visus
/
toolchain-gcc-arm-embedded

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2020 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Funding Free Software", the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
"GNU Free Documentation License".

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development. -->
<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Loop-Specific Pragmas (Using the GNU Compiler Collection (GCC))</title>

<meta name="description" content="Loop-Specific Pragmas (Using the GNU Compiler Collection (GCC))">
<meta name="keywords" content="Loop-Specific Pragmas (Using the GNU Compiler Collection (GCC))">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html#Top" rel="start" title="Top">
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Pragmas.html#Pragmas" rel="up" title="Pragmas">
<link href="Unnamed-Fields.html#Unnamed-Fields" rel="next" title="Unnamed Fields">
<link href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas" rel="prev" title="Function Specific Option Pragmas">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<a name="Loop_002dSpecific-Pragmas"></a>
<div class="header">
<p>
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> &nbsp; [<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>
</div>
<hr>
<a name="Loop_002dSpecific-Pragmas-1"></a>
<h4 class="subsection">6.62.17 Loop-Specific Pragmas</h4>

<dl compact="compact">
<dt><code>#pragma GCC ivdep</code></dt>
<dd><a name="index-pragma-GCC-ivdep"></a>

<p>With this pragma, the programmer asserts that there are no loop-carried
dependencies which would prevent consecutive iterations of
the following loop from executing concurrently with SIMD
(single instruction multiple data) instructions.
</p>
<p>For example, the compiler can only unconditionally vectorize the following
loop with the pragma:
</p>
<div class="smallexample">
<pre class="smallexample">void foo (int n, int *a, int *b, int *c)
{
  int i, j;
#pragma GCC ivdep
  for (i = 0; i &lt; n; ++i)
    a[i] = b[i] + c[i];
}
</pre></div>

<p>In this example, using the <code>restrict</code> qualifier had the same
effect. In the following example, that would not be possible. Assume
<em>k &lt; -m</em> or <em>k &gt;= m</em>. Only with the pragma, the compiler knows
that it can unconditionally vectorize the following loop:
</p>
<div class="smallexample">
<pre class="smallexample">void ignore_vec_dep (int *a, int k, int c, int m)
{
#pragma GCC ivdep
  for (int i = 0; i &lt; m; i++)
    a[i] = a[i + k] * c;
}
</pre></div>

</dd>
<dt><code>#pragma GCC unroll <var>n</var></code></dt>
<dd><a name="index-pragma-GCC-unroll-n"></a>

<p>You can use this pragma to control how many times a loop should be unrolled.
It must be placed immediately before a <code>for</code>, <code>while</code> or <code>do</code>
loop or a <code>#pragma GCC ivdep</code>, and applies only to the loop that follows.
<var>n</var> is an integer constant expression specifying the unrolling factor.
The values of <em>0</em> and <em>1</em> block any unrolling of the loop.
</p>
</dd>
</dl>


</body>
</html>