我明白了：

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

原因代码1200是：

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.

但为什么？

#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;
        }
    }
}