Issue 0CFP.16: P1: tgmath cbrt macro

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

Authors: WG14, Jim Thomas
Date: 2017-10-25
Reference document: N2178
Submitted against: Floating-point TS 18661 (C11 version, 2014-2016)
Status: Fixed
Fixed in: C23
Converted from: n2397.htm

Summary

This DR addresses a problem noted by Joseph Myers in emails SC22WG14.14743 and 14744:

… the example [in TS 18661-1 clause 16, for 7.25#6b – see below] implies that "#undef cbrtl" before calling the cbrt type-generic macro would mean it's not affected by constant rounding modes, but the actual normative text says "is affected by constant rounding modes" with no such caveat.

and

… neither example definition [in C11 or TS 18661-1] is valid because they might call a block-scope cbrtf / cbrtl; they need to avoid such a block-scope identifier, or a macro defined by the user, while still depending on whether expansion of the  
standard header cbrtf / cbrtl macros has been suppressed at that point.

The text in question is:

[6b] A type-generic macro corresponding to a function indicated in the table in 7.6.1a is affected by constant rounding modes (7.6.2). Note that the type-generic macro definition in the example in 6.5.1.1 does not conform to this specification. A conforming macro could be implemented as follows:

#define cbrt(X)     _Generic((X),                  \
               long double: cbrtl(X),         \
               default: _Roundwise_cbrt(X),   \
               float: cbrtf(X)                \
               )

where _Roundwise_cbrt() is equivalent to cbrt() invoked without macro-replacement suppression.

The cause of the problems is the use of cbrtl and cbrtf in the macro definition. The suggested TC below replaces these uses with _Roundwise_ prefixed identifiers similar to _Roundwise_cbrt.

Suggested Technical Corrigendum

In TS 18661-1, clause 16, replace:

#define cbrt(X)     _Generic((X),                  \
               long double: cbrtl(X),         \
               default: _Roundwise_cbrt(X),   \
               float: cbrtf(X)                \
               )

where _Roundwise_cbrt() is equivalent to cbrt() invoked without macro-replacement suppression.

with

#define cbrt(X)     _Generic((X),                       \
               long double: _Roundwise_cbrtl(X),   \
               default: _Roundwise_cbrt(X),        \
               float: _Roundwise_cbrtf(X)               \
               )

where _Roundwise_cbrtl(), _Roundwise_cbrt(), and _Roundwise_cbrtf() are equivalent to cbrtl(), cbrt(), and cbrtf(), respectively, invoked without macro-replacement suppression.

Comment from WG14 on 2019-05-03:

Oct 2017 meeting

Committee Discussion

There was considerable discussion on this issue. The first point is that the _Generic example cited that is proposed to be fixed was not intended to impose requirements, yet has elicited two fixes so far, this being a third. The second point is that the fix offered would likely elicit numerous compiler errors as stated and no longer serves its original intention of illustrating _Generic best practice usage. Lastly, lacking a clear simple example, is there a problem here that needs clarification becomes uncertain.

Apr 2018 meeting

Committee Discussion

A new document N2212 was presented with a much simpler proposed change. It was accepted by the committee.

Proposed Change

In TS 18661-1, clause 16, replace:

       #define cbrt(X) _Generic((X), \
                       long double: cbrtl(X), \
                       default: _Roundwise_cbrt(X), \
                       float: cbrtf(X) \
                       )

where _Roundwise_cbrt() is equivalent to cbrt() invoked without macro-replacement suppression.

with

       #define cbrt(X) _Generic((X), \
                       long double: _Roundwise_cbrtl, \
                       default: _Roundwise_cbrt, \
                       float: _Roundwise_cbrtf \
                       )(X)

where _Roundwise_cbrtl(), _Roundwise_cbrt(), and _Roundwise_cbrtf() are equivalent to cbrtl(), cbrt(), and cbrtf(), respectively, invoked without macro-replacement suppression.