[Bug lto/114241] New: False-positive -Wodr warning when using -flto and -fno-semantic-interposition

[Bug lto/114241] New: False-positive -Wodr warning when using -flto and -fno-semantic-interposition

[abbeyj+gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114241

            Bug ID: 114241
           Summary: False-positive -Wodr warning when using -flto and
                    -fno-semantic-interposition
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: abbeyj+gcc at gmail dot com
  Target Milestone: ---

When building the following example, a `-Wodr` warning is shown.  I don't think
the code violates the ODR so this warning is unexpected.  This reproducer
consists of 3 files.

d.h:
```
class A {
public:
  virtual ~A();
  virtual void a1();
};

class B : public A {
public:
  void a1();
  virtual void b1() = 0;
  virtual void b2() = 0;
};

class C : public B {
};

class D : public C {
public:
  void b1();
  void b2();
};
```


foo.cpp:
```
#include "d.h"

void foo() {
  D x;
}
```


bar.cpp:
```
#include "d.h"

void B::a1() {
}

void bar() {
  D y;
}
```


Build using:
g++ -fPIC -fno-semantic-interposition -flto -c foo.cpp
g++ -fPIC -fno-semantic-interposition -flto -c bar.cpp
g++ -shared foo.o bar.o -o lib.so


Output:
> d.h:14:7: warning: virtual table of type ‘struct C’ violates one definition rule [-Wodr]
>    14 | class C : public B {
>       |       ^
> d.h:14:7: note: the conflicting type defined in another translation unit has virtual table with more entries
>    14 | class C : public B {
>       |       ^


The warning message here is not great as it refers to "another translation
unit" but never names either of the two translation units involved.  In this
reduced example there are only two translation units but when there are many
translation units involved it is more difficult to figure out where the problem
is coming from.

The bigger issue is that I don't think this warning should have been issued at
all.  I can't see any way that the code is violating the ODR.

The earliest version that I've been able to reproduce with is 8.4

Godbolt link: https://godbolt.org/z/ncnqPMdzq

[Bug lto/114241] False-positive -Wodr warning when using -flto and -fno-semantic-interposition

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114241

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org
     Ever confirmed|0                           |1
      Known to fail|                            |13.2.1
   Last reconfirmed|                            |2024-03-06
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed also with GCC 13.  I guess we're optimizing things in a strange
(invalid) way before WPA and get confused because of that.

Honza?

[Bug lto/114241] False-positive -Wodr warning when using -flto and -fno-semantic-interposition

[hubicka at ucw dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114241

--- Comment #2 from Jan Hubicka <hubicka at ucw dot cz> ---
This indeed looks like bug caused by fact that the class is keyed into
one of the two units.

Outputting translation unit names is unfortunately hard, since they are
object files and often comming from .a archive where we know only
offset, but no original file name or something.

I had patch which passed through original filename attached to
TRALSATION_UNIT_DECL, but it also had problems, i.e. printing relative
filenames from wrong directory.

[Bug lto/114241] False-positive -Wodr warning when using -flto and -fno-semantic-interposition

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114241

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |hubicka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
mine. Will debug why the tables diverges.