This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: WG14, Jim Thomas
Date: 2016-09-10
Reference document: N2077
Submitted against: Floating-point TS 18661 (C11 version, 2014-2016)
Status: Fixed
Fixed in: C23
Converted from: n2397.htm
This is about the issue raised by Joseph Myers in email SC22WG14.14280:
TS 18661-1 says "Whether C assignment (6.5.16) (and conversion as if by assignment) to the same format is an IEC 60559 convertFormat or copy operation is implementation-defined, even if
<fenv.h>defines the macro
FE_SNANS_ALWAYS_SIGNAL(F.2.1).".Does this apply to function return, where the return type of the function is the same as the type of the expression passed to the return statement and no wider evaluation format is in use - that is, may this act as either convertFormat or copy? C11 F.6 clearly envisages that such a return statement may do a conversion to the same type in the case of wider evaluation formats. But 6.8.6.4#3 only refers to conversions "If the expression has a type different from the return type of the function in which it appears".
The specification, from F.3#3, quoted above is incomplete in that it doesn’t cover function returns, which are not assignments or conversions as if by assignment. As currently written, C11 + TS18661-1 might be read to exclude the possibility of using convertFormat in this case. A statement should be added to say that the implementation has the option to apply convertFormat to the return value. The change does not break existing implementations.
The effect of convertFormat would be that signaling NaNs would signal and noncanonical representations would be canonicalized. It is extremely unlikely that a program would depend on convertFormat not being used.
In Clause 8, to the text for C F.3#3:
[3] Whether C assignment (6.5.16) (and conversion as if by assignment) to the same format is an IEC 60559 convertFormat or copy operation is implementation-defined, even if
<fenv.h>defines the macroFE_SNANS_ALWAYS_SIGNAL(F.2.1).
append the sentence:
If the return expression of a
returnstatement is evaluated to the floating-point format of the return type, it is implementation-defined whether a convertFormat operation is applied to the result of the return expression.”
At the end of Clause 8, add:
In F.3#3, attach a footnote to the wording:
Whether C assignment (6.5.16) (and conversion as if by assignment) to the same format is an IEC 60559 convertFormat or copy operation
where the footnote is:
*) Where the source and destination formats are the same, convertFormat operations differ from copy operations in that convertFormat operations raise the “invalid” floating-point exception on signaling NaN inputs and do not propagate non-canonical encodings.
Comment from WG14 on 2018-10-18:
Oct 2016 meeting
The committee agrees that this is a defect and accepts the Suggested Technical Corrigendum
In Clause 8, to the text for C F.3#3:
[3] Whether C assignment (6.5.16) (and conversion as if by assignment) to the same format is an IEC 60559 convertFormat or copy operation is implementation-defined, even if
<fenv.h>defines the macroFE_SNANS_ALWAYS_SIGNAL(F.2.1).
append the sentence:
If the return expression of a
returnstatement is evaluated to the floating-point format of the return type, it is implementation-defined whether a convertFormat operation is applied to the result of the return expression.”
At the end of Clause 8, add:
In F.3#3, attach a footnote to the wording:
Whether C assignment (6.5.16) (and conversion as if by assignment) to the same format is an IEC 60559 convertFormat or copy operation
where the footnote is:
*) Where the source and destination formats are the same, convertFormat operations differ from copy operations in that convertFormat operations raise the “invalid” floating-point exception on signaling NaN inputs and do not propagate non-canonical encodings.