ISPC surprising code-gen edge case

gistfile1.txt

export void good(uniform int output[], uniform const int a[], uniform const int b[], uniform unsigned int count)
{
        count &= ~15;
        foreach (i = 0 ... count)
        {
                // This function works as you'd expect
                int x = a[i];
                int y = b[i];
                output[i] = (x > 0) ? x : y;
        }

}

export void bad(uniform int output[], uniform const int a[], uniform const int b[], uniform unsigned int count)
{
        count &= ~15;
        foreach (i = 0 ... count)
        {
                // This one doesn't: to preserve semantics of the program correctly, b[i] may only be
                // accessed in lanes where x > 0. Which means that this code generates a lot of conditional
                // control flow that users will likely be surprised by. :)
                int x = a[i];
                output[i] = (x > 0) ? x : b[i];
        }

}