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Date:	Tue, 06 Oct 2015 18:26:26 +0200
From:	Christian Franke <Christian DOT Franke AT t-online DOT de>
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To:	cygwin AT cygwin DOT com
Subject:	Re: Updated: gcc-5.2.0-1 (Test x86/x86_64)
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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.


BTW: segment sizes differ considerably:

$ size */smartctl.exe
    text    data     bss     dec     hex filename
  459725    6956   66320  533001   82209 gcc-4-old-strings/smartctl.exe
  451525    7424   66260  525209   80399 gcc-5-new-strings/smartctl.exe
  517337    8912   66196  592445   90a3d gcc-5-old-strings/smartctl.exe

Christian


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