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

["Václav Haisman" <vhaisman@gmail.com>]

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. :)

-- 
VH

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