The 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

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()

                                                                                                                                 from Linux Kernel Development Second Edition

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在linux中判斷語句經常會看到likely和unlikely，例如：

if(likely(value)){

}

else{

}

簡單從表面上看if(likely(value)) == if(value)，if(unlikely(value)) == if(value)。

也就是likely和unlikely是一樣的，但是實際上執行是不同的，加likely的意思是value的值為真的可能性更大一些，那麼執行if的機會大，而unlikely表示value的值為假的可能性大一些，執行else機會大一些。加上這種修飾，編譯成二進位制程式碼時likely使得if後面的執行語句緊跟著前面的程式，unlikely使得else後面的語句緊跟著前面的程式，這樣就會被cache預讀取，增加程式的執行速度，likely和unlikely的實現在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的一個預處理命令，其解釋如下：

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.