[Cfp-interest 2360] Re: Updated (V4) N2823

Fri Jan 21 22:54:29 PST 2022

First, what’s really the problem?

Do the freestanding implementations in question want to define __STDC_IEC_60559_BFP__ or __STDC_IEC_60559_DFP__? That is, do they want to declare support for Annex F? If not, there’s not a problem. If they do, …

Do freestanding implementations that want to declare support for Annex F want to provide all the features of Annex F except for the parts that require “global” storage? For example, do they want to provide all the math functions? If not, just addressing the “global” storage part isn’t a solution for them still wouldn’t allow them to declare support for Annex F.

So, we can focus on freestanding implementations that want to declare support for Annex F but be exempt just from supporting the parts that require global or thread storage. The potentially relevant such parts refer to the floating-point environment (flags and modes) and the variable errno (which have thread storage) and the floating-point numeric conversion functions in stdlib.h (which use locales whose maintenance requires “global” storage).

Freestanding implementations are not required to accept strictly conforming programs that use features of <errno.h> or <locale.h>. I understand this to imply such programs cannot access the variable errno or the locale, and hence freestanding implementations need not maintain “global” storage for errno or locales (even if they provide functions whose specification references errno or locales).

This leaves the floating-point environment. Maintaining storage for the environment is necessary only if the program can read (or test) flags or change modes. A simple. direct way of addressing this would be to not require freestanding implementations to accept strictly conforming programs that set the state of the FENV_ACCESS pragma to ON (7.6.1). This could be part 1 of a 3-part proposal (described further below).

As noted in previous email, we should also allow freestanding implementations to define math_errhandling to be 0 (7.12). This will allow the feature tests for errno and floating-point exceptions to work for freestanding implementations. Alternative 3 in V6 addresses this, but the 0 option should be allowed only for freestanding implementations -- and it’s not really an alternative because it’s only for part of the problem with fenv.h. It could be part 2 of the aforementioned 3-part proposal.

A separate issue is whether the floating-point numeric conversion functions should have the same option as math functions to report errors via errno or floating-point exceptions. Alternative 2 in V6 specifies the changes, but calling it an “alternative” seems confusing. It could be part 3 of the 3-part proposal.

Part 1 of the 3-part proposal addresses the main issue. Part 2 is a programming aid for freestanding implementations. Part 3 is for consistent error reporting.

- Jim Thomas

> On Jan 20, 2022, at 6:39 AM, Rajan Bhakta <rbhakta at us.ibm.com> wrote:
> 
> Hi Jim,
> 
> This paper did not make it into the schedule for the next C meeting so it is not a tight schedule at all. I do hope to get something in within the next couple of weeks to make it in time for the subsequent (post February) WG14 meeting though.
> 
> Regards,
> 
> Rajan Bhakta
> z/OS XL C/C++ Compiler Technical Architect
> ISO C Standards Representative (Canada, USA), PL22.11 Chair
> C/C++ Compiler Development
> rbhakta at us.ibm.com
> 
> IBM
> 
> <graycol.gif>"Jim Thomas" ---01/20/2022 12:24:39 AM---Rajan, the comments below are for V4. I believe most of them still apply to V6, but they are incompl
> 
> From: "Jim Thomas" <jaswthomas at sbcglobal.net>
> To: "Rajan Bhakta" <rbhakta at us.ibm.com>
> Cc: "CFP" <cfp-interest at ucbtest.org>
> Date: 01/20/2022 12:24 AM
> Subject: [EXTERNAL] Re: [Cfp-interest 2346] Updated (V4) N2823
> 
> 
> 
> 
> Rajan, the comments below are for V4. I believe most of them still apply to V6, but they are incomplete. It will probably be Friday before I can respond to V6 properly. How tight is your schedule on this? On Jan 19, 2022, at 3:25 PM, Rajan Bhakta ZjQcmQRYFpfptBannerStart 
> This Message Is From an External Sender 
> This message came from outside your organization. 
> ZjQcmQRYFpfptBannerEnd
> Rajan, the comments below are for V4. I believe most of them still apply to V6, but they are incomplete. It will probably be Friday before I can respond to V6 properly.  How tight is your schedule on this?
> On Jan 19, 2022, at 3:25 PM, Rajan Bhakta <rbhakta at us.ibm.com <mailto:rbhakta at us.ibm.com>> wrote:
> Once more into the breech! Responses inline in blue below.
> 
> (See attached file: N2823UpdateV6.html)
> 
> Regards,
> 
> Rajan Bhakta
> z/OS XL C/C++ Compiler Technical Architect
> ISO C Standards Representative (Canada, USA), PL22.11 Chair
> C/C++ Compiler Development
> rbhakta at us.ibm.com <mailto:rbhakta at us.ibm.com>
> 
> IBM
> 
> <graycol.gif>"Jim Thomas" ---01/19/2022 04:26:12 PM---There’s no need to mention <math.h> in “… when <stdlib.h>, <fenv.h> and <math.h> are included …”. Th
> 
> From:  "Jim Thomas" <jaswthomas at sbcglobal.net <mailto:jaswthomas at sbcglobal.net>>
> To:  "Rajan Bhakta" <rbhakta at us.ibm.com <mailto:rbhakta at us.ibm.com>>
> Cc:  "CFP" <cfp-interest at ucbtest.org <mailto:cfp-interest at ucbtest.org>>
> Date:  01/19/2022 04:26 PM
> Subject:  [EXTERNAL] Re: [Cfp-interest 2346] Updated (V4) N2823
> 
> 
> 
> 
> 
> There’s no need to mention <math.h> in “… when <stdlib.h>, <fenv.h> and <math.h> are included …”. The “inclusive version” of Alternative 1 is missing ", without the requirements to set errno (see 7.5) or modify the floating-point ZjQcmQRYFpfptBannerStart 
> This Message Is From an External Sender 
> This message came from outside your organization. 
> ZjQcmQRYFpfptBannerEnd
> There’s no need to mention <math.h> in “… when <stdlib.h>, <fenv.h> and <math.h> are included …”.
> 
> Agreed.
> 
> The “inclusive version” of Alternative 1 is missing ", without the requirements to set errno (see 7.5) or modify the floating-point environment (see 7.6),”.
> I thought that was the discussion we had today. Without requiring errno.h to be included, there is no way to access errno from a user program, meaning we do not need to say anything about setting it as the behaviour is not observable by a program.
> That was what I had thought you meant by "not excluding". i.e. no need to say no setting errno or modifying the floating point environment in the normative text. I did add in a line to mention the floating-point environment to the footnote as you suggested below.
> 
> Ah. I didn’t understand the whole intention for the “inclusive version”.
> 
> 
> Similar to [?] regarding errno, if the <fenv.h> functions to query flags or modify modes are not required, the implementation does not have to maintain thread local storage for the floating-point environment.
> 
> Regarding the [?] footnote, I don’t see that C mentions the as if rule. Also, the C term seems to be “thread storage”. Here’s a different shot at the footnote:
> [?] Since a conforming freestanding implementation is not required to accept program that use features in the contents of <errno.h>, the implementation need not provide thread storage for errno. Similarly, since a conforming freestanding implementation is not required to accept programs that use features in the contents of <fenv.h> for accessing floating-point exceptions or for changing the floating-point rounding mode, the implementation need not provide thread storage for the floating-point environment.
> 
> In 7.20 #20 we could allow freestanding implementations to define math_errhanding to 0, to indicate that neither errno nor floating-point exceptions are supported. 
> 
> The draft footnote above doesn’t mention locales. I’m not sure what to say.
> 
> 
> The following would be more parallel, and uses the same wording as for math overflow and underflow in 7.12.1:
> ;  if the integer expression math_errhandling & MATH_ERRNO is nonzero, whether errno acquires the value ERANGE is implementation defined; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, whether the "underflow" floating-point exception is raised is implementation-defined.
> Where do you see this being added? I can't see using it in alternative 1 as that is intended to have no error handling. Was this a proposal for alternative 2?
> 
> Yes. It’s for the underflow paragraph for alternative 2, for 7.22.1.5 and 7.29.4.1.1. 
> 
> Matching the overflow paragraph to 7.12.1 gives
> ...; if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the “overflow" floating-point exception is raised.
> 
> The overflow paragraph for 7.29.4.1.1 should match 7.22.1.5.
> If so, I like it as it allows implementations to avoid the exception case as well. This however would be a change to the existing definition which requires ERANGE for overflow. I did want to avoid changing the existing specification too much beyond allowing exceptions (as it seems we do for other functions).
> 
> Hmm. Isn’t it substantively the same as what you have? It makes the floating-point numeric conversion functions like the math functions with respect to errno and floating-point exceptions. Yes, it is a change from the current C draft. The change is like when we added floating-point exceptions as an alternative to errno for math.h functions.
> 
> -  Jim Thomas
> 
> The (unchanged) last sentence in the last change does not match what is in the draft.
> Updated. This is probably also what Fred meant. Sorry Fred, I misunderstood what you were referring to.
> 
> 
> - Jim Thomas
> On Jan 19, 2022, at 11:30 AM, Rajan Bhakta <rbhakta at us.ibm.com <mailto:rbhakta at us.ibm.com>> wrote:
> (See attached file: N2823UpdateV4.html)
> 
> Regards,
> 
> Rajan Bhakta
> z/OS XL C/C++ Compiler Technical Architect
> ISO C Standards Representative (Canada, USA), PL22.11 Chair
> C/C++ Compiler Development
> rbhakta at us.ibm.com <mailto:rbhakta at us.ibm.com>
> 
> IBM
> 
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