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

      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 <stdio.h>
#include<sys/time.h> 
#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。