he gcc C compiler has a built-in directive that optimizes conditional branches as either very likely taken or very unlikely taken. The compiler uses the directive to appropriately optimize the branch. The kernel wraps the directive in very easy-to-use macros, likely() and unlikely().
For example, consider an if statement such as the following:
if (foo) {
/* ... */
}
To mark this branch as very unlikely taken (that is, likely not taken):
/* we predict foo is nearly always zero ... */
if (unlikely(foo)) {
/* ... */
}
Conversely, to mark a branch as very likely taken:
/* we predict foo is nearly always nonzero ... */
if (likely(foo)) {
/* ... */
}
You should only use these directives when the branch direction is overwhelmingly a known priori or when you want to optimize a specific case at the cost of the other case. This is an important point: These directives result in a performance boost when the branch is correctly predicted, but a performance loss when the branch is mispredicted. A very common usage for unlikely() and likely() is error conditions. As one might expect, unlikely() finds much more use in the kernel because if statements tend to indicate a special case.
from Linux Kernel Development Second Edition
---------------------------------------------------------------------------------------------
在linux中判断语句经常会看到likely和unlikely，例如：
if(likely(value)){
}
else{
}
简单从表面上看if(likely(value)) == if(value)，if(unlikely(value)) == if(value)。
也就是likely和unlikely是一样的，但是实际上执行是不同的，加likely的意思是value的值为真的可能性更大一些，那么执行if的机会大，而unlikely表示value的值为假的可能性大一些，执行else机会大一些。加上这种修饰，编译成二进制代码时likely使得if后面的执行语句紧跟着前面的程序，unlikely使得else后面的语句紧跟着前面的程序，这样就会被cache预读取，增加程序的执行速度，likely和unlikely的实现在include/linux/compiler.h中：
9 #if __GNUC__ == 2 && __GNUC_MINOR__ < 96
10 #define __builtin_expect(x, expected_value) (x)
11 #endif
12
13 #define likely(x) __builtin_expect((x),1)
14 #define unlikely(x) __builtin_expect((x),0)
__builtin_expect是gcc的一个预处理命令，其解释如下：
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.
转自：http://www.cnblogs.com/yangzd/archive/2010/09/27/1837202.html