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