计算程序运行时间

最新推荐文章于 2024-04-10 10:29:46 发布

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#运行时间 #C++ #clock_t

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本文通过编写代码片段，对比编译器使用指针迭代和下标迭代实现字符串复制操作时的性能差异，评估两种方法在不同情况下的执行效率。

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测试编译器对于指针迭代和下标迭代是否生成相同的代码。

#include <iostream>
#include <time.h> // For clock()
#include <stddef.h> // For size_t
#include <string.h> // For memcpy()
 

void copy_with_pointers(char const *src, char *dst,
                        size_t n) {
   for (size_t k = 0; k!=n; ++k)
      *dst++ = *src++;
}

void copy_with_indices(char const *src, char *dst,
                       size_t n) {
   for (size_t k = 0; k!=n; ++k)
      dst[k] = src[k];
}

int main() {
   int const n_bytes = 100000;
   int const n_calls = 100;
   char *src = new char[n_bytes];
   char *dst = new char[n_bytes];

   // Reduce paging effects by accessing all bytes:
   memcpy(dst, src, n_bytes);
   clock_t start, end, reftime; // transform %s to div CLOCKS_PER_SEC 

   // Test 1 (reference time)
   start = clock();// 
   for (int k = 0; k!=n_calls; ++k)
      memcpy(dst, src, n_bytes);
   end = clock();
   reftime = end-start;
   std::cout << end - start << std::endl;

   // Test 2 (pointers)
   start = clock();
   for (int k = 0; k!=n_calls; ++k)
      copy_with_pointers(src, dst, n_bytes);
   end = clock();
   std::cout << end - start << std::endl;
   std::cout << "With pointers: "
        << static_cast<double>(end-start)/reftime
        << " times slower than memcpy.\n";

   // Test 3 (indices)
   start = clock();
   for (int k = 0; k!=n_calls; ++k)
      copy_with_indices(src, dst, n_bytes);
   end = clock();
   std::cout << end - start << std::endl;
   std::cout << "With indices: "
        << static_cast<double>(end-start)/reftime
        << " times slower than memcpy.\n";

   delete[] src;
   delete[] dst;
   return 0;
}

代码详解：

clock_t start, end;

start = clock();

/* code */

end = clcok();

times = end - start; //单位是clock_t（CPU时钟数） ,转换为秒需要除以CLOCKS_PER_SEC