!standard 4.1.4(12/1)                                  20-06-05  AI12-0362-2/01
!class Amendment 20-06-05
!status work item 20-06-05
!status received 20-06-95
!priority Low
!difficulty Easy
!subject Attributes for fixed point types
!summary

Implementations have permission to extend attributes defined only for floating
point types (such as Ceiling) to fixed point types. The semantics of any such 
extension are implementation-defined.

!problem

There is a growing interest in fixed-point with the emergence of machine 
learning and GPUs. It is nice if an algorithm can be moved between fixed 
and float without signifiant re-engineering. However, the attributes Floor
Ceiling, Rounding, Unbiased_Rounding, Machine_Rounding, and Truncation are 
missing for fixed point types. This forces unnecessary conversions to
floating point types, which can themselves introduce rounding effects.

!proposal

(See Summary.)

!wording

Replace 4.1.4 (12/1)

   An implementation may provide implementation-defined attributes; the
   identifier for an implementation-defined attribute shall differ from
   those of the language-defined attributes unless supplied for compatibility
   with a previous edition of this International Standard.

with

   An implementation may provide implementation-defined attributes;
   the identifier for such an implementation-defined attribute shall
   differ from those of the language-defined attributes.

   An implementation may extend the definition of a language-defined
   attribute by accepting uses of that attribute that would otherwise be
   illegal in the following cases:
     - in order to support compatibility with a previous edition of
       of this International Standard; or
     - in the case of a language-defined attribute whose
       prefix is required by this International Standard to be a floating
       point subtype, an implementation may accept an attribute_reference
       whose prefix is a fixed-point subtype; the semantics of such an
       attribute_reference are implementation-defined.

!discussion

A suggestion was made to define for fixed point types some of the attributes 
that are currently defined only for floating point types. These included the 
Ceiling, Floor, Rounding, Unbiased_Rounding, Machine_Rounding, and Truncation
attributes.

It was decided that it was too late in the Ada202x standardization process for
this specific change, and that furthermore there are some details that remain 
to be worked out.

In order to allow implementations to provide this functionality as a 
legal-but-implementation-dependent language extension, permission is granted 
to implementations to extend the semantics of language-defined attributes in 
this case.

!ASIS

[Not sure. It seems like some new capabilities might be needed,
but I didn't check - Editor.]

!ACATS test

ACATS B- and C-Tests are needed to check that the new capabilities are
supported.


!appendix

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