MSYS2: Export forwarders for `__cxa_begin_catch`, `__cxa_end_catch`, `__cxa_rethrow` and `__gxx_personality_seh0`

[qmfrederik]

libobjc2 uses native C++ exceptions on MinGW. The clang compiler will emit references to __cxa_begin_catch, __cxa_end_catch, __cxa_rethrow and __gxx_personality_seh0 for Objective C code which uses Objective C exceptions.

These symbols are defined in the C++ runtime, not in libobjc2. As a result, merely linking with libobjc2 is not sufficient. Objective C code such as GNUstep must be compiled with the LDFLAGS="-lgcc_s -lstdc++" or LDFLAGS="-lc++", depending on the environment.

This is tedious. Additionally, specifying -lc++ on the msys/clang64 environment causes linker errors:

 Linking library libgnustep-base ...
ld.lld: error: libc++.dll.a(libc++.dll): .idata$4 should not refer to special section 0

A similar error has been observed for other libraries

A solution for this is to define forwarding exports for __cxa_begin_catch, __cxa_end_catch, __cxa_rethrow and __gxx_personality_seh0. This is implemented by adding a eh_forwards.def file to the list of libobjc2 source files, which forwards the symbols to the actual C++ runtime. On MSYS2, the libstdc++ and libc++ runtimes are supported, which covers all MinGW environments: https://www.msys2.org/docs/environments/.

Forwarding exports are discussed here:

• https://learn.microsoft.com/en-us/cpp/build/reference/exports?view=msvc-170
• https://devblogs.microsoft.com/oldnewthing/20060719-24/?p=30473
• https://devblogs.microsoft.com/oldnewthing/20121116-00/?p=6073

[qmfrederik]

@MehdiChinoune - FYI. This is my best attempt at removing the need to set LDFLAGS (other than -fuse-ld=lld) on all platforms, and avoid a linker crash on clang64. One thing which bugs me is that I'd like to get the name of the shared library of the C++ runtime (e.g. libstdc++-6). I can use find_library(CXX_RUNTIME "stdc++" REQUIRED) but this will get me the name of the import lbirary (libstd++). Do you know of a way to get the actual runtime library name in CMake?

@davidchisnall It's a bit clunky, but it does work. The root problem this PR is trying to address is that ObjC code will generate calls to __cxa_begin_catch and friends, but they are not part of libobjc2, they are in the C++ runtime. An alternative is to declare wrapper functions in libobjc2, such as objc_begin_catch, which are just wrappers around the C++ runtime equivalent, and have clang emit calls to the objc_begin_catch instead?

[davidchisnall]

Option 2 might be better for modularity. Unfortunately, LLVM 18 has now branched and so we will have to support both options unless you can get the change in before it ships.

[qmfrederik]

Getting those changes in LLVM 18 while they have already declared release candidates feels like a stretch. What if I amend this PR so that it declares those functions, and we can work to get an update in the next release of LLVM? We'll have to be hybrid for a while, but MSYS2 is a fast-moving rolling release so the forwarders could be deprecated in the next 6 months or so.

I assume the API shape would look like this:

libobjc2 method	façade for
objc_begin_catch	__cxa_begin_catch
objc_end_catch	__cxa_end_catch
objc_exception_rethrow	__cxa_rethrow
__gnustep_objcxx_personality_seh0	__gxx_personality_seh0

The rethrow method is named objc_exception_rethrow and not objc_rethrow, in line with Apple's implementation. An alternative name for the personality method would be __objc_personality_v0 or __objc_personality_seh0, which would match the Apple implementation.

[qmfrederik]

Let's do it properly - #280