Stack unwinding on Windows (SEH) #291
Description
The tests are failing on Windows and I looked into the matter. The problem lies in setjmp/longjmp functions which unwind on Windows. I have implemented workaround here #265 (comment).
Goal: compatibility with SEH, so wasm trap can freely unwind through different stack frames: wasm, Rust, C++ and more.
Major work to be done: our compiler, cranelift, should provide us with unwind information
Firefox implementation: doesn't rely on SEH; it manually iterates over wasm frames and performs the clean up, and uses custom thunks to restore stack pointer.
Helpful references:
- unwind procedure and unwind info ABI
- nice example of manual analysis of unwind info generated by C# JIT
- talk about Structured Exception Handling (SEH) - CppCon 2018: James McNellis “Unwinding the Stack: Exploring How C++ Exceptions Work on Windows”, slides here
- patch based on one of my local branches with some scratch code; most interesting are
code_memory.rsandTrampolines.c. The whole thing is not supposed to work, it's just a nice reference for future with already written code. - analogous code in Firefox. My understanding is: this handler shouldn't be called anyway. Vectored Exception Handler handles the exceptions - overwrites the PC in stored context to custom handler and tells OS to continue execution. In wasmtime we currently unwind directly from the Vectored Exception Handler.