public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
* [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
end of thread, other threads:[~2023-08-21 19:59 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 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
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).