const static inline volatile unsigned char* __attribute__((always_inline)) World(void)
{
     register volatile const char * const str = "Hello World!\n";
     return str;   
}
const static inline size_t __attribute__((always_inline)) Size(const volatile unsigned char * restrict str)
{     
     return (const volatile size_t)strlen((const char *)str);   
}
const static void inline __attribute__((always_inline)) Hello(const volatile unsigned char * restrict str)
{    
     write(1, (const void *)str, Size(str));
}
extern signed int main(void) 
{    
     Hello(World());     
     return 0;
}

Главное это не заблудиться между restict, const * const и volatile :)