This loop has less efficient memory access patterns than a nearby outer loop. To fix: Reorder the loops if possible.

Example (original code)

…
for (int j = 0; j < N; j++)
    for (int k = 0; k < N; k++)
        c[i][j] = c[i][j] + a[i][k] * b[k][j];
…


void matmul(float *a[], float *b[], float *c[], int N) {
    for (int i = 0; i < N; i++)
        for (int j = 0; j < N; j++)
            for (int k = 0; k < N; k++)
                c[i][j] = c[i][j] + a[i][k] * b[k][j];
}

Example (revised code)

…
for (int k = 0; k < N; k++)
    for (int j = 0; j < N; j++)
        c[i][j] = c[i][j] + a[i][k] * b[k][j];
…


void matmul(float *a[], float *b[], float *c[], int N) {
    for (int i = 0; i < N; i++)
        for (int k = 0; k < N; k++)
            for (int j = 0; j < N; j++)
                c[i][j] = c[i][j] + a[i][k] * b[k][j];
}

Interchanging is not always possible because of dependencies, which can lead to different results.

Reorder loops