[Bug other/87199] New: Thread local storage dynamic initialization behaviour differs Linux vs macOS

[me at nvartolomei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87199

            Bug ID: 87199
           Summary: Thread local storage dynamic initialization behaviour
                    differs Linux vs macOS
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: me at nvartolomei dot com
  Target Milestone: ---

Hello,

Here is a test program that behaves different on Linux vs macOS.

main.c:
```
#include "lib.h"

int main() {
    getProfileEvents();
}
```

lib.cpp
```
#include "lib.h"

thread_local ThreadStatus thread_local_var = ThreadStatus();

ThreadStatus::ThreadStatus() {
    std::cout << "cons";

    doX();
}
```

lib.h
```
#pragma once

#include <memory>
#include <iostream>

class ThreadStatus {
public:
    int x = 0;

    ThreadStatus();

    static void doX() { std::cout << "test"; }
};

extern thread_local ThreadStatus thread_local_var;

static int getProfileEvents() {
    return thread_local_var.x;
}
```

Compiling it using the following command:

`g++-8 -g -std=c++17 -c -I . main.cpp && g++-8 -g  -std=c++17 -c -I . lib.cpp
&& g++-8 -o main main.o lib.o && ./main`

On a Linux machine the output is: "constest" (g++-8 (Ubuntu
8-20180414-1ubuntu2) 8.0.1 20180414 (experimental) [trunk revision 259383])
On a macOS machine the output is empty. (Homebrew GCC 8.2.0, also HEAD)

I found curious that moving doX definition from .h file to .cpp will make this
work correctly.