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* [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors
@ 2023-10-11 14:42 iamsupermouse at mail dot ru
2023-10-11 17:17 ` [Bug c++/111771] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: iamsupermouse at mail dot ru @ 2023-10-11 14:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771
Bug ID: 111771
Summary: Incorrect "is used uninitialized" warning, as if
zero-initialization didn't propagate through
user-provided default constructors
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: iamsupermouse at mail dot ru
Target Milestone: ---
Here's the code. GCC with `-Wall -O3` warns that `x` is used uninitialized,
even though it's zeroed.
struct A
{
A() {}
int x;
};
struct B : A {};
int main()
{
B b = B();
return b.x;
}
Since `B` doesn't have a user-provided default constructor, value-initializing
it like this performs zero-initialization, which propagates recursively
(http://eel.is/c++draft/dcl.dcl#dcl.init.general-6.2) over all members, zeroing
everything. Yet GCC incorrectly warns about `x` being uninitialized.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors
2023-10-11 14:42 [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors iamsupermouse at mail dot ru
@ 2023-10-11 17:17 ` pinskia at gcc dot gnu.org
2023-10-12 5:17 ` iamsupermouse at mail dot ru
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-10-11 17:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Did you miss that the implicit B constructor will just call A's constructor ?
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors
2023-10-11 14:42 [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors iamsupermouse at mail dot ru
2023-10-11 17:17 ` [Bug c++/111771] " pinskia at gcc dot gnu.org
@ 2023-10-12 5:17 ` iamsupermouse at mail dot ru
2023-10-12 7:29 ` rguenth at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: iamsupermouse at mail dot ru @ 2023-10-12 5:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771
--- Comment #2 from Egor <iamsupermouse at mail dot ru> ---
Before calling A's constructor, it will zero `x` anyway.
I was also surprised when I learned this yesterday, but it's what the standard
says.
1. `()` performs value-initialization on B:
http://eel.is/c++draft/dcl.dcl#dcl.init.general-16.4
2. Since B's ctor is not user-provided, that resolves to zero-initialization
followed by default-initialization:
http://eel.is/c++draft/dcl.dcl#dcl.init.general-9.1.2
3. Zero-initialization of B propagates to A, then propagates to `x` and zeroes
it, regardless of A having a user-provided constructor or not:
http://eel.is/c++draft/dcl.dcl#dcl.init.general-6.2
4. Lastly default-initialization of B calls B's constructor and in turn calls
A's constructor.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors
2023-10-11 14:42 [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors iamsupermouse at mail dot ru
2023-10-11 17:17 ` [Bug c++/111771] " pinskia at gcc dot gnu.org
2023-10-12 5:17 ` iamsupermouse at mail dot ru
@ 2023-10-12 7:29 ` rguenth at gcc dot gnu.org
2023-10-12 7:30 ` rguenth at gcc dot gnu.org
2023-10-12 16:34 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-10-12 7:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2023-10-12
Keywords| |diagnostic, wrong-code
Status|UNCONFIRMED |NEW
CC| |jason at gcc dot gnu.org
Known to fail| |13.2.1, 7.5.0
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
We have
int main ()
{
int D.2848;
{
struct B b;
try
{
b.D.2778.x = 0;
B::B (&b);
D.2848 = b.D.2778.x;
return D.2848;
but:
void B::B (struct B * const this)
{
_1 = &this->D.2778;
A::A (_1);
}
void A::A (struct A * const this)
{
*this = {CLOBBER};
so A::A invoked by B::B invalidates the initialized storage. Maybe a
"different" B::B should have been called (one not invoking A::A?).
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors
2023-10-11 14:42 [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors iamsupermouse at mail dot ru
` (2 preceding siblings ...)
2023-10-12 7:29 ` rguenth at gcc dot gnu.org
@ 2023-10-12 7:30 ` rguenth at gcc dot gnu.org
2023-10-12 16:34 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-10-12 7:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771
--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
-fno-lifetime-dse fixes the issue (and the diagnostic)
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/111771] Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors
2023-10-11 14:42 [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors iamsupermouse at mail dot ru
` (3 preceding siblings ...)
2023-10-12 7:30 ` rguenth at gcc dot gnu.org
@ 2023-10-12 16:34 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-10-12 16:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111771
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |DUPLICATE
Status|NEW |RESOLVED
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 108993.
*** This bug has been marked as a duplicate of bug 108993 ***
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-10-12 16:34 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2023-10-11 14:42 [Bug c++/111771] New: Incorrect "is used uninitialized" warning, as if zero-initialization didn't propagate through user-provided default constructors iamsupermouse at mail dot ru
2023-10-11 17:17 ` [Bug c++/111771] " pinskia at gcc dot gnu.org
2023-10-12 5:17 ` iamsupermouse at mail dot ru
2023-10-12 7:29 ` rguenth at gcc dot gnu.org
2023-10-12 7:30 ` rguenth at gcc dot gnu.org
2023-10-12 16:34 ` pinskia at gcc dot gnu.org
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