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Re: Signaling NaNs

From: Marc Nieper-Wißkirchen
Subject: Re: Signaling NaNs
Date: Fri, 10 Dec 2021 08:57:24 +0100

Hi Bruno,

thanks for replying so quickly!

My intention is neither to feed the signaling NaN into floating-point operations nor to cause it an exception to be raised.  What I really want to do is to model a type whose value is either a floating-point number (including infinities and the NaNs returned by the floating-point functions) or a sentinel value. Furthermore, I want this to fit into the size of a double.

The reason why I thought of using the bit pattern of a signaling NaN is that such a signaling NaN wouldn't be returned by the usual floating-point functions. A quiet NaN, which wouldn't either, would also work for my purpose.  This would correspond to the first of your three suggestions.  So I could reformulate my question to: "Is there a way to produce a (quiet) NaN that won't occur as a result of the C library functions?" This seems to be possible on most architectures but will need specific code for some architectures, which is why I thought of Gnulib.



Am Do., 9. Dez. 2021 um 22:49 Uhr schrieb Bruno Haible <bruno@clisp.org>:
Hi Marc,

> I have been searching through the list of modules but haven't been able to
> find it:  Does Gnulib offer a way to store a signaling NaN in a memory
> location (if supported by the platform)

It doesn't, because quiet NaNs are easier to work with.

> The forthcoming C2x standard will have FLT_SNAN, DBL_SNAN, and LDBL_SNAN,
> but before that there doesn't seem to be a portable way to get signaling
> NaNs.

... indeed, when you have a non-optimizing compiler, how to prevent the
compiler from generating instructions that produce a floating-point exception
earlier than desired?

> and some way to check a memory
> location whether it contains a signaling NaN?

Why would you need that? If you are using floating-point operations
(such as addition, square root, etc.) the signaling NaN will produce an
exception, as desired. Whereas if you are implementing some extra floating-
point operation by looking at the precise bit pattern (using integer
arithmetic), the standards [1][2] tell you which bit pattern to look for.

> In case the following problem is easier: What I really need is a bit
> pattern for a double that won't be returned by the usual floating-point
> functions.

When you look at [1] and [2], all bit patterns have a meaning.

So, you could
  - either use one of the many particular quiet NaN values, and hope that
    no other code produces it, or
  - (like what the IA64 CPU does in hardware) work with 65-bit units,
    where the 65th bit means "uninitialized", or
  - reserve a couple of extra bits for a type tag on every value, like
    what some Lisp implementations do (e.g. GNU clisp).
I think valgrind uses one of the latter two techniques as well.


[1] https://en.wikipedia.org/wiki/Single-precision_floating-point_format
[2] https://en.wikipedia.org/wiki/Double-precision_floating-point_format

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