[Emscripten-EH] Implement __cxa_throw in terms of _Unwind_RaiseException - #27498
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sbc100
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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? |
sbc100
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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... |
| $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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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