[Users] Prologation=NaN

[Ian Hinder]

On 18 Sep 2015, at 08:27, Roland Haas <rhaas at aei.mpg.de> wrote:

> Hello all,
> 
>> Don't use NaN for poison!  If you do, then you cannot tell the
>> difference between NaNs generated because of numerical problems, and
>> poison generated due to programming bugs.
> Not true really. There are multiple "nan" values. See eg
> http://en.cppreference.com/w/c/numeric/math/nan and if you use nan("17")
> for poison you will be able to tell it apart from the nan that you get
> out of 1./0 and sqrt(-1.) type errors.
> 
> Attached it is a simple C99 program to demonstrate this. Basically it
> means that to detect poison you need to use memcmp (which we already do)
> and that in floating point arithmetic a nan("17") propagates as a
> nan("17") when combined with other non-NaN numbers but may be wiped when
> combined with a NaN (which would also wipe a non-NaN poison value though).

Fascinating.  I had no idea that there was more than one NaN value.  The wikipedia page <https://en.wikipedia.org/wiki/NaN> explains it is more detail than the cppreference page.

The conversion from the character representation to the binary representation appears to not be defined by the C/C++ standard, so presumably is left up to the compiler / libc / platform.  Also, I suspect that this usage of NaN payloads is not well supported by most analysis software.

Apart from those concerns, yes, it would be very good to use a specific type of NaN to represent poison.

-- 
Ian Hinder
http://members.aei.mpg.de/ianhin

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