Re: support for bitwise comparison of floats

[Ben Pfaff]

Eric Blake <address@hidden> writes:

> According to Bruno Haible on 3/25/2007 9:57 AM:
>> Eric Blake wrote:
>>> /* return true iff the representation of d needs a leading '-' */
>>> bool
>>> is_negative (long double d)
>>> {
>>>   if (d == 0)
>>>     {
>>>       union {
>>>         long double d;
>>>         long l;
>>>       } u;
>>>       u.d = d;
>>>       u.l |= 1;
>>>       return u.d < 0;
>>>     }
>>>   return d < 0;
>>> }
[...]
>> It's safer to use   u.l |= 8;   since noone will put the sign bit at
>> bit 3 or 28 (except the HP-PA designers perhaps :-)).
>
> Yes, I like your idea of using a bit other than the LSB as a way to ensure
> that the sign bit is not hit.  But you also have to pick a bit that does
> not fall in the 2-byte padding of the 10-byte x86 long double.

I think that you can detect that you hit the padding by testing
whether u.d == 0.  If it's still 0, then you changed a padding
bit.
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