[CMake] CMake, Mingw-w64 32 bit exception handling.

[Arne Kjetil Andersen]

Greetings.

I'm a developer on a fairly large project where I'm using CMake version
3.9.1

I primarily work on linux, but also cross compiles for windows using
Mingw-w64 on my linux box.

I have encountered an issue which I'm having some trouble figuring out.
Running through some of my tests where an exception is thrown (on purpose)
the 32 bit version compiled with Mingw-w64-g++ version 7.1.1 just calls
terminate even though there are try catch blocks. Now mind you, this all
works fine on the native linux compiled version of my tests, and also the
64 bit windows version compiled with Mingw-w64-g++ version 7.1.1.

Going through all the projects CMakeLists.txt I could not find any reason
for this behavior, but tried to add -fexceptions as a compiler option in
the top most CMakeLists.txt file for the 32 bit mingw-w64 compiler.
Unfortunately this made no difference.

So investigating some more I took a look at the linklibs.rsp file generated
for that particular test executable, and noticed this entry:
-lgcc_eh -lgcc_eh

(yes it's twice, but that is not the issue, although that might be a cmake
bug?).
(also note - this option is also present for the 64 bit build files for
mingw-w64, but there it works as expected).

Now, removing those two library link options from the linklibs.rsp file
makes the 32 bit windows version of test application work as expected. I am
not sure what libgcc_eh.a actually does (tried searching for some
information, but had little luck actually figuring that out), but clearly
it has something to do with exception handling.

Now I figured I would create a small minimal example that would reproduce
this issue outside my projects source tree. So basically created a small
program that throws an exception, and catches that. Created a
CMakeLists.txt file with the same general options as my farily large
project, and had cmake generate the build files for 32 bit mingw-w64.
Inspecting the linklibs.rsp file I was surprised to see that "-lgcc_eh"
were nowhere to be found, and as such the 32 bit version of this test
worked fine.

So, my question is, does anyone know under which circumstances cmake will
add -lgcc_eh to linklibs.rsp, and is there any way I can prevent cmake from
doing so for the 32 bit mingw-w64 compiler?

Also, maybe I'm going about this issue the wrong way, and that my findings
mentioned above is not a good way of handling this. Or maybe this might be
a bug with the 32 bit mingw-w64 compiler?

I should probably also mention that the 32 bit version of Mingw-w64 uses
the sjlj exception handling mechanism.

Any help and pointers would be greatly appreciated - cause adding a step in
the developer documentation to go into the linklibs.rsp file to remove
-lgcc_eh is kind of a last resort.

Thanks for any input on this matter, and please let me know if attaching
CMakeOutput.log or other files would be beneficial.

Best Regards,
Arne Kjetil Andersen
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