[llvm-bugs] [Bug 51814] New: Mixing objects with thread_local in C++ and _Thread_local in C

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51814

            Bug ID: 51814
           Summary: Mixing objects with thread_local in C++ and
                    _Thread_local in C
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: simon.marchi at polymtl.ca
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Hi,

I'm not sure that this is really a bug, but if I could at least get an
explanation of what is happening I'd be really happy.  With clang 12, this
fails to build:

$ cat tls.c
extern __attribute__((visibility("hidden"))) _Thread_local const char
*logger_thread_name;

__attribute__((visibility("hidden"))) _Thread_local const char
*logger_thread_name;
$ cat main.cpp 
extern "C" {
  extern __attribute__((visibility("hidden")))
    // Works if you use _Thread_local
    // _Thread_local
    thread_local
    const char *logger_thread_name;
}

const char *func() {
    return logger_thread_name;
}
$ cat build.sh 
clang++ main.cpp -c -fPIC -DPIC
clang tls.c -c -fPIC -DPIC
clang++ main.o tls.o -shared
$ bash build.sh
/usr/bin/ld: main.o: relocation R_X86_64_PC32 against undefined hidden symbol
`_ZTH18logger_thread_name' can not be used when making a shared object
/usr/bin/ld: final link failed: bad value
clang-12: error: linker command failed with exit code 1 (use -v to see
invocation)

What we have here is:

 - a C source file defining a TLS variable using _Thread_local, with the
attribute visibility("hidden")
 - a C++ source file using this TLS variable, the declaration it sees is in an
extern "C", but uses "thread_local", and also has the attribute
visibility("hidden")
 - both 
 - we compile both using -fPIC and link them into a shared library
 - the linking fails because of an invalid relocation type

The link is successful is you either:

 - change the declaration in the C++ file to use the C keyword _Thread_local
instead
 - change the declaration in the C++ file to remove the visibility("hidden")
attribute

Is the code above doing things that we really shouldn't do?  Regardless of
that, is there some bug in clang, given that it produces a R_X86_64_PC32
relocation in a file built with -fPIC?

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