[Cfp-interest 2486] Re: 5.2.4.2.3 and IEC 60559

[Fred J. Tydeman]

On Tue, 2 Aug 2022 09:56:10 +0200 Vincent Lefevre wrote:
>
>On 2022-08-01 22:46:01 -0500, Fred J. Tydeman wrote:
>> It appears to me that section 5.2.4.2.3 assumes that decimal floating point (DFP) is IEC 60559 DFP..
>> Yet, I do not see such a requirement in 5.2.4.2.3.
>> 
>> I do see in 6.2.5#12 that the formats are IEC 60559 DFP.
>
>I think that should also be put in 5.2.4.2.3.
>
>BTW, does the standard clearly say that these types are optional?
>This just seems to be suggested by 5.2.4.2.3, but 6.2.5#12 says
>no more than "There are three decimal floating types, designated
>as _Decimal32, _Decimal64, and _Decimal128. Respectively, they
>have the IEC 60559 formats: decimal32, decimal64, and decimal128.
>Decimal floating types are real floating types.", which could be
>interpreted as such types being mandatory. Compare with _Complex,
>for which it is said "Complex types are a conditional feature that
>implementations need not support;".
>
>Moreover, it seems that if one of these 3 types is supported, then
>the 3 of them need to be supported. Is this wanted?

Page 183, 6.10.9.3 Conditional feature macros
__STDC_IEC_60559_DFP__ The integer constant 202311L, intended to indicate support of decimal
floating types and conformance to Annex F (IEC 60559 floating-point arithmetic) for
decimal floating-point arithmetic.

Page 474, B.6.1  Characteristics of decimal floating types
The following macros are provided only if the implementation defines __STDC_IEC_60559_DFP__ .
N is 32, 64 and 128.
DEC_INFINITY
 DECN_MANT_DIG
 DECN _MIN_EXP
 DECN _SNAN
DEC_NAN
 DECN_MAX_EXP
 DECN _MIN
DECN_EPSILON
 DECN_MAX
 DECN _TRUE_MIN

Page 547, #4 (H.2.1): "An implementation that defines __STDC_IEC_60559_DFP__
provides the decimal floating types _Decimal32 , _Decimal64 , and
_Decimal128 (6.2.5)."

Given the above, it seems to me that the first sentence in:
5.2.4.2.3 Characteristics of decimal floating types in <float.h>:

This subclause specifies macros in <float.h> that provide characteristics of decimal floating types
in terms of the model presented in 5.2.4.2.2.

should be changed to:

This subclause specifies macros in <float.h> that provide characteristics of IEC 60559 decimal floating types
(an optional feature) in terms of the model presented in 5.2.4.2.2.


---
Fred J. Tydeman        Tydeman Consulting
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+1 (702) 608-6093      Vice-chair of PL22.11 (ANSI "C")
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