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.)
