likely() unlikey()

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

#if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 96)
#define __builtin_expect(exp,c) (exp)
#endif

__builtin_expect在gcc的info手册中：

QUOTE:

-- Built-in Function: long __builtin_expect (long EXP, long C)
     You may use `__builtin_expect' to provide the compiler with branch
     prediction information.  In general, you should prefer to use
     actual profile feedback for this (`-fprofile-arcs'), as
     programmers are notoriously bad at predicting how their programs
     actually perform.  However, there are applications in which this
     data is hard to collect.

     The return value is the value of EXP, which should be an integral
     expression.  The value of C must be a compile-time constant.  The
      semantics of the built-in are that it is expected that EXP == C.
     For example:

          if (__builtin_expect (x, 0))
            foo ();

     would indicate that we do not expect to call `foo', since we
     expect `x' to be zero.  Since you are limited to integral
     expressions for EXP, you should use constructions such as

          if (__builtin_expect (ptr != NULL, 1))
            error ();

     when testing pointer or floating-point values.

Conclusion: 通知编译器处理条件分支时，将更可能发生的下一条指令安排在流水线预取流程上，充分利用CPU流水线，以便提高程序执行效率。