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<title>Function Multiversioning - Using the GNU Compiler Collection (GCC)</title>
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<a name="Function-Multiversioning"></a>
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Next: <a rel="next" accesskey="n" href="Namespace-Association.html#Namespace-Association">Namespace Association</a>,
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Previous: <a rel="previous" accesskey="p" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a>
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</div>
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<h3 class="section">7.8 Function Multiversioning</h3>
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<p><a name="index-function-versions-4235"></a>
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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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<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>
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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 Function Multiversioning</a> for more details.
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</body></html>
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