Add OpenMP* Support
To add OpenMP* support to your application, do the following:
- Add the appropriate OpenMPpragmasto your source code.
- Compile the application with the/Qopenmp(Windows*) or-qopenmp(Linux*) option.
- For applications with large local or temporary arrays, you may need to increase the stack space available at runtime. In addition, you may need to increase the stack allocated to individual threads by using theenvironment variable or by setting the corresponding library routines.OMP_STACKSIZE
You can set other environment variables to control multi-threaded code execution.
OpenMP
Pragma Syntax
Pragma
SyntaxTo add OpenMP support to your application, first
declare the OpenMP header and then
add appropriate OpenMP
pragmas
to your source code.
To declare the OpenMP header, add the following in your code:
#include <omp.h>
OpenMP
pragmas
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® oneAPI
.
DPC++/C++
CompilerThe following syntax illustrates using the
pragmas
in your source.
Example
|
---|
|
where:
- <prefix> - Required for all OpenMPpragmas. The prefix must be#pragma omp.
- <pragma> - A valid OpenMPpragma. Must immediately follow the prefix.
- [<clause>] - Optional. Clauses can be in any order and repeated as necessary, unless otherwise restricted.
- <newline> - A required component ofpragmasyntax. It precedes the structured block that is enclosed by thispragma.
The
pragmas
are interpreted as comments if you omit the
/Qopenmp
(Windows) or
-qopenmp
(Linux) option.
The following example demonstrates one way of using an OpenMP
pragma
to parallelize a loop.
Example
|
---|
|
Compile the Application
The
/Qopenmp
(Windows) or
-qopenmp
(Linux) option enables the parallelizer to generate multi-threaded code based on the OpenMP
pragmas
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) option works with both
-O0
(Linux) and
/Od
(Windows*) and with any optimization level of
O1
,
O2
and
O3
.
Specifying
-O0
(Linux) or
/Od
(Windows) with the
/Qopenmp
(Windows) or
-qopenmp
(Linux) option helps to debug OpenMP applications.
Compile your application using commands similar to those shown below:
Operating System
| Syntax Example
|
---|---|
Linux
| icpx -qopenmp
source_file |
Windows
| icx /Qopenmp
source_file |
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:
Operating System
| Extended Syntax Example
|
---|---|
Linux
| icpx -qopenmp -c parallel.cpp |
Windows
| icx /Qopenmp /c parallel.c |
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"
pragmas
must be compiled. For example:
Operating System
| Syntax Example
|
---|---|
Linux
| icpx -qopenmp -fopenmp-targets=spir64 offload.cpp |
Windows
| icx /Qopenmp /Qopenmp-targets=spir64 offload.c |
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