(j3.2006) (SC22WG5.5299) [ukfortran] Did we intend to prohibit this?

[Bill Long]

On Jul 3, 2014, at 12:56 PM, Van Snyder <van.snyder at jpl.nasa.gov> wrote:

> On Thu, 2014-07-03 at 11:14 +0100, N.M. Maclaren wrote:
>> On Jul 3 2014, Van Snyder wrote:
>> 
>>> As far as I can tell from looking at 7.1.12, the only function from
>>> IEEE_Arithmetic that's allowed in a constant expression is
>>> IEEE_Selected_Real_Kind.
>>> 
>>> Did we intend to prohibit all the others?
>>> 
>>> Is there a problem with admitting the inquiry functions, and admitting
>>> the others provided their arguments are constant expressions?
>> 
>> One issue that would have to be considered is what requirements we would
>> place on compile- and run-time options to control the modes.  IEEE 754
>> remains a very assembler-level specification, does not map well even
>> to C, and its arithmetic model is seriously incompatible with Fortran's.
>> This is probably resolvable, but isn't just a matter of relaxing the
>> restrictions.  I, for one, lack the enthusiasm to tackle this.
> 
> The reason this question arose was an attempt at the following:
> 
>  integer, parameter :: RK = kind(1.0e0)
>  real(rk), parameter :: SNaN = merge( IEEE_Value(1.0_rk,IEEE_Signaling_NaN), &
>                                     & 0.0_rk, IEEE_Support_DataType(1.0_rk) )
> 
> Are there at least a few functions from IEEE_Arithmetic, and the two
> inquiry functions from IEEE_Exceptions, that we could admit into
> constant expressions?  The functions that don't depend upon modes,
> especially the inquiry functions, ought not to be troublesome.

Note that the edits in interp F08/0104 do expand the list:

[152:7-8] section 7.1.12 Constant expression, paragraph 1, replace
bullet (8):

"(8) a reference to the transformational function
IEEE_SELECTED_REAL_KIND from the intrinsic module
IEEE ARITHMETIC (14), where each argument is a constant
expression,"

with:

"(8) a reference to a transformational function from the
intrinsic module IEEE_ARITHMETIC or IEEE_EXCEPTIONS (14), where
each argument is a constant expression,?

Some of the former Inquiry functions were reclassified as transformational by this interp, though apparently not the ones you need above. Perhaps you could propose expanding the list. I don?t  see a technical reason why the code you proposed could not work. 

Cheers,
Bill






> 
> While looking at this, I noticed that IEEE_Is_Finite appears twice in
> Table 14.1.  Is there a reason for this, or is it just a typo?
> 
>> Regards,
>> Nick.
>> 
> 
> 
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Bill Long                                                                       longb at cray.com
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