Date: Tue, 28 Sep 2021 20:33:32 +0000
I suspect it is mostly because nobody has done the work.
@Hubert<mailto:hubert.reinterpretcast_at_[hidden]> - could you offer some clarity on this part?
-- Gaby
From: Core <core-bounces_at_[hidden]> On Behalf Of Richard Smith via Core
Sent: Tuesday, September 28, 2021 1:07 PM
To: sg12_at_[hidden]
Cc: Richard Smith <richardsmith_at_[hidden]>; SG6 numerics <sci_at_[hidden]>; core_at_[hidden]
Subject: Re: [isocpp-core] [SG12] constexpr and FP exceptions (was: Implementation defining undefined behavior)
Yes, I think this would be an improvement, but it would also be a normative change. [expr.mul]/4 is very explicit and clear that division by zero is undefined behavior, even for floating-point types.
Is there a reason we haven't re-synced our floating-point specification with C's much more explicit and nailed-down formulation, other than that no-one has done the work? (I seem to recall Hubert mentioning that the C specification may not properly describe the PPC double-double representation in some ways. It would be nice to fix that if my recollection is right.)
On Tue, Sep 28, 2021 at 10:43 AM Gabriel Dos Reis via SG12 <sg12_at_[hidden]<mailto:sg12_at_[hidden]>> wrote:
That would be an improvement in clarity of the current (C++) wording. Though I don't know if 'should' would have the normative effect you're seeking...
-- Gaby
From: Core <core-bounces_at_[hidden]<mailto:core-bounces_at_[hidden]>> On Behalf Of Jason Merrill via Core
Sent: Tuesday, September 28, 2021 6:38 AM
To: Hubert Tong <hubert.reinterpretcast_at_[hidden]<mailto:hubert.reinterpretcast_at_[hidden]>>
Cc: Jason Merrill <jason_at_[hidden]<mailto:jason_at_[hidden]>>; SG6 numerics <sci_at_[hidden]<mailto:sci_at_[hidden]>>; C++ Core Language Working Group <core_at_[hidden]<mailto:core_at_[hidden]>>; sg12_at_[hidden]<mailto:sg12_at_[hidden]>
Subject: [isocpp-core] constexpr and FP exceptions (was: Implementation defining undefined behavior)
On Tue, Sep 21, 2021 at 11:48 AM Hubert Tong <hubert.reinterpretcast_at_[hidden]<mailto:hubert.reinterpretcast_at_[hidden]>> wrote:
On Tue, Sep 21, 2021 at 11:44 AM Herring, Davis via Core <core_at_[hidden]<mailto:core_at_[hidden]>> wrote:
> I'm thinking specifically about floating point division by zero, which
> sometimes hits a hardware exception and sometimes produces
> floating-point infinity (when is_iec559 is true).
>
> As with other cases of undefined behavior, this is made testable by
> constexpr. Can an implementation say that a construct that is
> undefined in the C++ standard is defined in that implementation, and
> therefore accept it in constexpr?
For this case in particular, SG6 (or at least Lawrence) has in the past recommended against supporting it <https://wiki.edg.com/bin/view/Wg21issaquah2016/CoreWorkingGroup#Core_issue_2168_Narrowing_conver<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.edg.com%2Fbin%2Fview%2FWg21issaquah2016%2FCoreWorkingGroup%23Core_issue_2168_Narrowing_conver&data=04%7C01%7Cgdr%40microsoft.com%7Cc36fdb31b66348be763408d982bb98a7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637684564515290002%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=FXLv%2FvJw75OpjW6a91BbXyuGLOlJT%2BfYAqQoK8SnztI%3D&reserved=0>> <https://lists.isocpp.org/sci/2016/03/0000.php<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.isocpp.org%2Fsci%2F2016%2F03%2F0000.php&data=04%7C01%7Cgdr%40microsoft.com%7Cc36fdb31b66348be763408d982bb98a7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637684564515299997%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=wrvH9T%2BuwhZy9fNcCcXlRYC5hLX2Tctn5mON5AvF6bI%3D&reserved=0>>.
It looks to me like those links are talking about narrowing, not division by zero. And narrowing isn't undefined, it's ill-formed.
For division by zero specifically, why doesn't the definition in IEC559 count as defined behavior, when we advertise that a type conforms to that standard? In C's Annex F, it seems to:
F.8.2 Translation
During translation the IEC 60559 default modes are in effect:
- The rounding direction mode is rounding to nearest.
- The rounding precision mode (if supported) is set so that results are not shortened.
- Trapping or stopping (if supported) is disabled on all floating-point exceptions.
Recommended practice:
The implementation should produce a diagnostic message for each translation-time floating-point exception, other than "inexact"; the implementation should then proceed with the translation of the program.
Jason
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@Hubert<mailto:hubert.reinterpretcast_at_[hidden]> - could you offer some clarity on this part?
-- Gaby
From: Core <core-bounces_at_[hidden]> On Behalf Of Richard Smith via Core
Sent: Tuesday, September 28, 2021 1:07 PM
To: sg12_at_[hidden]
Cc: Richard Smith <richardsmith_at_[hidden]>; SG6 numerics <sci_at_[hidden]>; core_at_[hidden]
Subject: Re: [isocpp-core] [SG12] constexpr and FP exceptions (was: Implementation defining undefined behavior)
Yes, I think this would be an improvement, but it would also be a normative change. [expr.mul]/4 is very explicit and clear that division by zero is undefined behavior, even for floating-point types.
Is there a reason we haven't re-synced our floating-point specification with C's much more explicit and nailed-down formulation, other than that no-one has done the work? (I seem to recall Hubert mentioning that the C specification may not properly describe the PPC double-double representation in some ways. It would be nice to fix that if my recollection is right.)
On Tue, Sep 28, 2021 at 10:43 AM Gabriel Dos Reis via SG12 <sg12_at_[hidden]<mailto:sg12_at_[hidden]>> wrote:
That would be an improvement in clarity of the current (C++) wording. Though I don't know if 'should' would have the normative effect you're seeking...
-- Gaby
From: Core <core-bounces_at_[hidden]<mailto:core-bounces_at_[hidden]>> On Behalf Of Jason Merrill via Core
Sent: Tuesday, September 28, 2021 6:38 AM
To: Hubert Tong <hubert.reinterpretcast_at_[hidden]<mailto:hubert.reinterpretcast_at_[hidden]>>
Cc: Jason Merrill <jason_at_[hidden]<mailto:jason_at_[hidden]>>; SG6 numerics <sci_at_[hidden]<mailto:sci_at_[hidden]>>; C++ Core Language Working Group <core_at_[hidden]<mailto:core_at_[hidden]>>; sg12_at_[hidden]<mailto:sg12_at_[hidden]>
Subject: [isocpp-core] constexpr and FP exceptions (was: Implementation defining undefined behavior)
On Tue, Sep 21, 2021 at 11:48 AM Hubert Tong <hubert.reinterpretcast_at_[hidden]<mailto:hubert.reinterpretcast_at_[hidden]>> wrote:
On Tue, Sep 21, 2021 at 11:44 AM Herring, Davis via Core <core_at_[hidden]<mailto:core_at_[hidden]>> wrote:
> I'm thinking specifically about floating point division by zero, which
> sometimes hits a hardware exception and sometimes produces
> floating-point infinity (when is_iec559 is true).
>
> As with other cases of undefined behavior, this is made testable by
> constexpr. Can an implementation say that a construct that is
> undefined in the C++ standard is defined in that implementation, and
> therefore accept it in constexpr?
For this case in particular, SG6 (or at least Lawrence) has in the past recommended against supporting it <https://wiki.edg.com/bin/view/Wg21issaquah2016/CoreWorkingGroup#Core_issue_2168_Narrowing_conver<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.edg.com%2Fbin%2Fview%2FWg21issaquah2016%2FCoreWorkingGroup%23Core_issue_2168_Narrowing_conver&data=04%7C01%7Cgdr%40microsoft.com%7Cc36fdb31b66348be763408d982bb98a7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637684564515290002%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=FXLv%2FvJw75OpjW6a91BbXyuGLOlJT%2BfYAqQoK8SnztI%3D&reserved=0>> <https://lists.isocpp.org/sci/2016/03/0000.php<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.isocpp.org%2Fsci%2F2016%2F03%2F0000.php&data=04%7C01%7Cgdr%40microsoft.com%7Cc36fdb31b66348be763408d982bb98a7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637684564515299997%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=wrvH9T%2BuwhZy9fNcCcXlRYC5hLX2Tctn5mON5AvF6bI%3D&reserved=0>>.
It looks to me like those links are talking about narrowing, not division by zero. And narrowing isn't undefined, it's ill-formed.
For division by zero specifically, why doesn't the definition in IEC559 count as defined behavior, when we advertise that a type conforms to that standard? In C's Annex F, it seems to:
F.8.2 Translation
During translation the IEC 60559 default modes are in effect:
- The rounding direction mode is rounding to nearest.
- The rounding precision mode (if supported) is set so that results are not shortened.
- Trapping or stopping (if supported) is disabled on all floating-point exceptions.
Recommended practice:
The implementation should produce a diagnostic message for each translation-time floating-point exception, other than "inexact"; the implementation should then proceed with the translation of the program.
Jason
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Received on 2021-09-28 15:34:04