Comment by rzimmerman

Try with gcc 6.2 options -Ofast -march=native:

  int square(int num) {
    int a = 0;
    for (int x = 0; x < num; x+=2) {
      if (!(x % 2)) {
        a += x;
      }
   }
    return a;
  }

All kinds of loop unrolling and vector instructions. Now remove the "!"