In my source files of implementation of standard library I can see many methods with names prefixed with `__builtin_` i.e: `__builtin_memmove`. What is the meaning of this? In what sense are these methods *built in*?

```
template<bool _IsMove>
    struct __copy_move<_IsMove, true, random_access_iterator_tag>
    {
      template<typename _Tp>
        static _Tp*
        __copy_m(const _Tp* __first, const _Tp* __last, _Tp* __result)
        {
      __builtin_memmove(__result, __first,
                sizeof(_Tp) * (__last - __first));
      return __result + (__last - __first);
    }
    };

```

Are these just a call to binary code? Debugger cannot step into it.