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* [Bug c++/111094] New: Spurious Wuninitialized swapping underlying bytes of object representation in move constructor
@ 2023-08-21 19:26 ed at catmur dot uk
2023-08-21 19:33 ` [Bug tree-optimization/111094] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: ed at catmur dot uk @ 2023-08-21 19:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111094
Bug ID: 111094
Summary: Spurious Wuninitialized swapping underlying bytes of
object representation in move constructor
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ed at catmur dot uk
Target Milestone: ---
struct S { short x, y; };
struct A {
A() {}
A(A&& rhs) {
auto p = reinterpret_cast<unsigned char*>(&s);
auto q = reinterpret_cast<unsigned char*>(&rhs.s);
for (int i = 0; i != sizeof s; ++i) {
auto t = p[i];
p[i] = q[i];
q[i] = t;
}
}
bool b = false;
S s;
};
A f() {
A a;
A b = static_cast<A&&>(a);
return b;
}
at -O3 -Wall:
In constructor 'A::A(A&&)',
inlined from 'A f()' at <source>:18:29:
<source>:9:23: warning: '*(__vector(4) unsigned char*)((char*)&a + offsetof(A,
A::s.S::x))' is used uninitialized [-Wuninitialized]
9 | p[i] = q[i];
| ~~~^
<source>: In function 'A f()':
<source>:17:7: note: 'a' declared here
17 | A a;
| ^
I'm reasonably sure that this usage of swapping underlying bytes is OK by
[basic.indet]/2.3.
(Motivation is e.g. optional with empty-on-move behavior and optimization for
trivial types.)
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/111094] Spurious Wuninitialized swapping underlying bytes of object representation in move constructor
2023-08-21 19:26 [Bug c++/111094] New: Spurious Wuninitialized swapping underlying bytes of object representation in move constructor ed at catmur dot uk
@ 2023-08-21 19:33 ` pinskia at gcc dot gnu.org
2023-08-21 19:34 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-21 19:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111094
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|c++ |tree-optimization
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is what the IR looks like:
a ={v} {CLOBBER};
*b_2(D).b = 0;
vect__14.11_28 = MEM <vector(4) unsigned char> [(unsigned char *)&a + 2B];
MEM <vector(4) unsigned char> [(unsigned char *)b_2(D) + 2B] =
vect__14.11_28;
a ={v} {CLOBBER(eol)};
return b_2(D);
Which is copying the unitialized parts of a.S to b.S.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/111094] Spurious Wuninitialized swapping underlying bytes of object representation in move constructor
2023-08-21 19:26 [Bug c++/111094] New: Spurious Wuninitialized swapping underlying bytes of object representation in move constructor ed at catmur dot uk
2023-08-21 19:33 ` [Bug tree-optimization/111094] " pinskia at gcc dot gnu.org
@ 2023-08-21 19:34 ` pinskia at gcc dot gnu.org
2023-08-21 19:43 ` pinskia at gcc dot gnu.org
2023-08-21 19:59 ` ed at catmur dot uk
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-21 19:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111094
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
But that does not mean GCC should not warn about this. It just means it will
NOT be undefined behavior ...
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/111094] Spurious Wuninitialized swapping underlying bytes of object representation in move constructor
2023-08-21 19:26 [Bug c++/111094] New: Spurious Wuninitialized swapping underlying bytes of object representation in move constructor ed at catmur dot uk
2023-08-21 19:33 ` [Bug tree-optimization/111094] " pinskia at gcc dot gnu.org
2023-08-21 19:34 ` pinskia at gcc dot gnu.org
@ 2023-08-21 19:43 ` pinskia at gcc dot gnu.org
2023-08-21 19:59 ` ed at catmur dot uk
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-21 19:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111094
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also I read 2.2 differently (https://eel.is/c++draft/basic.indet) .
Since the object which you are reading from is of type S and even though you
are using an `unsigned char` to read the value here, it will not just be an
Indeterminate but also will be undefined.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/111094] Spurious Wuninitialized swapping underlying bytes of object representation in move constructor
2023-08-21 19:26 [Bug c++/111094] New: Spurious Wuninitialized swapping underlying bytes of object representation in move constructor ed at catmur dot uk
` (2 preceding siblings ...)
2023-08-21 19:43 ` pinskia at gcc dot gnu.org
@ 2023-08-21 19:59 ` ed at catmur dot uk
3 siblings, 0 replies; 5+ messages in thread
From: ed at catmur dot uk @ 2023-08-21 19:59 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111094
Ed Catmur <ed at catmur dot uk> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #4 from Ed Catmur <ed at catmur dot uk> ---
Well, I feel like [basic.types.general] probably allows it.
But regardless, yeah, this is an opt-in diagnostic so it's fine if gcc warns on
things that are legal but dubious. We'll just have to be a bit smarter in how
we write this code. Thanks again.
^ permalink raw reply [flat|nested] 5+ messages in thread
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2023-08-21 19:26 [Bug c++/111094] New: Spurious Wuninitialized swapping underlying bytes of object representation in move constructor ed at catmur dot uk
2023-08-21 19:33 ` [Bug tree-optimization/111094] " pinskia at gcc dot gnu.org
2023-08-21 19:34 ` pinskia at gcc dot gnu.org
2023-08-21 19:43 ` pinskia at gcc dot gnu.org
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