C/C++变量值交换的两种方式

#include <stdio.h>
#include <time.h>
// 使用中间变量
time_t fun1()
{
	time_t t1 = time(0);
	int a = 1, b = 2;
	for (int i = 0; i < 1000000000 ; ++i)
	{	
		int c = a;
		a = b;
		b = c;
	}
	return time(0) - t1;
}
// 使用异或 原理：交换律 ( a ^ b) ^ b = a ^ (b ^ b) = a ^ 0 = a
time_t fun2()
{
	time_t t1 = time(0);
	int a = 1, b = 2;
	for (int i = 0; i < 1000000000 ; ++i)
	{
		a = a ^ b;
		b = a ^ b;
		a = a ^ b;
	}
	return time(0) - t1;
}

int main(int argc, char **argv)
{
	printf("fun1 use time %u\n",fun1());
	printf("fun2 use time %u\n",fun2());

	printf("press any key to exit...\n");
	getchar();
	return 0;
}

目测效率还是中间变量方式更高一些。

但是Why？