unnormal Intel 80-bit long doubles and isnanl

[Florian Weimer]

* Siddhesh Poyarekar:

> On 11/27/20 5:01 PM, Florian Weimer wrote:
>> I think the last part (the “bug”) is new.  I welcome a consensus along
>> those lines.  I just want to highlight this aspect.
>
> Should we consider fixing behaviour if the bug manifests in a user
> application and not in glibc itself?  i.e. a crash because glibc
> either returned the unnormal or misclassified the unnormal number?

I think in general, that's a bit like fixing buffer overflows in
applications.  It's just not possible with the current compilation
model.  So I find it difficult to come up with a general rule.

The nature of these non-normal numbers is that the CPU does not produce
them.  I think we should make sure that glibc doesn't, either, with
obvious exceptions such as memcpy.  But beyond that, I don't know.

> At the minimu ISTM that we should at least make the classification
> consistent with gcc.

Yes, I agree.

Thanks,
Florian
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