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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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<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 (&quot;default&quot;)))
int foo ()
{
  // The default version of foo.
  return 0;
}

__attribute__ ((target (&quot;sse4.2&quot;)))
int foo ()
{
  // foo version for SSE4.2
  return 1;
}

__attribute__ ((target (&quot;arch=atom&quot;)))
int foo ()
{
  // foo version for the Intel ATOM processor
  return 2;
}

__attribute__ ((target (&quot;arch=amdfam10&quot;)))
int foo ()
{
  // foo version for the AMD Family 0x10 processors.
  return 3;
}

int main ()
{
  int (*p)() = &amp;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 &quot;default&quot; 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.
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