[libunwind] Enable wasm target for exceptions

[yerzham]

Most minimal changes to enable libunwind to build for wasm by excluding __wasm__ from the Windows-style __declspec path in config.h and by recognizing wasm in assembly.h to skip the unsupported-target error. This unblocks building an exceptions-enabled wasi-sdk sysroot (see WebAssembly/wasi-sdk#565) and builds on prior attempts including yamt’s gist, #79667, and #140365.

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[llvmbot]

@llvm/pr-subscribers-libunwind

Author: Yerzhan Zhamashev (yerzham)

Changes

Most minimal changes to enable libunwind to build for wasm by excluding __wasm__ from the Windows-style __declspec path in config.h and by recognizing wasm in assembly.h to skip the unsupported-target error. This unblocks building an exceptions-enabled wasi-sdk sysroot (see WebAssembly/wasi-sdk#565) and builds on prior attempts including yamt’s gist, #79667, and #140365.

---

Full diff: https://github.com/llvm/llvm-project/pull/168449.diff

2 Files Affected:

• (modified) libunwind/src/assembly.h (+2)
• (modified) libunwind/src/config.h (+8-7)

diff --git a/libunwind/src/assembly.h b/libunwind/src/assembly.h index 84c9d526f1d75..9c35b6050a2bf 100644 --- a/libunwind/src/assembly.h +++ b/libunwind/src/assembly.h @@ -254,6 +254,8 @@ aliasname: \ #define NO_EXEC_STACK_DIRECTIVE // clang-format on +#elif defined(__wasm__) + #else #error Unsupported target diff --git a/libunwind/src/config.h b/libunwind/src/config.h index f017403fa2234..73162995f9293 100644 --- a/libunwind/src/config.h +++ b/libunwind/src/config.h @@ -75,13 +75,14 @@ #define _LIBUNWIND_EXPORT #define _LIBUNWIND_HIDDEN #else - #if !defined(__ELF__) && !defined(__MACH__) && !defined(_AIX) - #define _LIBUNWIND_EXPORT __declspec(dllexport) - #define _LIBUNWIND_HIDDEN - #else - #define _LIBUNWIND_EXPORT __attribute__((visibility("default"))) - #define _LIBUNWIND_HIDDEN __attribute__((visibility("hidden"))) - #endif +#if !defined(__ELF__) && !defined(__MACH__) && !defined(_AIX) && \ + !defined(__wasm__) +#define _LIBUNWIND_EXPORT __declspec(dllexport) +#define _LIBUNWIND_HIDDEN +#else +#define _LIBUNWIND_EXPORT __attribute__((visibility("default"))) +#define _LIBUNWIND_HIDDEN __attribute__((visibility("hidden"))) +#endif #endif #define STR(a) #a 

[zyedidia]

Compared to yamt's gist why do we not have #define NO_EXEC_STACK_DIRECTIVE this time? When I tried to use this patch to build an exceptions-enabled version of wasi-sdk (with your additional patch) I encountered a build error related to NO_EXEC_STACK_DIRECTIVE:

llvm-project/libunwind/src/UnwindRegistersRestore.S:1258:1: error: invalid instruction NO_EXEC_STACK_DIRECTIVE ^ 

After fixing that I then ran into the following errors when linking libc++abi.so:

wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.o: undefined symbol: _Unwind_RaiseException wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.o: undefined symbol: _Unwind_DeleteException wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.o: undefined symbol: _Unwind_RaiseException wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.o: undefined symbol: _Unwind_RaiseException wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_SetGR wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_SetGR wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_SetIP wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_GetLanguageSpecificData wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_GetIP wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_GetRegionStart wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_SetGR wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_SetGR wasm-ld: error: libcxxabi/src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o: undefined symbol: _Unwind_SetIP 

Are there further changes needed to fix this?

[yerzham]

I needed to clarify wasi-sdk had to use latest version of src/llvm-project, at least I tested since v21.1.5. Otherwise, usage of NO_EXEC_STACK_DIRECTIVE in UnwindRegistersRestore.S should be gated out since #92192 (then some modification at #162581). I do not think NO_EXEC_STACK_DIRECTIVE is ever used for wasm target after that, so it would be an unused declaration. As for the consequent errors, probably still related to outdated llvm source?