140 lines
5.3 KiB
HTML
140 lines
5.3 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<head>
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<title>Using the GNU Compiler Collection (GCC): Function Multiversioning</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Function Multiversioning">
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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="Namespace-Association.html#Namespace-Association" rel="next" title="Namespace Association">
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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="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>
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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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