为什么`do {} while(0);`这么快？

Question

我尝试了以下3个for循环:

#define loop 1000000000
NSDate *start;
NSDate *end;


// 1: empty for loop
start = [NSDate date];
for (NSInteger i = 0; i < loop; i++) {
}
end = [NSDate date];
NSLog(@"time interval: %f", [end timeIntervalSince1970] - [start timeIntervalSince1970]);


// 2: do-while for loop
start = [NSDate date];
for (NSInteger i = 0; i < loop; i++) {
    do {
    } while (0);
}
end = [NSDate date];
NSLog(@"time interval: %f", [end timeIntervalSince1970] - [start timeIntervalSince1970]);

// 3: @try-@finally for loop
start = [NSDate date];
for (NSInteger i = 0; i < loop; i++) {
    @try {
    }
    @finally {
    }
}
end = [NSDate date];
NSLog(@"time interval: %f", [end timeIntervalSince1970] - [start timeIntervalSince1970]);

我for在'07 Macbook Pro上测试了这三个循环大约10亿个.我记录了时间戳来计算执行持续时间.结果如下:

1: empty for loop => 2.947088 sec
2: do-while for loop => 2.581905 sec
3: @try-@finally for loop => 4.216685 sec

什么？do-while for循环比空for循环快!

为什么？

---

更新

我在for循环中添加了一些额外的代码(j ++):

NSInteger j;
#define loop 1000000000

// 1: empty for loop
j = 0
for (NSInteger i = 0; i < loop; i++) {
    j++;
}

// 2: do-while for loop
j = 0;
for (NSInteger i = 0; i < loop; i++) {
    do {
        j++;
    } while (0);
}

// 3: @try-@finally for loop
j = 0;
for (NSInteger i = 0; i < loop; i++) {
    @try {
        j++;
    }
    @finally {
    }
}

输出:

1: empty for loop => 2.590103 sec
2: do-while for loop => 2.138528 sec
3: @try-@finally for loop => 3.983589 sec

这3个比以前的代码快得多,而do-while for循环仍然是最快的.奇怪的!

Answer 1

不是.输出很大程度上取决于所使用的特定编译器以及有效的优化设置.任何值得盐的编译器都会优化掉两个循环.当我编译以下示例代码时,Clang和GCC都会在任何非零优化级别(即-O1上面)完全删除循环:

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

#define loop 1000000000

int main(void)
{
  struct timeval t1, t2, t3;
  gettimeofday(&t1, NULL);

  for (long i = 0; i < loop; i++) {
  }

  gettimeofday(&t2, NULL);

  for (long i = 0; i < loop; i++) {
    do {
    } while (0);
  }

  gettimeofday(&t3, NULL);

  int64_t d1 = (t2.tv_sec - t1.tv_sec) * 1000000ll + (t2.tv_usec - t1.tv_usec);
  int64_t d2 = (t3.tv_sec - t2.tv_sec) * 1000000ll + (t3.tv_usec - t2.tv_usec);

  printf("Empty for loop:    %lld.%06d\n", d1 / 1000000, (int)(d1 % 1000000));
  printf("do-while for loop: %lld.%06d\n", d2 / 1000000, (int)(d2 % 1000000));

  return 0;
}

在优化级别0( -O0,未优化的代码),这两个锵和其中或者两个环以相同的速度(在实验误差之内)运行GCC产生代码,或do-while循环运行稍慢由于额外的未优化的代码.我使用Clang 4.1和GCC 4.2.1在64位Mac上的结果:

Clang, 32-bit, -O0:
Empty for loop:    2.632714
do-while for loop: 2.633194

Clang, 64-bit, -O0:
Empty for loop:    2.632078
do-while for loop: 2.632046

Clang, 32-bit, -O1:
Empty for loop:    0.000000
do-while for loop: 0.000000

Clang, 64-bit, -O1:
Empty for loop:    0.000000
do-while for loop: 0.000000

GCC, 32-bit, -O0:
Empty for loop:    2.633221
do-while for loop: 2.633754

GCC, 64-bit, -O0:
Empty for loop:    2.778056
do-while for loop: 2.983421  (!!!)

GCC, 32-bit, -O1:
Empty for loop:    0.000001
do-while for loop: 0.000000

GCC, 64-bit, -O1:
Empty for loop:    0.000000
do-while for loop: 0.000000