test.c

void func1(int n, double *mat, double *sum) {
  int col, row;
  for (col = 0; col < n; col++) {
    sum[col] = 0;
  }

  for (col = 0; col < n; col++) {
    for (row = 0; row < n; row++) {
      sum[col] += mat[n * row + col];
    }
  }
}

void func2(int n, double *mat, double *sum) {
  int col, row;
  for (col = 0; col < n; col++) {
    sum[col] = 0;
  }

  for (row = 0; row < n; row++) {
    for (col = 0; col < n; col++) {
      sum[col] += mat[n * row + col];
    }
  }
}

void func3(int n, double *mat, double *sum, int B) {
  int col, row, b;
  for (col = 0; col < n; col++) {
    sum[col] = 0;
  }

  for (col = 0; col < n; col += B) {
    for (row = 0; row < n; row++) {
      for (b = 0; b < B; b++) {
        sum[col + b] += mat[n * row + (col + b)];
      }
    }
  }
}