             CALL SOME_WORK(I,N)
          END DO
  !$OMP END PARALLEL
        ...
        SUBROUTINE SOME_WORK(I,N)
  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP DO
          DO J =1, N
             CALL WORK(I,J)
          END DO
  !$OMP END PARALLEL
          RETURN
          END