Intel® C++ Compiler 16.0 User and Reference Guide
To add OpenMP* support to your application, do the following:
Add the appropriate OpenMP* pragmas to your source code.
Compile the application with [Q]openmp option.
For applications with large local or temporary arrays, you may need to increase the stack space available at run-time. In addition, you may need to increase the stack allocated to individual threads by using the KMP_STACKSIZE environment variable or by setting the corresponding library routines.
You can set other environment variables to control multi-threaded code execution.
To 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® C++ Compiler.
The following syntax illustrates using the pragmas in your source.
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Example |
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<prefix> <pragma> [<clause>, ...] <newline> |
where:
<prefix> - Required for all OpenMP* pragmas. The prefix must be #pragma omp.
<pragma> - A valid OpenMP* pragma. Must immediately follow the prefix.
[<clause>] - Optional. Clauses can be in any order and repeated as necessary, unless otherwise restricted.
[<newline>] - A required component of pragma syntax. It precedes the structured block which is enclosed by this pragma.
The pragmas are interpreted as comments if you omit the [Q]openmp option.
The following example demonstrates one way of using an OpenMP* pragma to parallelize a loop.
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Example |
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#include <omp.h>
void simple_omp(int *a){
int i;
#pragma omp parallel for
for (i=0; i<1024; i++)
a[i] = i*2;
}
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The [Q]openmp 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 [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:
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Operating System |
Syntax Example |
|---|---|
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Linux* |
icc -openmp source_file |
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OS X* |
icl -openmp source_file // with CLANG compiler icc -openmp source_file // with EDG compiler |
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Windows* |
icl /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:
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Operating System |
Extended Syntax Example |
|---|---|
|
Linux* |
icc -openmp -c parallel.cpp |
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OS X* |
icl -openmp -c parallel.cpp // with CLANG compiler icc -openmp -c parallel.cpp // with EDG compiler |
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Windows* |
icl /Qopenmp /c parallel.cpp |
The compiler might return a message similar to the following:
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Example |
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parallel.cpp(20) : (col. 3) 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.