Compiler Warnings

[Olson, Arthur David (NIH/NCI) [E]]

Here’s an "increment_overflow" replacement that doesn't get any complaints from gcc.
How can it be portably simplified?

								--ado

static int
increment_overflow(ip, j)
register int * const	ip;
register int		j;
{
	register int	i, halfsum, halfmod, bothneg;

	i = *ip;
	*ip += j;
	if (i == 0 || j == 0) {
		return 0;
	}
	if ((i > 0) != (j > 0)) {
		return 0;
	}
	bothneg = i < 0;
	if (bothneg) {
		if (i == INT_MIN || j == INT_MIN)
			return 1;
		i = -i;
		j = -j;
	}
	halfsum = (i / 2) + (j / 2);
	halfmod = (i % 2) + (j % 2);
	if (halfmod == 2) {
		++halfsum;
		halfmod = 0;
	}
	if (bothneg) {
		if (halfsum < -(INT_MIN / 2))
			return 0;
		if (halfsum > -(INT_MIN / 2))
			return 1;
		return ((INT_MIN % 2) == 0) ? halfmod : 0;
	} else {
		if (halfsum < (INT_MAX / 2))
			return 0;
		if (halfsum > (INT_MAX / 2))
			return 1;
		return ((INT_MAX % 2) == 0) ? halfmod : 0;
	}
}