NaN's (was Re: FLOATING NULL?)

[Jim Giles]

In article <1991Jun1.214347.11696 at zoo.toronto.edu>, henry at zoo.toronto.edu (Henry Spencer) writes:
|> [...]           It is simply a fact that in IEEE floating point, x!=x
|> is sometimes true, and optimizing it to `false' is a compiler bug.

I depends on how the `!=' operator is defined in C.  The only relational
predicate that evaluates `true' under IEEE FP is `unordered'.  So, if 
`!=' is defined as `?<>' (in the IEEE notation: `?' stands for `unordered?'), 
then it will evaluate `x!=x' as `true' if x is a NAN.  However, if `!=' is
defined as `<>', then it will _always_ evaluate `x!=x' as `false'. 

I can't find any specific reference in the ANSI C document which defines
which behaviour is appropriate.  The ANSI/IEEE document Std 754-1985 
(IEEE Standard for Binary Floating-Point Numbers) _recommends_ that the
_Fortran_ operator .NE. should be defined as `?<>'.  Neither the Fortran
nor the C ANSI documents (proposals, in the case of Fortran) mention
the issue at all.

J. Giles