为什么这个矩阵乘法算法比另一个更快？

Question

int mmult_omp(double *c,
           double *a, int aRows, int aCols,
           double *b, int bRows, int bCols, int numThreads)
{
  for (i = 0; i < aRows; i++) {
for (j = 0; j < bCols; j++) {
  c[i*bCols + j] = 0;
}
for (k = 0; k < aCols; k++) {
  for (j = 0; j < bCols; j++) {
                c[i*bCols + j] += a[i*aCols + k] * b[k*bCols + j];
  }
}

}

for (i = 0; i < aRows; i++) {
    for (j = 0; j < bCols; j++) {
    c[i*bCols + j] = 0;
    for (k = 0; k < aCols; k++) {
    c[i*bCols + j] += a[i*aCols + k] *  b[k*bCols + j];
  }
}

}

Why is the first algorithm faster than the second? I've used C's time library and the first algorithm is objectively faster than the second. Why is that?

Answer 1

This code is very hard to understand. I had to copy it and reformat it to see what loops were what. I'm not really sure why one is faster but here's a great resource to see why.

Here are links to inspect the assembly output:

link for #1
link for #2