Add OpenMP* Support

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 that is enclosed by this directive.

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

The directives are interpreted as comments if you omit the /Qopenmp (Windows) or -qopenmp (Linux and macOS*) 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 /Qopenmp (Windows) or -qopenmp (Linux and macOS*) 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 /Qopenmp (Windows) or -qopenmp (Linux and macOS*) option works with both -O0 (Linux and macOS*) and /Od (Windows*) and with any optimization level of O1, O2 and O3.

Specifying -O0 (Linux and macOS*) or /Od (Windows) with the /Qopenmp (Windows) or -qopenmp (Linux and macOS*) 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:

To build your application with target offload support (introduced since OpenMP 4.0) use compiler options to specify the target for which the regions marked with OpenMP "target" directives must be compiled. For example:

Refer to Get Started with OpenMP* Offload to GPU for the Intel® oneAPI DPC/C++ Compiler and Intel® Fortran Compiler for more information.

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.