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* [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results
@ 2021-10-24 17:27 Darrell.Wright at gmail dot com
2021-10-24 19:24 ` [Bug c++/102916] " pinskia at gcc dot gnu.org
` (13 more replies)
0 siblings, 14 replies; 15+ messages in thread
From: Darrell.Wright at gmail dot com @ 2021-10-24 17:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
Bug ID: 102916
Summary: cmath constexpr can lead to ODR violations/incorrect
results
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: Darrell.Wright at gmail dot com
Target Milestone: ---
Because many of the cmath functions are constexpr, even in conformance mode of
the compiler, this can lead to ODR issues. It is detectable in C++20 and gives
incorrect results.
#include <cmath>
template<auto F>
consteval auto foo( ) {
if constexpr (requires {
[]<int>() {}.template operator()<(F(), void(), 0)>();
}) {
return 0;
} else {
return 1;
}
}
auto a = foo<[]{ return std::sqrt( 4.0 ); }>( );
https://gcc.godbolt.org/z/M6zGTbTfM
gcc returns 0, clang/libstdc++ and MSVC return 1.
I believe that in conformance -std=c++20 -pedantic mode that gcc should not
mark these methods as constexpr
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug c++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
@ 2021-10-24 19:24 ` pinskia at gcc dot gnu.org
2021-10-24 19:57 ` Darrell.Wright at gmail dot com
` (12 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-24 19:24 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
> this can lead to ODR issues
I don't think it can the C++ standard allows a compiler to have an extended
const expressions IIRC.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug c++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
2021-10-24 19:24 ` [Bug c++/102916] " pinskia at gcc dot gnu.org
@ 2021-10-24 19:57 ` Darrell.Wright at gmail dot com
2021-10-24 20:18 ` Darrell.Wright at gmail dot com
` (11 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: Darrell.Wright at gmail dot com @ 2021-10-24 19:57 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #2 from Darrell Wright <Darrell.Wright at gmail dot com> ---
The constexpr value returned is different depending on the compiler. If one
uses clang and gcc this leads to an ODR issue as
void bar( ) {
if constexpr( foo<[]{ return std::sqrt( 4.0 ); }>( ) ) {
doA( );
} else {
doB( );
}
}
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug c++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
2021-10-24 19:24 ` [Bug c++/102916] " pinskia at gcc dot gnu.org
2021-10-24 19:57 ` Darrell.Wright at gmail dot com
@ 2021-10-24 20:18 ` Darrell.Wright at gmail dot com
2021-10-24 20:30 ` pinskia at gcc dot gnu.org
` (10 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: Darrell.Wright at gmail dot com @ 2021-10-24 20:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #3 from Darrell Wright <Darrell.Wright at gmail dot com> ---
Also http://eel.is/c++draft/library#constexpr.functions-1
An issue is that it's high level observable and not just an optimization
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug c++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (2 preceding siblings ...)
2021-10-24 20:18 ` Darrell.Wright at gmail dot com
@ 2021-10-24 20:30 ` pinskia at gcc dot gnu.org
2021-10-24 20:34 ` pinskia at gcc dot gnu.org
` (9 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-24 20:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Darrell Wright from comment #3)
> Also http://eel.is/c++draft/library#constexpr.functions-1
>
> An issue is that it's high level observable and not just an optimization
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2013
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug c++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (3 preceding siblings ...)
2021-10-24 20:30 ` pinskia at gcc dot gnu.org
@ 2021-10-24 20:34 ` pinskia at gcc dot gnu.org
2021-10-24 20:41 ` [Bug libstdc++/102916] " Darrell.Wright at gmail dot com
` (8 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-24 20:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #4)
> (In reply to Darrell Wright from comment #3)
> > Also http://eel.is/c++draft/library#constexpr.functions-1
> >
> > An issue is that it's high level observable and not just an optimization
>
> http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2013
So it looks GCC decided at the time when the defect report was filed to the C++
language working group, GCC changed to allow the constexpr and then the library
working group changed how to explicitly reject these instead.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (4 preceding siblings ...)
2021-10-24 20:34 ` pinskia at gcc dot gnu.org
@ 2021-10-24 20:41 ` Darrell.Wright at gmail dot com
2021-10-25 8:32 ` redi at gcc dot gnu.org
` (7 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: Darrell.Wright at gmail dot com @ 2021-10-24 20:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #6 from Darrell Wright <Darrell.Wright at gmail dot com> ---
Right, mostly it can fall under as-if(if it wasn't explicitly disallowed) but
because it's observable it can lead to some interesting behaviour differences
when libstdc++ is compiled with gcc and clang.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (5 preceding siblings ...)
2021-10-24 20:41 ` [Bug libstdc++/102916] " Darrell.Wright at gmail dot com
@ 2021-10-25 8:32 ` redi at gcc dot gnu.org
2021-10-25 8:35 ` redi at gcc dot gnu.org
` (6 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: redi at gcc dot gnu.org @ 2021-10-25 8:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
C++23 is making these constexpr anyway so I'm not very inclined to change this
now.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (6 preceding siblings ...)
2021-10-25 8:32 ` redi at gcc dot gnu.org
@ 2021-10-25 8:35 ` redi at gcc dot gnu.org
2021-10-25 9:32 ` redi at gcc dot gnu.org
` (5 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: redi at gcc dot gnu.org @ 2021-10-25 8:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And we already have a bug report about the presence of constexpr being
non-conforming. IIRC the conclusion has been "yes, but it's more useful to
support this than to strictly conform".
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (7 preceding siblings ...)
2021-10-25 8:35 ` redi at gcc dot gnu.org
@ 2021-10-25 9:32 ` redi at gcc dot gnu.org
2021-10-25 9:34 ` redi at gcc dot gnu.org
` (4 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: redi at gcc dot gnu.org @ 2021-10-25 9:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
See pr 49813. Jakub suggests we should remove 'constexpr' for strict modes at
least.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (8 preceding siblings ...)
2021-10-25 9:32 ` redi at gcc dot gnu.org
@ 2021-10-25 9:34 ` redi at gcc dot gnu.org
2021-10-25 13:29 ` Darrell.Wright at gmail dot com
` (3 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: redi at gcc dot gnu.org @ 2021-10-25 9:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Darrell Wright from comment #6)
> Right, mostly it can fall under as-if(if it wasn't explicitly disallowed)
Adding constexpr *is* explicitly disallowed though.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (9 preceding siblings ...)
2021-10-25 9:34 ` redi at gcc dot gnu.org
@ 2021-10-25 13:29 ` Darrell.Wright at gmail dot com
2022-11-30 14:52 ` redi at gcc dot gnu.org
` (2 subsequent siblings)
13 siblings, 0 replies; 15+ messages in thread
From: Darrell.Wright at gmail dot com @ 2021-10-25 13:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #11 from Darrell Wright <Darrell.Wright at gmail dot com> ---
(In reply to Jonathan Wakely from comment #7)
> C++23 is making these constexpr anyway so I'm not very inclined to change
> this now.
That is good to hear, I thought I had read/heard that there was a lot of
roadblocks on that and the behaviour of things like rounding modes.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (10 preceding siblings ...)
2021-10-25 13:29 ` Darrell.Wright at gmail dot com
@ 2022-11-30 14:52 ` redi at gcc dot gnu.org
2022-11-30 14:53 ` redi at gcc dot gnu.org
2024-05-21 3:46 ` pinskia at gcc dot gnu.org
13 siblings, 0 replies; 15+ messages in thread
From: redi at gcc dot gnu.org @ 2022-11-30 14:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Darrell Wright from comment #2)
> The constexpr value returned is different depending on the compiler. If one
> uses clang and gcc this leads to an ODR issue as
>
> void bar( ) {
> if constexpr( foo<[]{ return std::sqrt( 4.0 ); }>( ) ) {
> doA( );
> } else {
> doB( );
> }
> }
This is EXTREMELY contrived. You've basically set up a program where you detect
is std::sqrt is constexpr, and then said you get different results depending
whether std::sq
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (11 preceding siblings ...)
2022-11-30 14:52 ` redi at gcc dot gnu.org
@ 2022-11-30 14:53 ` redi at gcc dot gnu.org
2024-05-21 3:46 ` pinskia at gcc dot gnu.org
13 siblings, 0 replies; 15+ messages in thread
From: redi at gcc dot gnu.org @ 2022-11-30 14:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |WONTFIX
Status|UNCONFIRMED |RESOLVED
--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #12)
> This is EXTREMELY contrived. You've basically set up a program where you
> detect is std::sqrt is constexpr, and then said you get different results
> depending whether std::sq
... rt is constexpr. Well, yeah.
Don't do that then.
Nobody forces your code to check whether std::sqrt is constexpr, so just don't.
And if your code really chooses between invoking feed_cat() and
launch_missiles() based on that condition, you're screwed in C++23 anyway.
So just don't do that.
^ permalink raw reply [flat|nested] 15+ messages in thread
* [Bug libstdc++/102916] cmath constexpr can lead to ODR violations/incorrect results
2021-10-24 17:27 [Bug c++/102916] New: cmath constexpr can lead to ODR violations/incorrect results Darrell.Wright at gmail dot com
` (12 preceding siblings ...)
2022-11-30 14:53 ` redi at gcc dot gnu.org
@ 2024-05-21 3:46 ` pinskia at gcc dot gnu.org
13 siblings, 0 replies; 15+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-05-21 3:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102916
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |hstong at ca dot ibm.com
--- Comment #14 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 115171 has been marked as a duplicate of this bug. ***
^ permalink raw reply [flat|nested] 15+ messages in thread
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