Issue 0441: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 2

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

Authors: WG 14, Joseph Myers
Date: 2013-07-21
Reference document: N1730
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm

Summary

Some issues with floating point in C11 have been identified as part of the UK review of the N1711 draft of TS 18661-1. While such issues relate to the general area of C bindings to IEC 60559:2011, and so could be addressed in the TS on that basis, since the issues also apply to C11 as-is it may be more appropriate to address some or all of these issues as Defect Reports with a view to having a normative fix in a future TC to C11 rather than only having a fix in conjunction with the new bindings.

Issue 2: Definition of FLT_ROUNDS

The C11 definition of FLT_ROUNDS is inadequate in that it refers to floating-point addition but does not say addition of what type. If long double is not an IEC 60559 type, it may not fully support all rounding modes even though they are supported by other types. (This is an actual issue with real implementations using non-IEC 60559 types for long double.) A suitable fix would be:

In 5.2.4.2.2#8, insert "for type float" after "floating-point addition". At the end of F.2#1, insert "The value of FLT_ROUNDS applies to all IEC 60559 types supported by the implementation, but may not apply to non-IEC 60559 types.".

Comment from WG14 on 2017-11-03:

Oct 2013 meeting

The committee adopted a Proposed Committee Response that has been substantially revised.

Oct 2014 meeting

Committee Discussion

The Proposed Committee Response was revised for accuracy and more detailed information, and is provided below.

Proposed Committee Response (obsolete)

The committee regards the existing definition of FLT_ROUNDS as intended to apply to types float, double and long double. However, if all three types cannot support the same set of rounding modes, the implementation needs to set FLT_ROUNDS to -1 meaning indeterminable.

As has been pointed out, in Annex F, only the types float and double need be IEC 60559 types. If long double is not an IEC 60559 type (for example, a pair of doubles), it may not support the same set of rounding modes as float and double. In this case, having FLT_ROUNDS apply to float and double (but not long double) would result in a value of 0, 1, 2, or 3 and would provide new and useful information to the programmer.

However, this behavioral change could also break existing programs, and as such the committee prefers to leave as is for this revision of the Standard.

Apr 2015 meeting

Committee Discussion

The Proposed Committee Response was revised yet again based on the input that since FLT_ROUNDS in existing practice is universally coded as 1, the proposed changes won’t affect existing practice.

Proposed Committee Response

The implementation of long double, for example, may significantly differ from IEC floating types and may not support the same choices as would otherwise be possible for FLT_ROUNDS. All known implementations define FLT_ROUNDS as the value 1 (round to nearest). and as such exempting non-IEC long double behavior allows the potential for implementations to provide the full range of possible values for IEC floating types.

Proposed Technical Corrigendum

At the end of F.2#1, insert

The value of FLT_ROUNDS applies to all IEC 60559 types supported by the implementation, but need not apply to non-IEC 60559 types.