[Cfp-interest 1251] Re: SNAN question

[Fred J. Tydeman]

On Mon, 4 Feb 2019 12:02:01 -0800 Jim Thomas wrote:
>
>> 7.12#7 has:
>> 
>> If a signaling NaN macro is used
>> for initializing an object of the same type that has 
>> static or thread-local storage duration, the
>> object is initialized with a signaling NaN value.
>
>This specification seems ambiguous. 

Agreed.

>Does "is used for initializing" include being in any expression that is used as an 
>initializer, or just being the initializer? The former would include not only -SNANF but also 1.0 + SNAN. 

clang appears to allow any expression with an SNAN macro as being an SNAN.
gcc appears to only allow SNAN, +SNAN, -SNAN as being an SNAN.

>I think our intention 
>was the latter. Assuming so, then the original example would allow but not require a signaling NaN result. To make this 
>intention clear, we could change 7.12#7 to 
>
>If a signaling NaN macro is used as the initializer
>for initializing an object of the same type that has 
>static or thread-local storage duration, the
>object is initialized with a signaling NaN value.

Or, perhaps:

If a (optionally signed) signaling NaN macro is used as the initializer
for initializing an object of the same type that has 
static or thread-local storage duration, the
object is initialized with a signaling NaN value.


Since unary minus does not trigger an SNAN, it seems that -SNAN
should be valid where ever SNAN is allowed.


---
Fred J. Tydeman        Tydeman Consulting
tydeman at tybor.com      Testing, numerics, programming
+1 (702) 608-6093      Vice-chair of PL22.11 (ANSI "C")
Sample C99+FPCE tests: http://www.tybor.com
Savers sleep well, investors eat well, spenders work forever.