[Cfp-interest 2233] Re: contradiction about the INFINITY macro

[Jim Thomas]

> On Oct 5, 2021, at 5:38 PM, Vincent Lefevre <vincent at vinc17.net> wrote:
> 
> On 2021-10-05 11:13:59 -0700, Jim Thomas wrote:
>> I suggest changing 
>> 
>> 	else to a positive constant of type float that overflows at translation time
>> 
>> to
>> 
>> 	else to an expression of type float that overflows with positive sign at translation time
>> 
>> This is intended to allow the implementation to use whatever magic is needed.
>> 
>> I had lost track of this issue and neglected to put it on the September CFP agenda. If we can converge on a resolution, and if no one objects, I can submit a paper with our agreed suggested change before the deadline. Please send any comments ASAP.
> 
> Wouldn't it be better to require INFINITY to be defined if and only if
> the type float supports infinity, so that this can be tested?
> 
> In the comp.std.c thread "contradiction about the INFINITY macro"
> https://groups.google.com/g/comp.std.c/c/XCzEiwD9n_A I started,
> Keith Thompson said:
> 
>  (I wonder if it would have been more useful to require that INFINITY not
>  be defined unless it can be defined as an actual infinity, but I haven't
>  given it a lot of thought.)
> 
> Note that in the current standard and with the suggested change, if
> infinity is not supported, then the use of INFINITY yields an overflow
> at translation time, which implies undefined behavior (even if the
> value will never be used). This is rather useless.
> 
> Moreover, shouldn't infinity be supported for all FP types or none?
> Keith Thompson said:
> 
>  The INFINITY and NAN macros are defined only for float.  I think the
>  assumption is that either all floating-point types support infinities,
>  or none do, and likewise for NaNs.  On the other hand, fpclassify() is a
>  macro that can be applied to an expression of any floating-point type.

If float includes infinity (or quiet NaN) then so must double and long double because all float values must be included in double and all double values must be included in long double. Any conversion of the float INFINITY (or NAN) to double or long double will raise no exceptions and can be done at translation time.

It is allowed for a wider standard floating type to have infinity (or NaN) but not a narrower type. C doesn’t have infinity and NaN macros for that. Is this an issue for some implementation?

- Jim Thomas

> 
> -- 
> Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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