Two Linux Kernel Macros: likely and unlikely

These two macro is defined in /include/linux/complier.h.

/* Somewhere in the middle of the GCC 2.96 development cycle, we implemented
   a mechanism by which the user can annotate likely branch directions and
   expect the blocks to be reordered appropriately.  Define __builtin_expect
   to nothing for earlier compilers.  */

#if __GNUC__ == 2 && __GNUC_MINOR__ < 96
#define __builtin_expect(x, expected_value) (x)
#endif

#define likely(x)    __builtin_expect((x),1)
#define unlikely(x)    __builtin_expect((x),0)

(the above source code extracted from linux kernel code version 2.4.37 )

Thanks to the __builtbin_expect(a, b) is a comparion operator, and it's defination may be as fellow:
#define __builtbin_expect(a, b) (((a) == (b)) ? (1) : (0)), we can define likely and unlikely as fellow:
#define likely(x) ((!!(x) == (1)) ? (1) : (0))
#define unlikely(x) ((!!(x) == (0)) ? (1) : (0))

useful link:
http://mail.nl.linux.org/kernelnewbies/2006-10/msg00045.html