pjscott

  for (long i = 2; i <= rounds + 2; i++) {
      x *= -1.0;
      pi += x / (2.0 * i - 1.0);
  }

  rounds += 2u; // do this outside the loop
  for (unsigned i=2u; i < rounds; ++i) // use ++i instead of i++
  {
      double x = -1.0 + 2.0 * (i & 0x1); // allows vectorization
      pi += (x / (2u * i - 1u)); // double / unsigned = double
  }