关于__builtin_expect

先上代码

11 #include <stdio.h>
12 
13 #define likely(x) __builtin_expect(!!(x), 1)
14 #define unlikely(x) __builtin_expect(!!(x), 0)
15 
16 int main(int argc, const char* argv[]) {
17     int count = 10;
18     if (likely(count > 1))
19         printf("It is ok\n");
20     else
21         printf("It isn't ok\n");
22 
23     int tmp = !!5;
24     printf("tmp = %d\n", tmp);
25 
26     tmp = !!0;
27     printf("tmp = %d\n", tmp);
28     return 0;
29 }

再上输出结果

It is ok
tmp = 1
tmp = 0

结论

1 __builtin_expect是gcc提供的，目的是为了优化代码
2 __builtin_expect(Exp, N)，意思是Exp == N的概率很大
3 !!的写法，是为了确保最后表达式的值 = 0 or 1