numeric.c:1406
static VALUE
fix_div(x, y)
    VALUE x, y;
{
    if (FIXNUM_P(y)) {
	long div;

	fixdivmod(FIX2LONG(x), FIX2LONG(y), &div, 0);
	return LONG2NUM(div);
    }
    return rb_num_coerce_bin(x, y);
}
