144 lines
6.5 KiB
HTML
144 lines
6.5 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<!-- Copyright (C) 1999-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The GNU Fortran Compiler: OpenMP</title>
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<meta name="description" content="The GNU Fortran Compiler: OpenMP">
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<meta name="keywords" content="The GNU Fortran Compiler: OpenMP">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" rel="up" title="Extensions implemented in GNU Fortran">
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<link href="OpenACC.html#OpenACC" rel="next" title="OpenACC">
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<link href="CONVERT-specifier.html#CONVERT-specifier" rel="prev" title="CONVERT specifier">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="OpenMP"></a>
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<div class="header">
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<p>
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Next: <a href="OpenACC.html#OpenACC" accesskey="n" rel="next">OpenACC</a>, Previous: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="p" rel="prev">CONVERT specifier</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="OpenMP-1"></a>
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<h4 class="subsection">6.1.16 OpenMP</h4>
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<a name="index-OpenMP-1"></a>
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<p>OpenMP (Open Multi-Processing) is an application programming
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interface (API) that supports multi-platform shared memory
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multiprocessing programming in C/C++ and Fortran on many
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architectures, including Unix and Microsoft Windows platforms.
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It consists of a set of compiler directives, library routines,
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and environment variables that influence run-time behavior.
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</p>
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<p>GNU Fortran strives to be compatible to the
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<a href="http://openmp.org/wp/openmp-specifications/">OpenMP Application Program Interface v4.0</a>.
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</p>
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<p>To enable the processing of the OpenMP directive <code>!$omp</code> in
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free-form source code; the <code>c$omp</code>, <code>*$omp</code> and <code>!$omp</code>
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directives in fixed form; the <code>!$</code> conditional compilation sentinels
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in free form; and the <code>c$</code>, <code>*$</code> and <code>!$</code> sentinels
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in fixed form, <code>gfortran</code> needs to be invoked with the
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<samp>-fopenmp</samp>. This also arranges for automatic linking of the
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GNU Offloading and Multi Processing Runtime Library
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<a href="http://gcc.gnu.org/onlinedocs/libgomp/index.html#Top">libgomp</a> in <cite>GNU Offloading and Multi Processing Runtime
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Library</cite>.
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</p>
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<p>The OpenMP Fortran runtime library routines are provided both in a
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form of a Fortran 90 module named <code>omp_lib</code> and in a form of
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a Fortran <code>include</code> file named <samp>omp_lib.h</samp>.
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</p>
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<p>An example of a parallelized loop taken from Appendix A.1 of
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the OpenMP Application Program Interface v2.5:
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</p><div class="smallexample">
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<pre class="smallexample">SUBROUTINE A1(N, A, B)
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INTEGER I, N
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REAL B(N), A(N)
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!$OMP PARALLEL DO !I is private by default
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DO I=2,N
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B(I) = (A(I) + A(I-1)) / 2.0
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ENDDO
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!$OMP END PARALLEL DO
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END SUBROUTINE A1
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</pre></div>
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<p>Please note:
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</p><ul>
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<li> <samp>-fopenmp</samp> implies <samp>-frecursive</samp>, i.e., all local arrays
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will be allocated on the stack. When porting existing code to OpenMP,
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this may lead to surprising results, especially to segmentation faults
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if the stacksize is limited.
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</li><li> On glibc-based systems, OpenMP enabled applications cannot be statically
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linked due to limitations of the underlying pthreads-implementation. It
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might be possible to get a working solution if
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<code>-Wl,--whole-archive -lpthread -Wl,--no-whole-archive</code> is added
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to the command line. However, this is not supported by <code>gcc</code> and
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thus not recommended.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="OpenACC.html#OpenACC" accesskey="n" rel="next">OpenACC</a>, Previous: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="p" rel="prev">CONVERT specifier</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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