Build compiler-rt with C++ exception tag definition

[kketch]

I tried compiling wasi-sdk's sysroot to build a C++ library for wasm32-wasip1 by following these instructions: https://github.com/WebAssembly/wasi-sdk/blob/main/CppExceptions.md.

However when running the resulting WASM module it tries to import a non existent exception tag: __cpp_exception.

I found out that LLVM's compiler-rt also needs to be built with -fwasm-exceptions otherwise the compiled object does not define the __cpp_exception tag used for C++ exceptions.

[alexcrichton]

Thanks for the PR! I was just digging into this myself yesterday and there's been a few updates on this in the meantime. The problem here is that the exceptions-testing in CI is done with LLVM prior to 22, and this change you're running into happened in LLVM 22. This means that the exceptions-support for wasi-sdk is broken with LLVM 22, which is what the submodule in this repository contains.

The "true fix" here is a PR I sent yesterday upstream to LLVM -- llvm/llvm-project#185770 -- which moves the definition of __cpp_exception to libunwind rather than compiler-rt. I say "true fix" here because long-term I'd like to be able to support a single sysroot which both does and doesn't support C++ exceptions. This change, however, would prevent that from happening because compiler-rt is target-specific and doesn't get a chance to have multiple versions.

That's all basically a long way of saying -- thanks for the PR, I agree this is broken, but if you wouldn't mind I'd prefer to not land this solution. I'll try to follow-up with some submodule-patches today to get exceptions working once wasi-sdk-31 is released.

[kketch]

Thanks, after opening this PR I noticed the exceptions test was failing locally but not on CI, that makes more sense now.

[alexcrichton]

#606 is the PR I wanted to make, and that should fix exceptions with LLVM 22 by including upstream patches.