Re: C++11 thread_local implementation issue on Cygwin/AMD64

[Václav Haisman <vhaisman at gmail dot com>]

On 9 February 2016 at 15:23, Václav Haisman <vhaisman@gmail.com> wrote:
> On 19 May 2015 at 10:52, Václav Haisman <vhaisman@gmail.com> wrote:
>> On 19 January 2015 at 15:42, Václav Haisman wrote:
>>>
>>> Hi.
>>>
>>> I have hit an issue with thread-local storage variables on
>>> Cygwin/AMD64, I do not see it with Cygwin/i686.
>>>
>>> I am having linking issues when using `thread_local` keyword in Cygwin
>>> with its GCC 4.8.3 and GCC 4.9.2. This is derived from log4cplus. The
>>> test case is split into three files:
>>>
>>> File def.hxx:
>>>
>>> ~~~~
>>> #include <string>
>>>
>>> namespace N
>>> {
>>>   struct S { std::string str; };
>>>   // extern declaration in a header
>>>   extern thread_local S * ptd;
>>>
>>>   // accessing the extern declared ptd here
>>>   inline
>>>   S * get_ptd ()
>>>   {
>>>     if (! ptd)
>>>       ptd = new S;
>>>     return ptd;
>>>   }
>>> } // namespace N
>>> ~~~~
>>>
>>> File def.cxx:
>>>
>>> ~~~~
>>> #include "def.hxx"
>>>
>>> namespace N
>>> {
>>>   // definition of ptd
>>>   thread_local S * ptd = nullptr;
>>> } // namespace N
>>> ~~~
>>>
>>> File use.cxx:
>>>
>>> ~~~~
>>> #include "def.hxx"
>>>
>>> namespace N
>>> {
>>>   __declspec(dllexport)
>>>   void * foo ()
>>>   {
>>>     // invoking inline get_ptd() function to get the value in ptd
>>>     return get_ptd ();
>>>   }
>>> }
>>> ~~~~
>>>
>>> Now, when I compile each .cxx with `g++ -std=gnu++11
>>> -fvisibility=hidden -c use.cxx def.cxx` and then try to link with `g++
>>> -shared -o cygtest.dll use.o def.o`, I get the following error from
>>> linker:
>>>
>>> ~~~~
>>> use.o:use.cxx:(.text$_ZTWN1N3ptdE[_ZTWN1N3ptdE]+0x15): relocation
>>> truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init
>>> function for N::ptd'
>>> collect2: error: ld returned 1 exit status
>>> ~~~~
>>>
>>> The nm -C ./def.o output confirms that:
>>>
>>> ~~~~
>>> `--> nm -C ./def.o
>>> 0000000000000000 b .bss
>>> 0000000000000000 d .data
>>> 0000000000000000 r .rdata
>>> 0000000000000000 r .rdata$zzz
>>> 0000000000000000 t .text
>>> 0000000000000008 r __emutls_t._ZN1N3ptdE
>>> 0000000000000000 D __emutls_v._ZN1N3ptdE
>>> 0000000000000000 r std::piecewise_construct
>>> ~~~~
>>>
>>> As you can see, the ptd thread-local variable initialization function
>>> is not defined anywhere. The use.o references this initialization
>>> function (see bottom of the listing):
>>>
>>> ~~~~
>>> `--> nm -C ./use.o
>>> 0000000000000000 b .bss
>>> 0000000000000000 d .data
>>> 0000000000000000 i .drectve
>>> 0000000000000000 p .pdata
>>> 0000000000000000 p .pdata$_ZN1N1SC1Ev
>>> 0000000000000000 p .pdata$_ZN1N7get_ptdEv
>>> 0000000000000000 p .pdata$_ZTWN1N3ptdE
>>> 0000000000000000 r .rdata
>>> 0000000000000000 r .rdata$.refptr.__emutls_v._ZN1N3ptdE
>>> 0000000000000000 r .rdata$.refptr._ZTHN1N3ptdE
>>> 0000000000000000 r .rdata$zzz
>>> 0000000000000000 R .refptr.__emutls_v._ZN1N3ptdE
>>> 0000000000000000 R .refptr._ZTHN1N3ptdE
>>> 0000000000000000 t .text
>>> 0000000000000000 t .text$_ZN1N1SC1Ev
>>> 0000000000000000 t .text$_ZN1N7get_ptdEv
>>> 0000000000000000 t .text$_ZTWN1N3ptdE
>>> 0000000000000000 A .weak._ZTHN1N3ptdE._ZN1N1SC1Ev
>>> 0000000000000000 r .xdata
>>> 0000000000000000 r .xdata$_ZN1N1SC1Ev
>>> 0000000000000000 r .xdata$_ZN1N7get_ptdEv
>>> 0000000000000000 r .xdata$_ZTWN1N3ptdE
>>>                  U __emutls_get_address
>>>                  U __emutls_v._ZN1N3ptdE
>>>                  U __gxx_personality_seh0
>>>                  U __real__ZdlPv
>>>                  U __real__Znwm
>>>                  U _Unwind_Resume
>>>                  U operator delete(void*)
>>> 0000000000000000 T N::S::S()
>>> 0000000000000000 T N::foo()
>>> 0000000000000000 T N::get_ptd()
>>>                  U std::basic_string<char, std::char_traits<char>,
>>> std::allocator<char> >::basic_string()
>>>                  U operator new(unsigned long)
>>> 0000000000000000 r std::piecewise_construct
>>>                  w TLS init function for N::ptd
>>> 0000000000000000 T TLS wrapper function for N::ptd
>>> ~~~~
>>>
>>> Now, this code seems to work well on Linux with both GCC and Clang.
>>>
>>> Is this a GCC problem on Cygwin?
>>> Am I using extern thread_local wrong?
>>>
>>> My experiments show that not using the extern keyword seems to fix the
>>> issue. But I am not sure if that does not introduce two ptd
>>> thread-local variables in two TUs.
>>>
>>> See also http://stackoverflow.com/q/28023728/341065
>>
>>
>> This is also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64697.
>
> Annual ping. :)

Annual ping for 2017. :)

-- 
VH

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