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| <h3 class="section">7.8 Function Multiversioning</h3> |
| |
| <p><a name="index-function-versions-3527"></a> |
| With the GNU C++ front end, for target i386, 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. |
| |
| <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> |
| <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. |
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