Warning: _Unwind_RaiseException is not correctly implemented

[Midar]

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.69 ()
Debian clang version 19.1.7 (3)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin

I'm currently trying to use exceptions in Objective-C using emscripten. I've already worked around the missing _Unwind_GetIP (why is this missing to begin with?) in my EH personality, however, ultimately, it fails as soon as an exception is actually thrown with:

Warning: _Unwind_RaiseException is not correctly implemented

With the core part of the unwinder missing for exceptions, how do C++ exceptions even work? How am I supposed to work around this? I've found __throw_exception_with_stack_trace, however, trying to use that, I get a linker error.

[hmelder]

I'm currently porting libobjc2 to emscripten and hit the same issue when using emscripten exceptions.

First, not all mandatory functions specified in the Itanium ABI are implemented. __cxa_get_globals is missing, for example.

After stubbing out calls to unimplemented functions, I'm also greeted by the warning.

I cannot find any documentation on the actual implementation of Emscripten exceptions. @aheejin it would be great if you can provide more details, or point me to the relevant source files.

[hmelder]

Seems like the libunwind implementation is basically just a stub

emscripten/src/lib/libcore.js

Line 1680 in 7f262d8

err('Warning: _Unwind_RaiseException is not correctly implemented');

[hoodmane]

Presumably you need to compile and link with -fexceptions or -fwasm-exceptions to get a real libunwind.

[hmelder]

That would be -fwasm-exceptions. However, this requires some changes in our personality function, as native WebAssembly exceptions are a bit weird.

[Midar]

__cxa_get_globals missing in the unwinder is not strange, but expected. You're mixing up C++ and the unwinder here.

-fwasm-exceptions wasn't fully implemented by Clang last time I checked.

[hmelder]

__cxa_get_globals missing in the unwinder is not strange, but expected. You're mixing up C++ and the unwinder here.

I know. I was refering to the second part of the Itanium ABI here.

-fwasm-exceptions wasn't fully implemented by Clang last time I checked.

Hmm I was able to compile a simple C++ program with -fwasm-exceptions -mllvm wasm-use-legacy-eh=false. Clang currently crashes during instruction selection when compiling an Objective-C try-catch block with WASM exceptions.

[Midar]

Yeah, the ObjC support for WASM exceptions is completely missing in Clang. I don't think it crashes but throws an exception that it's not implemented. However, it would be possible for emscripten to do something similar for ObjC as it does for C++ without WASM exceptions, which are fairly new. This ticket is about making the current hacks for C++ more generic so ObjC can also use them.

[sbc100]

If you, or anyone else would like to take a crack at fixing _Unwind_RaiseException then patches would be most welcome.

At this point most folks a likely to be focused on Wasm exceptions as the future so investing in making emscripten-exceptions more feature complete less likely to happen (unless someobody like yourself decides to focus on it).

Regarding _Unwind_RaiseException specifically. It looks like it outputs a warning and then calls __cxa_throw, which I assume is not quite correct, otherwise there would not be warning there? Have you tried simply commenting out the warning to see how far you get? Presumably _Unwind_RaiseException is not required to implement C++ exceptions which is why the warning here persists until today. Any idea why its needed for ObjC but not for C++ exceptions?

[Midar]

_Unwind_RaiseException is most definitely required for C++ exceptions to work. I suspect emscripten is just stubbing exceptions out more broadly for C++, so the standard way of throwing C++ exceptions is never even used. The ticket was mainly to figure out how C++ exceptions are made to work in emscripten, as it is clearly not via the Itanium Exceptions API.

[sbc100]

_Unwind_RaiseException is most definitely required for C++ exceptions to work. I suspect emscripten is just stubbing exceptions out more broadly for C++, so the standard way of throwing C++ exceptions is never even used. The ticket was mainly to figure out how C++ exceptions are made to work in emscripten, as it is clearly not via the Itanium Exceptions API.

I see yes. I believe the answer is that emscripten uses this setjmp-longjmp style exceptions handling and it gets lowered away in this pass: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp.

It looks like there are pretty extensive docs in the comments of that file.