为什么这两个相同的函数具有截然不同的执行时间

Question

我有两个相互复制的C函数。在下面的代码中，我打印出它们执行所需的时间。第一个函数(无论它是哪个副本(总是比第二个函数需要更长的时间来执行。为什么？

#include <stdio.h>
#include <time.h> // for clock_t
int binsearch_old (int x, int v[], int n);
int binsearch_new (int x, int v[], int n);
void main ()
{
  int x = 4;
  int v[10] = { 1,2,3,4,5,6,7,8,9,10 };
  int n = 10;
  clock_t begin_old = clock();
  printf("nbinsearch_old :: position: %in", binsearch_old(x, v, n));
  clock_t end_old = clock();
  double time_spent_old = (double)(end_old - begin_old) / CLOCKS_PER_SEC;
  printf("time spent old: %fn", time_spent_old);
  clock_t begin_new = clock();
  printf("nbinsearch_new :: position: %in", binsearch_new(x, v, n));
  clock_t end_new = clock();
  double time_spent_new = (double)(end_new - begin_new) / CLOCKS_PER_SEC;
  printf("time spent new: %fn", time_spent_new);
}
int binsearch_old (int x, int v[], int n)
{
  int low, high, mid;
  low = 0;
  high = n - 1;
  while (low <= high) {
    mid = (low + high) / 2;
    if ( x < v[mid])
      high = mid - 1;
    else if (x > v[mid])
      low = mid + 1;
    else //found match
      return mid;
  }
  return -1; // no match
}
int binsearch_new (int x, int v[], int n)
{
  int low, high, mid;
  low = 0;
  high = n - 1;
  while (low <= high) {
    mid = (low + high) / 2;
    if (x < v[mid])
      high = mid - 1;
    else if (x > v[mid])
      low = mid + 1;
    else //found match
      return mid;
  }
  return -1; // no match
}

gcc test.c和./a.out之后，您将看到类似以下内容的内容：

binsearch_old :: position: 3
time spent old: 0.000115
binsearch_new :: position: 3
time spent new: 0.000007

而且那个时代之间的关系是稳定的！第一个总是比第二个大，而且通常相差很多。这是怎么回事？

Answer 1

您也在测量打印时间。 您不应该计算printf的执行时间。

  clock_t begin_old = clock();
  int val = binsearch_old(x, v, n);
  clock_t end_old = clock();
  printf("nbinsearch_old :: position: %in", val);
  double time_spent_old = (double)(end_old - begin_old) / CLOCKS_PER_SEC;
  printf("time spent old: %fn", time_spent_old);

如果你不算，那么你应该问自己why does printf have different times for similar calls ?.

Answer 2

实现print所需的所有功能的"预热"并非不可忽视。逐步浏览 C 运行时库源代码，亲自查看。因此，您向printf发出的第一次调用通常比后续调用慢。

对printf的调用在计时范围内，这是实现中的错误。

从计时中删除printf调用将等于结果。