This is equivalent to

  while (1) {
    calc(x++);
  }

which is an infinite loop, since the expression x <= max is always true

Heh! Good catch! That is what I get for not testing. After UINT_MAX, x wraps around to 0.

Or if you only wanted to iterate halfway:

    unsigned int x = 0;

    while (x < ~x) {
        calc(x++);
    }

Or going down:

    unsigned int x = ~0;

    while (x > ~x) {
        calc(x--);
    }

By "halfway", did you mean "off by one"?

no