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133 lines
5.1 KiB
HTML
133 lines
5.1 KiB
HTML
4 years ago
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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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<html>
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<!-- Copyright (C) 1988-2018 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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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Loop-Specific Pragmas (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Loop-Specific Pragmas (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Loop-Specific Pragmas (Using the GNU Compiler Collection (GCC))">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Pragmas.html#Pragmas" rel="up" title="Pragmas">
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<link href="Unnamed-Fields.html#Unnamed-Fields" rel="next" title="Unnamed Fields">
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<link href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas" rel="prev" title="Function Specific Option Pragmas">
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</style>
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</head>
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<body lang="en">
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<a name="Loop_002dSpecific-Pragmas"></a>
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<div class="header">
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<p>
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Previous: <a href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas" accesskey="p" rel="prev">Function Specific Option Pragmas</a>, Up: <a href="Pragmas.html#Pragmas" accesskey="u" rel="up">Pragmas</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="Loop_002dSpecific-Pragmas-1"></a>
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<h4 class="subsection">6.61.16 Loop-Specific Pragmas</h4>
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<dl compact="compact">
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<dt><code>#pragma GCC ivdep</code></dt>
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<dd><a name="index-pragma-GCC-ivdep"></a>
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<p>With this pragma, the programmer asserts that there are no loop-carried
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dependencies which would prevent consecutive iterations of
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the following loop from executing concurrently with SIMD
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(single instruction multiple data) instructions.
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</p>
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<p>For example, the compiler can only unconditionally vectorize the following
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loop with the pragma:
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</p>
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<div class="smallexample">
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<pre class="smallexample">void foo (int n, int *a, int *b, int *c)
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{
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int i, j;
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#pragma GCC ivdep
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for (i = 0; i < n; ++i)
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a[i] = b[i] + c[i];
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}
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</pre></div>
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<p>In this example, using the <code>restrict</code> qualifier had the same
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effect. In the following example, that would not be possible. Assume
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<em>k < -m</em> or <em>k >= m</em>. Only with the pragma, the compiler knows
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that it can unconditionally vectorize the following loop:
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</p>
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<div class="smallexample">
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<pre class="smallexample">void ignore_vec_dep (int *a, int k, int c, int m)
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{
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#pragma GCC ivdep
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for (int i = 0; i < m; i++)
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a[i] = a[i + k] * c;
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}
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</pre></div>
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</dd>
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<dt><code>#pragma GCC unroll <var>n</var></code></dt>
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<dd><a name="index-pragma-GCC-unroll-n"></a>
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<p>You can use this pragma to control how many times a loop should be unrolled.
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It must be placed immediately before a <code>for</code>, <code>while</code> or <code>do</code>
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loop or a <code>#pragma GCC ivdep</code>, and applies only to the loop that follows.
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<var>n</var> is an integer constant expression specifying the unrolling factor.
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The values of <em>0</em> and <em>1</em> block any unrolling of the loop.
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</p>
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</dd>
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</dl>
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</body>
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</html>
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