sign changes when going from int to float and back

Consider the following code, which is an SSCCE of my actual problem:

#include <iostream>

int roundtrip(int x)
{
    return int(float(x));
}

int main()
{
    int a = 2147483583;
    int b = 2147483584;
    std::cout << a << " -> " << roundtrip(a) << '\n';
    std::cout << b << " -> " << roundtrip(b) << '\n';
}

The output on my computer (Xubuntu 12.04.3 LTS) is:

2147483583 -> 2147483520
2147483584 -> -2147483648

Note how the positive number b ends up negative after the roundtrip. Is this behavior well-specified? I would have expected int-to-float round-tripping to at least preserve the sign correctly...

Hm, on ideone, the output is different:

2147483583 -> 2147483520
2147483584 -> 2147483647

Did the g++ team fix a bug in the meantime, or are both outputs perfectly valid?