#define likely(x) __builtin_expect(!!(x), 1) 性能测试-白红宇

#define likely(x) __builtin_expect(!!(x), 1) 性能测试

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发布时间：2019-05-25

本文共 1454 字，大约阅读时间需要 4 分钟。

看linux源码或者一些优秀组件的源码，经常碰到likely和unlikely, 其实很简单，无非就是显式告诉编译器怎么去优化。有兴趣的话，可以看看对应的汇编。下面，我们来实际测试一下likely/unlikely的性能优化效果：

#include 
   
    #include
    
      #define likely(x) __builtin_expect(!!(x), 1)#define unlikely(x) __builtin_expect(!!(x), 0)#define N (1000 * 1000 * 1000)int64_t printTime(int64_t iT1) {	struct timeval t2;	gettimeofday(&t2, NULL);	int64_t iT2 = t2.tv_sec * 1000000 + t2.tv_usec;	int64_t gap = iT2 - iT1;	printf("gap is %lld\n", gap);	return t2.tv_sec * 1000000 + t2.tv_usec;}int fun1(){	int a = 0;	for (int i = 0; i < N; i++) 	{		if (i < 0)		{			a--;		}		else		{			a++;		}	}	return a;}int fun2(){	int a = 0;	for (int i = 0; i < N; i++) 	{		if (i >= 0) 		{			a++;		}		else		{			a--;		}	}	return a;}int fun3()  // 逻辑等价于fun1{	int a = 0;	for (int i = 0; i < N; i++) 	{		if (unlikely(i) < 0)		{			a--;		}		else		{			a++;		}	}	return a;}int fun4()  // 逻辑等价于fun2{	int a = 0;	for (int i = 0; i < N; i++) 	{		if (likely(i) >= 0) 		{			a++;		}		else		{			a--;		}	}	return a;}int main(){	struct timeval t;	int64_t iT = 0;	gettimeofday(&t, NULL);	fun1();	iT = printTime(t.tv_sec * 1000000 + t.tv_usec);	fun2();	iT = printTime(iT);	fun3();	iT = printTime(iT);	fun4();	iT = printTime(iT);	return 0;}

运行三次，结果：

taoge:~$ ./a.out

gap is 2777564

gap is 2432064

gap is 1788922

gap is 1872011

taoge:~$ ./a.out

gap is 2792285

gap is 2384819

gap is 1787595

gap is 1829156

taoge:~$ ./a.out

gap is 2798899

gap is 2423693

gap is 1786800

gap is 1830484

不多说。

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