c语言的If与Switch效率比较

Posted 2022-12-08 雪靡

最近看到文章¹表示Switch比If慢，这里就来测试一下是否属实。

立刻打开VS，编写以下代码：

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#define MAX_LOOP 100000000
int nums[5];
void switchFunc()

	int i, r;
	srand((unsigned)time(NULL));
	for (i = 0; i < MAX_LOOP; i++)
	
		r = rand() % 5;
		switch (r)
		
		case 3:
			nums[3]++;
			break;
		case 1:
			nums[1]++;
			break;
		case 2:
			nums[2]++;
			break;
		case 0:
			nums[0]++;
			break;
		default:
			nums[4]++;
			break;
		
	


void ifFunc()

	int i, r;
	srand((unsigned)time(NULL));
	for (i = 0; i < MAX_LOOP; i++)
	
		r = rand() % 5;
		if (r == 3)
			nums[3]++;
		else if (r == 1)
			nums[1]++;
		else if (r == 2)
			nums[2]++;
		else if (r == 0)
			nums[0]++;
		else
			nums[4]++;
	


int main()

	clock_t start, finish, i;
	double if_time, switch_time, if_total_time = 0, switch_total_time = 0;
	int test_time = 10;
	printf("Start test.\\n");
	printf(" \\tswitch time\\tif time\\n");
	for (i = 0; i < test_time; i++)
	
		memset(nums, 0, sizeof(nums));
		start = clock();
		switchFunc();
		finish = clock();
		switch_time = (double)1000.0 * (finish - start) / CLOCKS_PER_SEC;
		switch_total_time += switch_time;
		start = clock();
		ifFunc();
		finish = clock();
		if_time = (double)1000.0 * (finish - start) / CLOCKS_PER_SEC;
		if_total_time += if_time;
		printf("#%d\\t%0.0fms\\t\\t%0.0fms\\n", i + 1, switch_time, if_time);
	
	printf("\\n");
	printf("total\\t%0.0fms\\t\\t%0.0fms\\n", switch_total_time / test_time, if_total_time / test_time);
	return 0;

使用VS2022编译，在ReleaseX84环境下运行结果为：

Start test.
        switch time     if time
#1      3929ms          3673ms
#2      3590ms          3584ms
#3      3606ms          3587ms
#4      3604ms          3585ms
#5      3601ms          3589ms
#6      3589ms          3586ms
#7      3605ms          3584ms
#8      3596ms          3581ms
#9      3591ms          3592ms
#10     3605ms          3593ms

total   3632ms          3595ms

测试结果表明，switch确实比if慢。这个原因来自CPU 分支预测²。
编译器根据case的数量和case值的稀疏程度来翻译switch语句，当case情况比较多(例如4个以上)，并且值的范围跨度比较小时，就会使用跳转表。

构建的跳转表为

.text:00415A6C jpt_4159A7      dd offset $LN10_0       ; DATA XREF: _switchFunc+97↑r
.text:00415A6C                 dd offset $LN8          ; jump table for switch statement
.text:00415A6C                 dd offset $LN9_0
.text:00415A6C                 dd offset $LN7

增加了条件到0-9以后，swtich比if快。

修改后的代码如下：

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#define MAX_LOOP 100000000
int nums[10];
void switchFunc()

	int i, r;
	srand((unsigned)time(NULL));
	for (i = 0; i < MAX_LOOP; i++)
	
		r = rand() % 10;
		switch (r)
		
		case 8:
			nums[8]++;
			break;
		case 3:
			nums[3]++;
			break;
		case 7:
			nums[7]++;
			break;
		case 1:
			nums[1]++;
			break;
		case 4:
			nums[4]++;
			break;
		case 2:
			nums[2]++;
			break;
		case 0:
			nums[0]++;
			break;
		case 6:
			nums[6]++;
			break;
		case 5:
			nums[5]++;
			break;
		default:
			nums[9]++;
			break;
		
	


void ifFunc()

	int i, r;
	srand((unsigned)time(NULL));
	for (i = 0; i < MAX_LOOP; i++)
	
		r = rand() % 10;
		if (r == 8)
			nums[8]++;
		else if (r == 3)
			nums[3]++;
		else if (r == 7)
			nums[7]++;
		else if (r == 1)
			nums[1]++;
		else if (r == 4)
			nums[4]++;
		else if (r == 2)
			nums[2]++;
		else if (r == 0)
			nums[0]++;
		else if (r == 6)
			nums[6]++;
		else if (r == 5)
			nums[5]++;
		else
			nums[9]++;
	


int main()

	clock_t start, finish, i;
	double if_time, switch_time, if_total_time = 0, switch_total_time = 0;
	int test_time = 10;
	printf("Start test.\\n");
	printf(" \\tswitch time\\tif time\\n");
	for (i = 0; i < test_time; i++)
	
		memset(nums, 0, sizeof(nums));
		start = clock();
		switchFunc();
		finish = clock();
		switch_time = (double)1000.0 * (finish - start) / CLOCKS_PER_SEC;
		switch_total_time += switch_time;
		start = clock();
		ifFunc();
		finish = clock();
		if_time = (double)1000.0 * (finish - start) / CLOCKS_PER_SEC;
		if_total_time += if_time;
		printf("#%d\\t%0.0fms\\t\\t%0.0fms\\n", i + 1, switch_time, if_time);
	
	printf("\\n");
	printf("total\\t%0.0fms\\t\\t%0.0fms\\n", switch_total_time / test_time, if_total_time / test_time);
	return 0;

结果如下：

Start test.
        switch time     if time
#1      3962ms          3900ms
#2      3732ms          3787ms
#3      3750ms          3794ms
#4      3738ms          3794ms
#5      3727ms          3796ms
#6      3735ms          3785ms
#7      3729ms          3797ms
#8      3725ms          3799ms
#9      3744ms          3800ms
#10     3739ms          3784ms

total   3758ms          3804ms

结论

当条件分支比较少的情况下，if比较快，当条件分支比较多的情况下，swtich比较快。

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