Why is sorting a std::vector of std::tuple's faster than sorting a vector of std::arrays?

Question

I was curious to see whether sorting a vector <vector<int>> would be slower than sorting a vector <array <int, 3>>. The dimensions of the vector is 1000000 by 3, and below is my driver code implementing this:

#include <iostream>
#include <fstream>
#include <cmath>
#include <algorithm>
#include <vector>

using namespace std;

int main()
{
    vector <vector<int>> v(1000000, vector <int> (3));

    srand(time(nullptr));
    for(int i = 0; i < 1000000; ++i){
        for(int j = 0; j < 3; ++j){
            v[i][j] = rand();
        }
    }

    double start = clock();
    sort(v.begin(), v.end());
    cout << (clock()-start)/(CLOCKS_PER_SEC/1000) << endl;

    return 0;
}

Compiling with g++ -O3 sorting_test.cxx with gcc 7.5.0, I get a runtime of around 300 ms. Declaring v as a vector <array <int, 3>> halved the runtime to around 149 ms.

However, declaring v as a vector <tuple<int, int, int>> beat out both of the above options, with an average runtime of approximately 100 ms.

I can somewhat understand why the array option is faster than the vector option (array size is a constant expression, unlike the vector), but I have no idea why the tuple would beat both of them. Can somebody please explain this to me?

The code that fills the tuple <int, int, int>s is

srand(time(nullptr));
for(int i = 0; i < 1000000; ++i){
    get <0> (v[i]) = rand();
    get <1> (v[i]) = rand();
    get <2> (v[i]) = rand();
}

Answer 1

While the disassembly for the whole programs are too large, this demonstrates the core difference between operator< for array and tuple: https://godbolt.org/z/h1Y33e

Essentially, in the tuple version, you have a fixed comparison of 3 elements whereas in the array version, you have a loop.

Though I'm surprised that the compiler did not unroll the loop.

Edit: looks like clang does optimize them to both, non-loop code: https://godbolt.org/z/cMExTb (I did not fully read it, but I only see forward jumps)