Preventing parallelization

You can prevent parallelization on a loop-by-loop basis using the no_parallel directive or pragma. The form of this directive and pragma is shown in Table 6-4 “Form of no_parallel directive and pragma”.

Table 6-4 Form of no_parallel directive and pragma

Language	Form
Fortran	`C$DIR NO_PARALLEL`
C	`#pragma _CNX no_parallel`

Use these directives to prevent parallelization of the loop that immediately follows them. Only parallelization is inhibited; all other loop optimizations are still applied.

Example 6-8 no_parallel

The following Fortran example illustrates the use of no_parallel:

      DO I = 1, 1000
C$DIR   NO_PARALLEL
        DO J = 1, 1000
          A(I,J) = B(I,J)
        ENDDO
      ENDDO

In this example, parallelization of the J loop is prevented. The I loop can still be parallelized.

The +Onoautopar compiler option is available to disable automatic parallelization but allows parallelization of directive-specified loops. Refer to Chapter 7 “Controlling optimization”, and Chapter 9 “Parallel programming techniques”, for more information on +Onoautopar.

Preventing parallelization

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