Issue 0CFP.20: P1: changes for obsolescing DECIMAL_DIG

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: WG14, C Floating Point Group
Date: 2018-03-16
Reference document: N2211
Submitted against: Floating-point TS 18661 (C11 version, 2014-2016)
Status: Fixed
Fixed in: C23
Cross-references: 0CFP.22
Converted from: n2397.htm

Summary

N2211 described changes in C11 and TS 18661 to remove references to DECIMAL_DIG, which CR501 is expected to obsolesce. The changes that apply to C11 are collected in N2253 as an update to the suggested TC in CR501. The changes that apply to TS 18661-1 compose the CR in this document. The remaining change is for TS 18661-3, which will be covered by a CR in a subsequent document.

Suggested Technical Corrigendum

In 7.1, omit:

Change footnote 361) from:

361)  If the minimum-width IEC60559 extended format (64 bits of precision) is supported, DECIMAL_DIG shall be at least 21. If IEC 60559 double (53 bits of precision) is the widest IEC 60559 format supported, then DECIMAL_DIG shall be at least 17. (By contrast, LDBL_DIG and DBL_DIG are 18 and 15, respectively, for these formats.)

to:

361)  If the minimum-width IEC 60559 binary64-extended format (64 bits of precision) is supported, DECIMAL_DIG shall be at least 21. If IEC 60559 binary64 (53 bits of precision) is the widest IEC 60559 format supported, then DECIMAL_DIG shall be at least 17. (By contrast, LDBL_DIG and DBL_DIG are 18 and 15, respectively, for these formats.)

In 10.1, change:

After F.5#2, insert:

[2a] The <float.h> header defines the macro

CR_DECIMAL_DIG

if and only if __STDC_WANT_IEC_60559_BFP_EXT__ is defined as a macro at the point in the source file where <float.h> is first included. If defined, CR_DECIMAL_DIG expands to an integral constant expression suitable for use in #if preprocessing directives whose value is a number such that conversions between all supported types with IEC 60559 binary formats and character sequences with at most CR_DECIMAL_DIG significant decimal digits are correctly rounded. The value of CR_DECIMAL_DIG shall be at least DECIMAL_DIG + 3. If the implementation correctly rounds for all numbers of significant decimal digits, then CR_DECIMAL_DIG shall have the value of the macro UINTMAX_MAX.

[2b] Conversions of types with IEC 60559 binary formats to character sequences with more than CR_DECIMAL_DIG significant decimal digits shall correctly round to CR_DECIMAL_DIG significant digits and pad zeros on the right.

[2c] Conversions from character sequences with more than CR_DECIMAL_DIG significant decimal digits to types with IEC 60559 binary formats shall correctly round to an intermediate character sequence with CR_DECIMAL_DIG significant decimal digits, according to the applicable rounding direction, and correctly round the intermediate result (having CR_DECIMAL_DIG significant decimal digits) to the destination type. The “inexact” floating-point exception is raised (once) if either conversion is inexact. (The second conversion may raise the “overflow” or “underflow” floating-point exception.)

In F.5#2c, attach a footnote to the wording:

The “inexact” floating-point exception is raised (once) if either conversion is inexact.

where the footnote is:

*) The intermediate conversion is exact only if all input digits after the first CR_DECIMAL_DIG digits are 0.

to:

Replace the content of F.5 with:

[1] The <float.h> header defines the macro

CR_DECIMAL_DIG

if and only if __STDC_WANT_IEC_60559_BFP_EXT__ is defined as a macro at the point in the source file where <float.h> is first included. If defined, CR_DECIMAL_DIG expands to an integral constant expression suitable for use in #if preprocessing directives whose value is a number such that conversions between all supported IEC 60559 binary formats and character sequences with at most CR_DECIMAL_DIG significant decimal digits are correctly rounded. The value of CR_DECIMAL_DIG shall be at least M + 3, where M is the maximum value of the T_DECIMAL_DIG macros for IEC 60559 binary formats. If the implementation correctly rounds for all numbers of significant decimal digits, then CR_DECIMAL_DIG shall have the value of the macro UINTMAX_MAX.

[2] Conversions of types with IEC 60559 binary formats to character sequences with more than CR_DECIMAL_DIG significant decimal digits shall correctly round to CR_DECIMAL_DIG significant digits and pad zeros on the right.

[3] Conversions from character sequences with more than CR_DECIMAL_DIG significant decimal digits to types with IEC 60559 binary formats shall correctly round to an intermediate character sequence with CR_DECIMAL_DIG significant decimal digits, according to the applicable rounding direction, and correctly round the intermediate result (having CR_DECIMAL_DIG significant decimal digits) to the destination type. The “inexact” floating-point exception is raised (once) if either conversion is inexact. (The second conversion may raise the “overflow” or “underflow” floating-point exception.)

[4] The specification in this subclause assures conversion between IEC 60559 binary format and decimal character sequence follows all pertinent recommended practice. It also assures conversion from IEC 60559 format to decimal character sequence with at least T_DECIMAL_DIG digits and back, using to-nearest rounding, is the identity function, where T is the macro prefix for the format.

[5] Functions such as strtod that convert character sequences to floating types honor the rounding direction. Hence, if the rounding direction might be upward or downward, the implementation cannot convert a minus-signed sequence by negating the converted unsigned sequence.

In F.5#3, attach a footnote to the wording:

The “inexact” floating-point exception is raised (once) if either conversion is inexact.

where the footnote is:

*) The intermediate conversion is exact only if all input digits after the first CR_DECIMAL_DIG digits are 0.

Comment from WG14 on 2019-05-03:

Apr 2018 meeting

Committee Discussion

The paper was only briefly discussed.

This resolution is tied to the Floating Point CR 22 as well as to the C2x DR 501.

Oct 2018 meeting

Committee Discussion

A new paper N2254 was split out from N2211 that incorporates directly the Suggested Technical Corrigendum already extracted above.

The committee accepts the Suggested Technical Corrigendum as the Proposed Change to resolve this issue.

Proposed Change

In 7.1, omit:

Change footnote 361) from:

361)  If the minimum-width IEC60559 extended format (64 bits of precision) is supported, DECIMAL_DIG shall be at least 21. If IEC 60559 double (53 bits of precision) is the widest IEC 60559 format supported, then DECIMAL_DIG shall be at least 17. (By contrast, LDBL_DIG and DBL_DIG are 18 and 15, respectively, for these formats.)

to:

361)  If the minimum-width IEC 60559 binary64-extended format (64 bits of precision) is supported, DECIMAL_DIG shall be at least 21. If IEC 60559 binary64 (53 bits of precision) is the widest IEC 60559 format supported, then DECIMAL_DIG shall be at least 17. (By contrast, LDBL_DIG and DBL_DIG are 18 and 15, respectively, for these formats.)

In 10.1, change:

After F.5#2, insert:

[2a] The <float.h> header defines the macro

CR_DECIMAL_DIG

if and only if __STDC_WANT_IEC_60559_BFP_EXT__ is defined as a macro at the point in the source file where <float.h> is first included. If defined, CR_DECIMAL_DIG expands to an integral constant expression suitable for use in #if preprocessing directives whose value is a number such that conversions between all supported types with IEC 60559 binary formats and character sequences with at most CR_DECIMAL_DIG significant decimal digits are correctly rounded. The value of CR_DECIMAL_DIG shall be at least DECIMAL_DIG + 3. If the implementation correctly rounds for all numbers of significant decimal digits, then CR_DECIMAL_DIG shall have the value of the macro UINTMAX_MAX.

[2b] Conversions of types with IEC 60559 binary formats to character sequences with more than CR_DECIMAL_DIG significant decimal digits shall correctly round to CR_DECIMAL_DIG significant digits and pad zeros on the right.

[2c] Conversions from character sequences with more than CR_DECIMAL_DIG significant decimal digits to types with IEC 60559 binary formats shall correctly round to an intermediate character sequence with CR_DECIMAL_DIG significant decimal digits, according to the applicable rounding direction, and correctly round the intermediate result (having CR_DECIMAL_DIG significant decimal digits) to the destination type. The “inexact” floating-point exception is raised (once) if either conversion is inexact. (The second conversion may raise the “overflow” or “underflow” floating-point exception.)

In F.5#2c, attach a footnote to the wording:

The “inexact” floating-point exception is raised (once) if either conversion is inexact.

where the footnote is:

*) The intermediate conversion is exact only if all input digits after the first CR_DECIMAL_DIG digits are 0.

to:

Replace the content of F.5 with:

[1] The <float.h> header defines the macro

CR_DECIMAL_DIG

if and only if __STDC_WANT_IEC_60559_BFP_EXT__ is defined as a macro at the point in the source file where <float.h> is first included. If defined, CR_DECIMAL_DIG expands to an integral constant expression suitable for use in #if preprocessing directives whose value is a number such that conversions between all supported IEC 60559 binary formats and character sequences with at most CR_DECIMAL_DIG significant decimal digits are correctly rounded. The value of CR_DECIMAL_DIG shall be at least M + 3, where M is the maximum value of the T_DECIMAL_DIG macros for IEC 60559 binary formats. If the implementation correctly rounds for all numbers of significant decimal digits, then CR_DECIMAL_DIG shall have the value of the macro UINTMAX_MAX.

[2] Conversions of types with IEC 60559 binary formats to character sequences with more than CR_DECIMAL_DIG significant decimal digits shall correctly round to CR_DECIMAL_DIG significant digits and pad zeros on the right.

[3] Conversions from character sequences with more than CR_DECIMAL_DIG significant decimal digits to types with IEC 60559 binary formats shall correctly round to an intermediate character sequence with CR_DECIMAL_DIG significant decimal digits, according to the applicable rounding direction, and correctly round the intermediate result (having CR_DECIMAL_DIG significant decimal digits) to the destination type. The “inexact” floating-point exception is raised (once) if either conversion is inexact. (The second conversion may raise the “overflow” or “underflow” floating-point exception.)

[4] The specification in this subclause assures conversion between IEC 60559 binary format and decimal character sequence follows all pertinent recommended practice. It also assures conversion from IEC 60559 format to decimal character sequence with at least T_DECIMAL_DIG digits and back, using to-nearest rounding, is the identity function, where T is the macro prefix for the format.

[5] Functions such as strtod that convert character sequences to floating types honor the rounding direction. Hence, if the rounding direction might be upward or downward, the implementation cannot convert a minus-signed sequence by negating the converted unsigned sequence.

In F.5#3, attach a footnote to the wording:

The “inexact” floating-point exception is raised (once) if either conversion is inexact.

where the footnote is:

*) The intermediate conversion is exact only if all input digits after the first CR_DECIMAL_DIG digits are 0.