Re: checking for Not a Number?

["Joe Smith" <unknown_kev_cat@...>]

"Luiz Henrique de Figueiredo" <lhf@tecgraf.puc-rio.br> wrote in message 
20061221072046.A23167@lua.tecgraf.puc-rio.br">news:20061221072046.A23167@lua.tecgraf.puc-rio.br...
> > Directly compairing floating point numbers for equality is *always* a bad
> > idea.
>
> Only if the numbers are supposed to be the same number but are the result
> of two different computations.
>
> > For example, you can find that a floating point value which is not a
> > NaN is not equal to itself.
>
> I find this very surprising. And possibily wrong, ie against IEEE 754.
> The explanation that followed made no sense to me. It would make sense for
> floating point values held in *different* variables, but not for the 
> *same*
> variable. It seems to me that the gcc docs is spreading unnecessary FUD.
> But I must be missing something...
I've heard this same explanation elsewere too. (Somebody saw this problem in 
production code).
Apparently, the registers used in the FPU are oversized, and thus 
calculations performed there-in have morie bits than the datatype should.
Thus when compairing a value in the register to the truncated value in the 
memory, they can be non-equal.
I lack access to the IEEE 754 standard, and cannot make heads or tails of 
much of the ISO C standard when it talks about floating point numbers, so 
I'm not certain if this behavior violates either standard, but I've seen 
this documented more than once.

On the other hand, it is highly unlikely that the code listed would ever be 
affected by this, but it is still troubling.