[Releasing] MinGW 4.8.1 upgrade for 5.2 - InterlockedCompareExchange conflict

[Thiago Macieira]

On terça-feira, 3 de setembro de 2013 12:25:14, Koehne Kai wrote:
> It looks like one cannot run the current Qt 5.1.0 , Qt 5.1.1 MinGW that was
> built with mingw-builds 4.8.0 rev2 on top of a newer gcc 4.8.1 runtime. The
> reason is an 'InterlockedCompareExchange at 12' symbol that apparently was
> accidentally part of the toolchain [1], and which at least some ICU / Qt
> libs now require. 

Sorry, after reading the full thread again in the mingw ML, I'm confused. The 
reports in the mingw ML are that the libraries would export an extra symbol 
that should not have been exported.

Is that correct: an *export* from the DLL? Not an *import* from another DLL 
(kernel32.dll or a mingw DLL)?


I have a fairly recent build of Qt 5.2 with the toolchain at [1]. It shows me 
this:

$ for f in *.dll; do objdump -p $f | grep -B2 Interlocked; done
        DLL Name: libstdc++-6.dll
        vma:  Hint/Ord Member-Name Bound-To
        e6b4        0  InterlockedCompareExchange at 12
        DLL Name: libstdc++-6.dll
        vma:  Hint/Ord Member-Name Bound-To
        106cc       0  InterlockedCompareExchange at 12
[and others]

That means it's an *import* of the symbol, from libstdc++. So, IIUC, there was 
a problem with the headers, causing the symbol to get exported from some libs, 
including and especially the libstdc++ that got shipped with that release.

Then, for some reason, code used InterlockedCompareExchange, which the mingw 
headers had as a regular function as opposed to a 
 static inline __attribute__((__always_inline__,__gnu_inline__,__artificial__)) 
intrinsic (see GCC's *intrin.h files). That meant our DLLs now import the 
symbol from libstdc++.

My suggestion for resolution: don't break BC. Keep the symbol exported from 
the MinGW libraries where it got exported. From what I can tell, it's 
libgomp-1.dll, libquadmath-0.dll, libssp-0.dll and libstdc++-6.dll.

[1] http://download.qt-project.org/development_releases/prebuilt/mingw_32/x32-4.8.0-release-posix-dwarf-rev2-runtime.7z

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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