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Subject:	Re: Updated: gcc-5.2.0-1 (Test x86/x86_64)
To:	cygwin AT cygwin DOT com
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From:	David Stacey <drstacey AT tiscali DOT co DOT uk>
Message-ID:	<56144739.7040602@tiscali.co.uk>
Date:	Tue, 6 Oct 2015 23:12:09 +0100
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On 06/10/15 18:51, Ken Brown wrote:
> On 10/6/2015 12:26 PM, Christian Franke wrote:
>> cyg Simple wrote:
>>> On 9/30/2015 7:36 PM, David Stacey wrote:
>>>> On 30/09/15 23:34, JonY wrote:
>>>>> On 10/1/2015 00:05, David Stacey wrote:
>>>>>> On 30/09/15 12:15, JonY wrote:
>>>>>>> gcc-5.2.0-1 has been uploaded for 32bit and 64bit Cygwin.
>>>>>>>
>>>>>>> This is the first series of the 5.x releases, and should be
>>>>>>> considered
>>>>>>> as experimental as such.
>>>>>> Have you managed to work around the ABI change in gcc-5 [1], or will
>>>>>> this require a mass rebuild at the point gcc-5 becomes 'current'?
>>>>>>
>>>>>> [1] 
>>>>>> -http://developerblog.redhat.com/2015/02/05/gcc5-and-the-c11-abi/
>>>>> As far as I know, every gcc release will break C++ ABI, so it would
>>>>> mean
>>>>> rebuilding everything C++.
>>>> According to the Red Hat blog above, the last time g++ caused an ABI
>>>> change was back in the 3.x days, so it hasn't happened for a while. Ah
>>>> well, we have maintainers for most packages in Cygwin, so we'll 
>>>> have to
>>>> co-ordinate a rebuild.
>>> Regardless, JonY is correct.  Every C++ release, regardless of the
>>> vendor, causes an ABI break with shared libraries and the naming of the
>>> object elements (mangled names).
>>
>> Probably not in this 4.X -> 5.X case. Otherwise the new cygstdc++-6.dll
>> should IMO be renamed to -6.1, -7 or similar.
>>
>> A diff of exported symbols of cygstdc++-6.dll 4.9.3-1 and 5.2.0 shows
>> that the new version only adds new symbols.
>>
>>
>> Results of a quick build check with smartmontools package:
>>
>> - Existing package which was built with g++ 4.9.2 runs with 5.2.0
>> cygstdc++-6.dll and cyggcc_s-1.dll.
>>
>> - A rebuild with g++ 5.2.0 does not run with 4.9.3 cygstdc++-6.dll due
>> to missing symbols. This is as expected because 5.X adds a new C++11
>> conforming std::string which is used by default
>> (https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html).
>>
>> - A rebuild with g++ 5.2.0 and CXXFLAGS=-D_GLIBCXX_USE_CXX11_ABI=0 runs
>> with old 4.9.3 cygstdc++-6.dll and cyggcc_s.dll.
>>
>> So there is probably no need to rebuild existing C++ packages.
>
> What about the following scenario: Package P links against library L. 
> Library L is rebuilt to use the new ABI.  Isn't it possible that 
> package P will then have to be rebuilt as well?

My understanding is that if 'L' is built with -D_GLIBCXX_USE_CXX11_ABI=0 
then P wouldn't need to be rebuilt (I haven't tested that, though). 
However, this compiler switch loses some C++11 features of the STL, so 
it isn't a magic cure.

Dave.


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