<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
<html>
|
<!-- Copyright (C) 1988-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
|
<head>
|
<title>Using the GNU Compiler Collection (GCC): Function Multiversioning</title>
|
|
<meta name="description" content="Using the GNU Compiler Collection (GCC): Function Multiversioning">
|
<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Function Multiversioning">
|
<meta name="resource-type" content="document">
|
<meta name="distribution" content="global">
|
<meta name="Generator" content="makeinfo">
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
<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="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions">
|
<link href="Namespace-Association.html#Namespace-Association" rel="next" title="Namespace Association">
|
<link href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" rel="prev" title="C++ Attributes">
|
<style type="text/css">
|
<!--
|
a.summary-letter {text-decoration: none}
|
blockquote.smallquotation {font-size: smaller}
|
div.display {margin-left: 3.2em}
|
div.example {margin-left: 3.2em}
|
div.indentedblock {margin-left: 3.2em}
|
div.lisp {margin-left: 3.2em}
|
div.smalldisplay {margin-left: 3.2em}
|
div.smallexample {margin-left: 3.2em}
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
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.nocodebreak {white-space:nowrap}
|
span.nolinebreak {white-space:nowrap}
|
span.roman {font-family:serif; font-weight:normal}
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
ul.no-bullet {list-style: none}
|
-->
|
</style>
|
|
|
</head>
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
<a name="Function-Multiversioning"></a>
|
<div class="header">
|
<p>
|
Next: <a href="Namespace-Association.html#Namespace-Association" accesskey="n" rel="next">Namespace Association</a>, Previous: <a href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" accesskey="p" rel="prev">C++ Attributes</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</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>
|
</div>
|
<hr>
|
<a name="Function-Multiversioning-1"></a>
|
<h3 class="section">7.8 Function Multiversioning</h3>
|
<a name="index-function-versions"></a>
|
|
<p>With the GNU C++ front end, for x86 targets, you may specify multiple
|
versions of a function, where each function is specialized for a
|
specific target feature. At runtime, the appropriate version of the
|
function is automatically executed depending on the characteristics of
|
the execution platform. Here is an example.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">__attribute__ ((target ("default")))
|
int foo ()
|
{
|
// The default version of foo.
|
return 0;
|
}
|
|
__attribute__ ((target ("sse4.2")))
|
int foo ()
|
{
|
// foo version for SSE4.2
|
return 1;
|
}
|
|
__attribute__ ((target ("arch=atom")))
|
int foo ()
|
{
|
// foo version for the Intel ATOM processor
|
return 2;
|
}
|
|
__attribute__ ((target ("arch=amdfam10")))
|
int foo ()
|
{
|
// foo version for the AMD Family 0x10 processors.
|
return 3;
|
}
|
|
int main ()
|
{
|
int (*p)() = &foo;
|
assert ((*p) () == foo ());
|
return 0;
|
}
|
</pre></div>
|
|
<p>In the above example, four versions of function foo are created. The
|
first version of foo with the target attribute "default" is the default
|
version. This version gets executed when no other target specific
|
version qualifies for execution on a particular platform. A new version
|
of foo is created by using the same function signature but with a
|
different target string. Function foo is called or a pointer to it is
|
taken just like a regular function. GCC takes care of doing the
|
dispatching to call the right version at runtime. Refer to the
|
<a href="http://gcc.gnu.org/wiki/FunctionMultiVersioning">GCC wiki on
|
Function Multiversioning</a> for more details.
|
</p>
|
|
|
|
</body>
|
</html>
|
