Beginning with OpenMP

Wang Xianbing (updated: Tuesday, 10 February 2004)

OpenMP Program Development Tool

Intel C++ Compiler for Linux, installed in /opt/intel_cc_80 on access0 to access13.

The Intel C++ Compiler Usage

Invoking the Compiler from the Command Line

  1. set the environment

  2. invoke the compiler using icc or icpc. The icpc invocation assumes any preprocessed source files are C++ while the icc command assumes they are C.

Set the Environment Variables

    [user@access0]$ source /opt/intel_cc_80/bin/ccvars.sh


    or


    [user@access0]$ source /opt/intel_cc_80/bin/ccvars.csh

If you want the script to run automatically when you login, add the same command to the end of your startup file.

Sample .bash_profile entry for iccvars.sh:

#set environment vars for Intel C++ compiler
source /opt/intel_cc_80/bin/iccvars.sh

Invoking the Compiler with icc or icpc

Each invocation includes the C++ run-time libraries and header files. Use the -no_cpprt option if you do not want the C++ run-time libraries and headers.

Parallel Processing with OpenMP

To compile with OpenMP, you need to prepare your program by annotating the code with OpenMP directives. The Intel C++ Compiler first processes the application and produces a multithreaded version of the code which is then compiled. The output is a executable program with the parallelism implemented by threads that execute parallel regions or constructs.

Compiling with OpenMP

/home/rstaff/wangxb/test.c is an example program. First, copy the program into your home directory. Then, follow the steps below.

[user@access0]$ icc -openmp -c test.c
[user@access0]$ icc -openmp -o test test.o

-openmp Option

The -openmp option enables the parallelizer to generate multithreaded code based on the OpenMP directives. -openmp option works with both -O0 (no optimization) and any optimization level of -O1, -O2 (default) and -O3. Specifying -O0 with -openmp helps to debug OpenMP applications.

OpenMP Directive Format and Syntax

An OpenMP directive has the form:

#pragma omp directive-name [clause, ...] newline

where:

  • #pragma omp -- Required for all OpenMP directives.

  • directive-name -- A valid OpenMP directive. Must appear after the pragma and before any clauses.

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

  • newline -- Required. Proceeds the structured block which is enclosed by this directive.

OpenMP Diagnostics

The -openmp_report{0|1|2} option controls the OpenMP parallelizer's diagnostic levels 0, 1, or 2 as follows:

  • -openmp_report0 = no diagnostic information is displayed.

  • -openmp_report1 = display diagnostics indicating loops, regions, and sections successfully parallelized.

  • -openmp_report2 = same as -openmp_report1 plus diagnostics indicating MASTER constructs, SINGLE constructs, CRITICAL constructs, ORDERED constructs, ATOMIC directives, etc. are successfully handled.

The default is -openmp_report1

More Information

The official OpenMP website is www.OpenMP.org.