[Cfp-interest 1850] Re: Negative zero

[Jim Thomas]

In a recent post to STDS-754, Vincent Lefèvre wrote:
> I think that the terminology should be:
>  * negative: equivalent to "less than zero", i.e. "< 0";
>  * negative sign: can be used for any FP number; -0 has a negative
>    sign, but NaN does not have a sign;
>  * negative sign bit: can be used for any FP datum.
> 

Do we agree with this interpretation of “negative”? 

(I do, except for the last item. Do qNaN and sNaN data in an extended format necessarily have a sign bit? A sign bit pertains to encoding and 754 doesn’t specify an encoding for extended formats. I think this is a 754 issue.)

Does C require opposite-signed zeros to compare equal? Does Annex F say this explicitly? Annex F says it implicitly by reference to IEC 60559, so it should be editorial to say it explicitly in Annex F.

We might want to add a (foot)note to point out that negative zero and NaNs with a negative sign bit are not negative values.

Have we been careful enough about distinguishing the sign of a value and the bit sign of the representation? For example, 7.12.3.7 says

	The signbit macro returns a nonzero value if and only if the sign of its argument value is negative.

Rabbit hole?

- Jim Thomas

> On Nov 9, 2020, at 2:53 PM, Damian McGuckin <damianm at esi.com.au> wrote:
> 
> 
> Fred,
> 
> On Mon, 9 Nov 2020, Fred J. Tydeman wrote:
> 
>> C in <math.h> 7.12.7.5 The pow functions has:
>> A domain error occurs if x is finite and negative
>> and y is finite and not an integer value.
> 
> The IEEE 754 standard sats
> 
> pow(+/-0, y) is +INFINITY and signals Divide by Zero for y<0 finite and not odd integral
> 
> pow(+/-0, y) is +0.0 for y>0 finite and not an odd integer (and F.10.4.5 matches this)
> 
>> Is -0.0 negative?
>> Is pow(-0.0, 1.5) a domain error?
> 
> No. See the standard and F.10.4.5.
> 
>> F.10.4.5
>> The pow functions has: pow(?0, y) returns +0 for y > 0 and not an odd integer.
> 
>> Should "negative" be changed to "less than zero" in the above?
> 
> I believe you are correct.
> 
> BUT we have to add something to handle the case for x = -0.0 as special.
> 
> I think the above is clear and complete. Apologies if I am did not nail it
> 
> Regards - Damian
> 
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