Re: An error-catching question

[Michel Hack <hack@xxxxxxxxxxxxxx>]

Juergen Wolff von Gudenberg wrote (and others have referenced):
> my suggestion is well known, See Motion 7
>   use [+Nan,+nan] for emptyset
>   use [-nan,-nan] for NaI

Careful here!  Encoding information (e.g. Empty vs NaI) in the sign bit
of a NaN is very dangerous in 754-based environments, as a NaN sign is
defined to be meaningless, and may not be propagated predictably.  For
example, the sign of NaN*Num may be the XOR of the operand signs, even
though the NaN payload will be preserved.  (For NaN*NaN one of the two
payloads will be preserved -- which one is in general NOT predictable.)

Michel.
---Sent: 2010-09-10 18:38:57 UTC