Kahan states this in a 1997 paper:
«[…]An SNaN may be moved ( copied ) without incident, but any
other arithmetic operation upon an SNaN is an INVALID operation (
and so is loading one onto the ix87's stack ) that must trap or
else produce a new nonsignaling NaN. ( Another way to turn an
SNaN into a NaN is to turn 0xxx...xxx into 1xxx...xxx with a
logical OR.) Intended for, among other things, data missing from
statistical collections, and for uninitialized variables[…]»
( http://www.eecs.berkeley.edu/~wkahan/ieee754status/IEEE754.PDF)
x87 is legacy, it predates IEEE754 by 5 years and should be
forgotten.
Note also that the string representation for a signalling nan is
"NANS", so it reasonable to save it to file if you need to
represent missing data. "NAN" represents 0/0, sqrt(-1), not
missing data.
I'm not really sure how it can be interpreted differently?
Ola.
