Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: NaN's (was Re: FLOATING NULL?)
Message-ID: <1991Jun1.214347.11696@zoo.toronto.edu>
Date: Sat, 1 Jun 1991 21:43:47 GMT
References: <13223@uhccux.uhcc.Hawaii.Edu> <1991May28.153655.24199@zoo.toronto.edu> <2726@root44.co.uk> <14@tdatirv.UUCP>
Organization: U of Toronto Zoology

In article <14@tdatirv.UUCP> sarima@tdatirv.UUCP (Stanley Friesen) writes:
>>... there is a standard way of testing for NaN.
>>If x is a floating point variable, then (x != x) ...
>
>But what if the compiler notices you are asking for a self comparison and
>optimizes it away?  Then this will fail unless x is volatile.
>Since NaN's fall in the area of unspecified/undefined behavior I believe
>that such an optimization is legal under the ANSI standard.

But not under the IEEE FP standard.  Compilers need to be *very* careful
about optimizing floating-point operations if they are to provide a fully
IEEE-conforming environment.  A good many seemingly-innocuous operations
run into trouble.  It is simply a fact that in IEEE floating point, x!=x
is sometimes true, and optimizing it to `false' is a compiler bug.
-- 
"We're thinking about upgrading from    | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 to SunOS 3.5."              |  henry@zoo.toronto.edu  utzoo!henry