Why isn't the Visual C++ auto-vectorizer vectorizing this simple loop?

Question

I can't figure out why Visual C++ can't auto-vectorize this loop... any ideas?

I get:

testvec.cpp:12: info C5002: loop not vectorized due to reason '1200'

where reason code 1200 is:

Loop contains loop-carried data dependences that prevent vectorization. Different iterations of the loop interfere with each other such that vectorizing the loop would produce wrong answers, and the auto-vectorizer cannot prove to itself that there are no such data dependences.

But why?

#include <stdlib.h>

int main(int argc, char *argv[])
{
    int const n = argc;
    double
        *const p1 = (double *)malloc(n * n * sizeof(*p1)),
        *const p2 = (double *)malloc(n * n * sizeof(*p2));
    for (int j = 0; j < n; ++j)
    {
        double const sj = p1[n * j];
        for (int i = 0; i < n; ++i)
        {
            double const sum = p1[i] + sj, old = p1[i + n * j];
            p2[i + n * j] = sum < old ? sum : old;
        }
    }
}

Answer 1

I finally found how to fix it... seems like the multiplication in n * j is the culprit.
Hoisting it outside as int nj = n * j; and using nj in the inner loop instead fixes the problem.

I still don't know why this happens though.

If anyone knows, please post it!