NoteTo run the Intel® compiler in OpenMP mode, invoke the compiler with the -openmp (Linux* and Mac OS*) or /Qopenmp (Windows*) option using a command structured similar to the following:
Platform |
Description |
|---|---|
|
Linux and Mac OS |
ifort -openmp input_file |
|
Windows |
ifort /Qopenmp input_file |
Before you run the multi-threaded code, you can set the number of desired threads in the OpenMP environment variable, OMP_NUM_THREADS. For more information, see the OpenMP Environment Variables section. The Intel Extension Routines topic describes the OpenMP extensions to the specification that have been added by Intel to the Intel® compiler.
The -openmp (Linux and Mac OS) or /Qopenmp (Windows) option enables the parallelizer to generate multithreaded code based on the OpenMP directives. The code can be executed in parallel on uniprocessor, multiprocessor, and dual-core processor systems.
The openmp option works with both -O0 (Linux and Mac OS) and /Od (Windows) and with any optimization level of -O1, -O2 and -O3. (Linux and Mac OS) or /O1, /O2 and /O3 (Windows).
Specifying -O0 (Linux and Mac OS) or /Od (Windows) with the OpenMP option helps to debug OpenMP applications.
NoteIntel® Itanium®-based systems: Specifying this option implies -opt-mem-bandwith1 (Linux and Mac OS) or /Qopt-mem-bandwidth1 (Windows).
OpenMP directives use a specific format and syntax. The following syntax and example help illustrate how to use the directives with your source.
Syntax |
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<prefix> <directive> [<clause>[[,]<clause>...]] |
where:
<prefix>: Required for OpenMP. For fixed form source input, the prefix is !$OMP or C$OMP. For free form source input, the prefix is !$OMP only.
<directive>: Required must immediately follow the prefix; for example: !$OMP PARALLEL
[<clause>]: Must be specified if a directive uses one or more clauses
[,]: Commas between the <clause>s are optional.
Since OpenMP directives begin with an exclamation point, the directives are interpreted as comments if you omit the openmp option.
The following example demonstrates one way of using an OpenMP* directive to parallelize loops within parallel regions.
Example |
|---|
|
subroutine simple_omp(a, N) integer :: N, a(N) !$OMP PARALLEL DO do i = 1, N a(i) = i*2 end do end subroutine simple_omp |
Assume that you compile the sample above, using the commands similar to the following, where -c (Linux and Mac OS) or /c (Windows) instructs the compiler to compile the code without generating an executable:
Platform |
Description |
|---|---|
|
Linux and Mac OS |
ifort -openmp -c parallel.f90 |
|
Windows |
ifort /Qopenmp /c parallel.f90 |
The compiler might return a message similar to the following:
parallel.f90(20) : (col. 6) remark: OpenMP DEFINED LOOP WAS PARALLELIZED.
The OpenMP constructs defining a parallel region have one of the following syntax forms:
Example |
|---|
|
!$OMP <directive> <structured block of code> !$OMP END <directive> or !$OMP <directive> <structured block of code> or !$OMP <directive> |
where <directive> is the name of a particular OpenMP directive.