An interesting feature of G++ 4.8:

http://gcc.gnu.org/wiki/FunctionMultiVersioning

It allows to write various versions of a function, for different CPUs.

To work it uses new built-in functions like __builtin_cpu_supports and __builtin_cpu_is, run-time dispatch, the attribute syntax, plus the syntax to specify target optimizations for each function:

http://gcc.gnu.org/wiki/FunctionSpecificOpt

Bye,
bearophile