Adding OpenMP* Support to your Application

OpenMP Directive Syntax

To add OpenMP* support to your application, first add appropriate OpenMP* directives to your source code.

OpenMP* directives use a specific format and syntax. Intel Extension Routines to OpenMP* describes the OpenMP* extensions to the specification that have been added to the Intel® Fortran Compiler.

The following syntax illustrates using the directives in your source.

<prefix> <directive> [<clause>[[,]<clause>...]]

where:

• <prefix> - Required for all OpenMP* directives. For free form source input, the prefix is !$OMP only; for fixed form source input, the prefix is !$OMP or C$OMP.

• <directive> - A valid OpenMP* directive. Must immediately follow the prefix; for example: !$OMP PARALLEL.

• [<clause>] - Optional. Clauses can be in any order and repeated as necessary, unless otherwise restricted.

• [<newline>] - A required component of directive syntax. It precedes the structured block which is enclosed by this directive.

• [,]: Optional. Commas between more than one <clause> are optional.

The directives are interpreted as comments if you omit the [Q]openmp option.

The OpenMP* constructs defining a parallel region have one of the following syntax forms:

!$OMP <directive>
  <structured block of code> 
!$OMP END <directive> 
                  # OR 
!$OMP <directive>
  <structured block of code> 
                  # OR 
!$OMP <directive>

The following example demonstrates one way of using an OpenMP* directive to parallelize a loop.

subroutine simple_omp(a, N)
  use omp_lib
  integer :: N, a(N) 
!$OMP PARALLEL DO
  do i = 1, N
    a(i) = i*2
  end do 
end subroutine simple_omp

Compile the Application

The [Q]openmp option enables the parallelizer to generate multi-threaded code based on the OpenMP* directives in the source. The code can be executed in parallel on single processor, multi-processor, or multi-core processor systems.

The [Q]openmp option works with both -O0 (Linux* and OS X*) and /Od (Windows*) and with any optimization level of O1, O2 and O3.

Specifying -O0 (Linux* and OS X*) or /Od (Windows*) with the OpenMP* option helps to debug OpenMP* applications.

Compile your application using commands similar to those shown below:

Assume that you compile the sample above, using commands similar to the following, where the c option instructs the compiler to compile the code without generating an executable:

The compiler might return a message similar to the following:

parallel.f90(20) : (col. 6) remark: OpenMP DEFINED LOOP WAS PARALLELIZED.

Configure the OpenMP* Environment

Before you run the multi-threaded code, you can set the number of desired threads using the OpenMP* environment variable, OMP_NUM_THREADS.