Switching to DWARF2 exceptions

[Kalev Lember]

Hello,

We've talked about switching from SJLJ exceptions to DWARF2 for a while. 
Last time when we were asked to switch over [1], it was too late in 
Fedora release cycle to do it without compromising stability. However, 
now that F-13 cycle has begun, I think it would be right time to switch.

Switching to DWARF2 is needed to fix binary compatibility with upstream 
mingw gcc 4 compiler. Besides that, SJLJ has runtime cost in the common 
case when no exceptions are thrown; DWARF2 doesn't have that problem. 
MinGW upstream also thinks that DWARF2 should be made default in FSF GCC 
in the future [2].

I have prepared needed changes to do the switchover. My tests show that 
throwing exceptions between mingw DLLs works correctly with DWARF2. But 
DWARF2 also includes a regression: unwinding through DLLs that use SEH 
exceptions (Visual Studio compilers produce that) no longer works. To 
work around that, upstream mingw has a patch to annotate functions which 
support unwinding with SEH with __attribute__((seh_aware)). I've tested 
that patch and it really does fix binary compatibility with binaries 
produced by Visual Studio.

After the switchover mingw32-gcc will no longer provide 
"mingw32(libgcc_s_sjlj-1.dll)". There are a number of mingw32- packages 
that require that; I'll need to rebuild those in correct depencency 
order to fix broken deps. The affected packages are:

$ sudo repoquery --disablerepo=* --enablerepo=rawhide --qf 
'%{sourcerpm}' --whatrequires 'mingw32(libgcc_s_sjlj-1.dll)' | sort | 
uniq | sed 's/-[^-]*-[^-]*\.src.rpm//'
mingw32-boost
mingw32-cairomm
mingw32-cppunit
mingw32-enchant
mingw32-gettext
mingw32-glibmm24
mingw32-gtkmm24
mingw32-hunspell
mingw32-libglademm24
mingw32-libsigc++20
mingw32-libsqlite3x
mingw32-libtiff
mingw32-libxml++
mingw32-pangomm
mingw32-plib
mingw32-plotmm
mingw32-pthreads
mingw32-qt
mingw32-qwt
mingw32-zfstream

I'll request commit access for each of those in pkgdb and rebuild the 
affected packages myself. I'm not very interested in having tons of 
broken dependencies introduced in rawhide, and the only way to avoid 
that is to rebuild the packages quickly.

Also note that mingw32-qwt currently fails to rebuild in rawhide and 
will have a broken dep; Thomas Sailer said that qwt author thinks it's a 
bug in Qt itself.


[1] https://bugzilla.redhat.com/show_bug.cgi?id=510949
[2] http://thread.gmane.org/gmane.comp.gnu.mingw.devel/3331/focus=3348

-- 
Kalev