Add Support for WASM Exceptions

[woodsmc]

There are plans to add support for exceptions in WAMR. Are there any plans to add support to the WASI-SDK tool chain to support Native WASM Exceptions?

(edit: fix link)

[sunfishcode]

The rough plan is to look at Emscripten's port of libunwind to see if it can be either ported to upstream LLVM, or if not, ported to wasi-sdk. I don't know of anyone working on this yet.

It's also worth noting that Wasm Exceptions are not yet standardized; currently the proposal is in phase 3.

[woodsmc]

We hope to have an initial contribution to WAMR with exception support in the next few months. There is such a swathe of C++ software in-particular that is excluded from the WASM ecosystem because of the lack of exception support. It would be awesome to see a solution.

Although, digging through the code that is generated by the emscripten tool chain we can see, that at least for C++ it generates a single WASM exception throw and catch. This mimics almost exactly the approach used by traditional C++ compilers, but it means that the majority of the, quite elegant and nuanced behavior described in the WASM Exception proposal is rarely used. - This might be different for other programming languages - I've not checked.

[sbc100]

As Dan mentioned, the easy part is enabling exceptions in the compiler (e.g. passing -fwasm-exceptions). The more compex/nuanced part is getting libc++abi/libunwind configured correctly.

To see that current differences from upstream that were made in emscripten you can take a look here: https://github.com/emscripten-core/emscripten/blob/main/system/lib/libcxxabi/readme.txt. Specifically check out the link at the end that shows you the diffs from upstream llvm 16: llvm/llvm-project@llvmorg-16.0.4...emscripten-core:emscripten-libs-16

[woodsmc]

Whooha... Rockstar's both of you, and I've got some reading to do. Thanks for the pointers. Let me take a look.

[abrown]

There's also a draft PR with initial support for this in #198, which you may be interested in.

[aheejin]

I have a plan to upstream the current libc++abi/libunwind changes to the upstream LLVM. I think it can happen within a couple months if having this is a priority; the timeline can depend on the receptions from the reviewers though.

By the way there is a discussions going on (WebAssembly/exception-handling#280) which can change the spec itself. I don't think the library side will change much with this though.

[aheejin]

Thank you for the wait. Wasm EH's libcxxabi/libunwind changes have now been upstreamed:
llvm/llvm-project@e6cbba7
llvm/llvm-project@058222b

[sroussey]

What does -fwasm-exceptions do? Adding or removing that makes no difference, I still end up with

wasm-ld: error: /var/folders/74/djsf8lv965l2tkgr29ybpz4h0000gn/T/lib-858918.o: undefined symbol: __cxa_allocate_exception
wasm-ld: error: /var/folders/74/djsf8lv965l2tkgr29ybpz4h0000gn/T/lib-858918.o: undefined symbol: __cxa_free_exception
wasm-ld: error: /var/folders/74/djsf8lv965l2tkgr29ybpz4h0000gn/T/lib-858918.o: undefined symbol: __cxa_throw

thx

[anuraaga]

Just wanted to check, would WASM exceptions make sense for implementing C setjmp / longjmp (i.e., how postgres uses them to implement exceptions), or are they too different and only make sense with language-native exceptions like in C++? If the latter, is there currently no plan that would be able to support setjmp / longjmp in WASI?

[whitequark]

I think you should be able to lower SjLj to exceptions.

[sbc100]

Indeed, llvm already uses wasm exception to implement SJLJ, when wasm exceptions are enabled, so it should just work once we enable exceptions.

[sunfishcode]

Indeed, wasm exceptions is how we've always envisioned setjmp/longjmp would work.

[anuraaga]

Thanks for confirming! I implemented stack unwind for wazero

wazero/wazero#1808

and look forward to trying it out with wasi SDK (from my reading of the thread it isn't quite ready yet for use with the published SDKs)

[cpetig]

What does -fwasm-exceptions do? Adding or removing that makes no difference, I still end up with

wasm-ld: error: /var/folders/74/djsf8lv965l2tkgr29ybpz4h0000gn/T/lib-858918.o: undefined symbol: __cxa_allocate_exception
wasm-ld: error: /var/folders/74/djsf8lv965l2tkgr29ybpz4h0000gn/T/lib-858918.o: undefined symbol: __cxa_free_exception
wasm-ld: error: /var/folders/74/djsf8lv965l2tkgr29ybpz4h0000gn/T/lib-858918.o: undefined symbol: __cxa_throw

thx

I found that llvm 17 was branched off end of June and above exception patches were merged mid September, so we need to wait for llvm 18 (should branch off today, first RC on Friday) or upgrade to a git version.