Question

Having the following unions:

union {double first_d; uint64 first;};
union {double second_d; uint64 second;};
...
first_d = <a double value>
second_d = <a double value>

Does the output of following comparisons:

if(first_d > second_d)
    // CASE 1 OUTPUT
else if(first_d < second_d)
    // CASE 2 OUTPUT
else
    // CASE 3 OUTPUT

always the same for the following?

if(first> second)
    // CASE 1' OUTPUT
else if(first < second)
    // CASE 2' OUTPUT
else
    // CASE 3' OUTPUT

Answer 1

Nope. Here’s a counter-example using NaNs:

int main()
{

    union {double first_d; uint64 first;};
    union {double second_d; uint64 second;};

    first  = 0x7ff8000000000001;
    second = 0x7ff8000000000002;

    if(first_d > second_d)
        printf("greater\n");
    else if(first_d < second_d)
        printf("less\n");
    else
        printf("other\n");

    if(first > second)
        printf("greater\n");
    else if(first < second)
        printf("less\n");
    else
        printf("other\n");

    return 0;
}

Output:

other
less

---

I’ll also mention that type-punning via unions isn’t 100% standard conformant. So you could also say it’s undefined behavior.