Built-in testing for signaling nan?

[N.M. Maclaren]

>> Yes, due to the poor quality of the IEEE 754 specifications.  In 1984,
>> the distinction was left completely unspecified (even in intent).  In
>> 2008, there is a recommendation (no more) that the top bit of the payload
>> is used, with no specification of what to do if that is zero (which is
>> the most obvious default).  That appears to be the Intel specification,
>> though it may not have always been so even for x86.  I have certainly
>> seen more than one convention for different architectures, but have no
>> idea how many are extant nor how many are documented in the relevant
>> architecture manuals.
>
>In GCC, the qnan_msb_set bit of struct real_format is what specifies the 
>convention in use for a particular floating-point mode.

A single bit necessarily conveys at most one bit of information.
That can indicate whether the MSB being zero is the criterion;
it cannot indicate what the criterion is if that is not the case.


Regards,
Nick Maclaren.