[Bug c++/107535] New: Shouldn't -fvisibility=hidden hide C++17 inline static variables?

[Bug c++/107535] New: Shouldn't -fvisibility=hidden hide C++17 inline static variables?

[junchao.zhang at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107535

            Bug ID: 107535
           Summary: Shouldn't -fvisibility=hidden hide C++17 inline static
                    variables?
           Product: gcc
           Version: 11.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: junchao.zhang at gmail dot com
  Target Milestone: ---

I think inline static variables are global. However, they could be hidden by
-fvisibility=hidden.
This is dangerous, since with C++17 inline static variables,  they will appear
in headers and be included in *.cpp files. If the *.cpp files are compiled with
-fvisibility=hidden, each will get their own storage.

$ cat foo.hpp 
struct Foo {static inline int s = 100; };
extern __attribute__ ((visibility ("default"))) void DumpLibFoo();

$ cat foo.cpp
#include <stdio.h>
#include <foo.hpp>
void DumpLibFoo() { printf("In libfoo, Foo::s (%p) = %d\n", &Foo::s, Foo::s); }

$ cat test.cpp
#include <stdio.h>
#include <foo.hpp>

int main() {
  Foo::s = 200;
  printf("In main,   Foo::s (%p) = %d\n", &Foo::s, Foo::s);

  DumpLibFoo();
  return 0;
}

-----
gcc -std=c++17 -c -fPIC -I./ test.cpp -fvisibility=hidden
gcc -std=c++17 -c -fPIC -I./ foo.cpp
gcc -o libfoo.so -shared foo.o
gcc -std=c++17 -o test test.o -Wl,-rpath ./ -L ./ -lfoo

./test
In main,   Foo::s (0x5604c2741010) = 200
In libfoo, Foo::s (0x7f4e5173d028) = 100

[Bug c++/107535] -fvisibility=hidden should not hide C++17 inline static variables

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107535

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ABI

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
kind-of a pilot error, maybe we should diagnose those cases instead?

[Bug c++/107535] -fvisibility=hidden should not hide C++17 inline static variables

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107535

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> kind-of a pilot error,

Yes, this seems to be exactly what the code asks for. If you compile the class
with hidden visibility, its static member isn't shared between libfoo.so and
the rest of the program.

I think this fixes it:
struct __attribute__ ((visibility ("default"))) Foo {static inline int s = 100;
};

> maybe we should diagnose those cases instead?

So warn that DumpLibFoo has global visibility but refers to a global variable
that's potentially reachable from outside the current library, but that the
global variable's visibility doesn't match? Or something else?