C ++ - 使用Visual Studio进行奇怪的时间测量

Question

我试图计算编译器生成的一些低级汇编代码的性能。然而，在某些时刻，我得到了一些我不理解的奇怪结果。所以这是我的代码：

#include <iostream>
#include <chrono>
#include <vector>

//---------------------------------------------------------------------------------------

int main()
{
    std::srand(time(NULL));
    constexpr unsigned vectorSize = 1000u;
    constexpr unsigned loopCount = 1000000u;
    std::vector<int> vec1(vectorSize);
    std::vector<int> vec2(vectorSize);
    for (unsigned i = 0u; i < vectorSize; ++i)
    {
        vec1[i] = std::rand();
    }
    for (unsigned i = 0u; i < vectorSize; ++i)
    {
        vec2[i] = std::rand();
    }

    std::chrono::time_point<std::chrono::high_resolution_clock> start;
    std::chrono::time_point<std::chrono::high_resolution_clock> end;
    long long ms;

    //---------------------------------------------------------------------------------------
    start = std::chrono::high_resolution_clock::now();

    for (unsigned j = 0u; j < loopCount; ++j)
    {
        for (unsigned i = 0u; i < vec2.size(); ++i)
        {
            vec2[i] = vec1[i];
        }
    }

    end = std::chrono::high_resolution_clock::now();
    ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
    std::cout << "Evaluation took: " << ms << " ms" << std::endl;

    //---------------------------------------------------------------------------------------

    start = std::chrono::high_resolution_clock::now();

    for (unsigned j = 0u; j < loopCount; ++j)
    {
        for (unsigned i = 0u; i < vec2.size(); ++i)
        {
            vec2[i] = vec1[i];
        }
    }

    end = std::chrono::high_resolution_clock::now();
    ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
    std::cout << "Evaluation took: " << ms << " ms" << std::endl;

    //---------------------------------------------------------------------------------------

    start = std::chrono::high_resolution_clock::now();

    for (unsigned j = 0u; j < loopCount; ++j)
    {
        for (unsigned i = 0u; i < vec2.size(); ++i)
        {
            vec2[i] = vec1[i];
        }
    }

    end = std::chrono::high_resolution_clock::now();
    ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
    std::cout << "Evaluation took: " << ms << " ms" << std::endl;

    //---------------------------------------------------------------------------------------

    start = std::chrono::high_resolution_clock::now();

    for (unsigned j = 0u; j < loopCount; ++j)
    {
        for (unsigned i = 0u; i < vec2.size(); ++i)
        {
            vec2[i] = vec1[i];
        }
    }

    end = std::chrono::high_resolution_clock::now();
    ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
    std::cout << "Evaluation took: " << ms << " ms" << std::endl;

    //---------------------------------------------------------------------------------------

    start = std::chrono::high_resolution_clock::now();

    for (unsigned j = 0u; j < loopCount; ++j)
    {
        for (unsigned i = 0u; i < vec2.size(); ++i)
        {
            vec2[i] = vec1[i];
        }
    }

    end = std::chrono::high_resolution_clock::now();
    ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
    std::cout << "Evaluation took: " << ms << " ms" << std::endl;

    //---------------------------------------------------------------------------------------

    std::cout << "Press enter to exit..." << std::endl;
    std::cin.get();
    return 0;
}

主要＆＃34;工作＆＃34;在这个程序中，在循环中完成，其中向量vec1的元素被简单地分配给向量vec2。内部for循环条件旨在不允许编译器进行SIMD优化。我通过复制代码重复了5次代码，所以我希望每个拷贝都能得到或多或少相同的时间测量。但结果却不同：

  

评估时间：425毫秒
  评估时间：694毫秒
  评估耗时：462毫秒
  评估耗时：441毫秒
  评估耗时：710毫秒
  按enter退出...

在发布模式下使用Visual Studio 2015进行构建后的情况。令人惊讶的是，GCC的计算时间非常相似，但是使用Visual我每次都会得到这种奇怪的模式而且它总是一样的。

所以我想问一下我的代码是否以某种方式被破坏了，我错过了什么？或许这只是编译器奇怪的行为？ 在任何类型的基准测试中获得正确的时间测量非常重要，所以我不想从一开始就犯错误。