test.cpp

#include <stdio.h>
#include <vector>
#include <time.h>

using namespace std;

void fun1(int *x, int n)
{
    for (int i = 1; i<= n; i++)
        x[i] += x[i - 1];
}

void fun2(int *x, int n)
{
    for (int i = 0; i < n; i++)
        x[i + 1] += x[i];
}

int main() {
    timespec before, after;
    vector<int> x(100000000);
    clock_gettime(CLOCK_MONOTONIC_RAW, &before);
    fun1(x.data(), x.size());
    clock_gettime(CLOCK_MONOTONIC_RAW, &after);

    printf("%d\n", (after.tv_sec - before.tv_sec) * 1000*1000*1000 + after.tv_nsec - before.tv_nsec);

    clock_gettime(CLOCK_MONOTONIC_RAW, &before);
    fun2(x.data(), x.size());
    clock_gettime(CLOCK_MONOTONIC_RAW, &after);

    printf("%d\n", (after.tv_sec - before.tv_sec) * 1000*1000*1000 + after.tv_nsec - before.tv_nsec);
}