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From: jlg@cochiti.lanl.gov (Jim Giles)
Newsgroups: comp.lang.c
Subject: Re: NaN's (was Re: FLOATING NULL?)
Message-ID: <24890@lanl.gov>
Date: 1 Jun 91 23:53:34 GMT
References: <13223@uhccux.uhcc.Hawaii.Edu> <1991May28.153655.24199@zoo.toronto.edu> <2726@root44.co.uk> <14@tdatirv.UUCP> <1991Jun1.214347.11696@zoo.toronto.edu>
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In article <1991Jun1.214347.11696@zoo.toronto.edu>, henry@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