[Emscripten-EH] Implement __cxa_throw in terms of _Unwind_RaiseException - #27498
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I could potentially split this change up, land an NFC that just moves the old unwind code into its own file? Would that be useful? |
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__cxa_throw in terms of _Unwind_RaiseException
Extract `_Unwind_*` functions and `uncaughtExceptionCount` / `exceptionLast` JS state from libcore.js and libexceptions.js into a dedicated `libunwind.js`. This new libunwind.js is included as long as wasm EH is not being used. Also, invert the dependency between `__cxa_throw` and `_Unwind_RaiseException` so that `__cxa_throw` now calls `_Unwind_RaiseException` rather than the other way around. This is important as it allows C programs (or rather non-C++ programs like Rust) to call `_Unwind_RaiseException` without linking as C++. Inspired by emscripten-core#27496
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The reason we see a light code size saving here is because |
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Funny how I can work on compilers for decades and not be aware of how/when libunwind function are linked / used... |
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| $uncaughtExceptionCount: '0', | ||
| #if !DISABLE_EXCEPTION_CATCHING | ||
| $exceptionLast: null, | ||
| #endif |
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In libcxxabi native code (and thus in Wasm EH), managing information like uncaught exception count and last exception is done by libcxxabi, not libunwind. It is true in libcxxabi/libunwind that __cxa_throw calls _Unwind_RaiseException, but all exception status managing is done within libcxxabi. Wouldn't it be consistent to manage it within libexceptions.js and leave libunwind simple?
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Sure, we can make it consistent in that way. I'll make a followup PR.
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#27496) This makes `-fwasm-exceptions` link the Wasm EH runtime independently of C++ linking. Wasm EH objects can be produced by non-C++ frontends - notably rustc, whose precompiled std uses Wasm exceptions for unwinding and which invokes `emcc` (not `em++`) as its linker. Since `DEFAULT_TO_CXX` was disabled by default in #27469, any rust-driven emcc link now fails with: ``` wasm-ld: error: libstd-*.rlib: undefined symbol: __cpp_exception wasm-ld: error: libpanic_unwind-*.rlib: undefined symbol: _Unwind_RaiseException ``` because both libunwind and the `__cpp_exception` tag (previously defined in libc++abi) were only linked under `LINK_AS_CXX`. The tag and the unwinding runtime are language-agnostic runtime support for the Wasm EH LLVM lowering, not part of the C++ ABI surface. This PR now includes #27498 as its base, which provides the same `_Unwind_*` linkability for the non-Wasm-EH modes on the JS side. What's implemented: * The `__cpp_exception` tag definition moves from libc++abi (`__cpp_exception.S`) into libunwind's `Unwind-wasm.c` as inline asm, matching upstream llvm/llvm-project#185770. * libunwind is linked whenever `WASM_EXCEPTIONS` is enabled, rather than only when linking as C++. Test coverage extends `test_libunwind` from #27498 with `wasm_eh` and `wasm_legacy_eh` variants (completing its TODO), and adds `test_cpp_exception_tag` linking an asm object that throws the tag directly from a C caller - the same reference pattern as rustc-generated catch/throw sites - which fails to link without this change. The now-unneeded `DEFAULT_TO_CXX` workaround is dropped from `test_wasi_with_sjlj`. Existing C++ Wasm EH coverage (`core0.test_exceptions_*`) passes unchanged. _Made with AI assistance under my review_
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emscripten-core#27498 moved EH status management (`uncaughtExceptionCount` and `exceptionLast` from `libexceptions.js` to `libunwind.js`, but because in the native libcxxabi and libunwind, the management is done within libcxxabi, I think it'd be more consistent if we do the same for the JS libraries.
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#27498 moved EH status management (`uncaughtExceptionCount` and `exceptionLast` from `libexceptions.js` to `libunwind.js`, but because in the native libcxxabi and libunwind, the management is done within libcxxabi, I think it'd be more consistent if we do the same for the JS libraries.
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It turns out the libunwind symbols are supposed to always be available, enen non-C++ programs. This means that
__cxa_throwshould be implemented in terms of_Unwind_RaiseExceptionand not the other way around._Unwind_Resume__cxa_throwand other exception-handling APIs in terms of the lower level_UnwindAPI.Inspired by #27496