为什么分配给统一变量变慢

Question

在下面的代码中，我动态分配了两个int数组，并且只初始化其中一个。
为什么将值分配给初始化变量比将其分配给未初始化变量更快？ （约0.4纳秒，约4 ns，使用带有-O3标志的g ++ 7.4.0编译，n = 10000）

#include <chrono>
#include <iostream>

#define NOW std::chrono::high_resolution_clock::now
#define DURATION_CAST(x)                                                       \
    std::chrono::duration_cast<std::chrono::nanoseconds>(x).count()

int main(int argc, char* argv[])
{
    int n = atoi(argv[1]);
    srand(time(nullptr));

    int* x = new int[n];
    int* y = new int[n];

    for (int i = 0; i < n; ++i)
    {
        x[i] = rand() / (RAND_MAX / 2);
    }

    auto t0 = NOW();
    for (int i = 0; i < n; ++i)
        x[i] = 1;

    auto t1 = NOW();
    for (int i = 0; i < n; ++i)
        y[i] = 1;

    auto t2 = NOW();
    double dt1 = DURATION_CAST(t1 - t0);
    double dt2 = DURATION_CAST(t2 - t1);

    std::cout << "Average time: " << dt1 / n << " ns" << std::endl;
    std::cout << "Average time: " << dt2 / n << " ns" << std::endl;

    delete[] x;
    delete[] y;

    return 0;
}

Here's a version with additional print statements，以防编译器优化重要部分。