How to build libunwind for wasm?

[trcrsired]

error: "Unsupported architecture."

FAILED: src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj
/home/cqwrteur/llvm_toolchains/x86_64-pc-linux-gnu/bin/clang++ --target=wasm32-wasi --sysroot=/home/cqwrteur/toolchains/sysroots/wasm32-wasi/wasm32-wasi -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBUNWIND_IS_BAREMETAL -I/home/cqwrteur/projects/llvm-project/libunwind/include -Wno-array-bounds -Wno-cast-function-type -Wno-uninitialized -fuse-ld=lld -Wno-misleading-indentation -Wno-global-constructors -Wno-unused-command-line-argument -O3 -DNDEBUG -std=c++11 -Werror=return-type -funwind-tables -nostdinc++ -D_DEBUG -UNDEBUG -D_LIBUNWIND_IS_NATIVE_ONLY -Wall -Wextra -Wnewline-eof -Wshadow -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wundef -Wunused-template -Wformat-nonliteral -Wno-user-defined-literals -Wno-covered-switch-default -Wno-suggest-override -Wno-error -pedantic -fno-rtti  -fstrict-aliasing -fno-exceptions -fno-rtti -MD -MT src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj -MF src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj.d -o src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj -c /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:12:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/include/libunwind.h:16:
/home/cqwrteur/projects/llvm-project/libunwind/include/__libunwind_config.h:184:4: error: "Unsupported architecture."
  184 | #  error "Unsupported architecture."
      |    ^
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:12:
/home/cqwrteur/projects/llvm-project/libunwind/include/libunwind.h:71:17: error: use of undeclared identifier '_LIBUNWIND_CONTEXT_SIZE'
   71 |   uint64_t data[_LIBUNWIND_CONTEXT_SIZE];
      |                 ^
/home/cqwrteur/projects/llvm-project/libunwind/include/libunwind.h:76:17: error: use of undeclared identifier '_LIBUNWIND_CURSOR_SIZE'
   76 |   uint64_t data[_LIBUNWIND_CURSOR_SIZE];
      |                 ^
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:30:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/AddressSpace.hpp:23:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/EHHeaderParser.hpp:17:
/home/cqwrteur/projects/llvm-project/libunwind/src/DwarfParser.hpp:69:26: error: use of undeclared identifier '_LIBUNWIND_HIGHEST_DWARF_REGISTER'
   69 |     kMaxRegisterNumber = _LIBUNWIND_HIGHEST_DWARF_REGISTER
      |                          ^
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:31:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/UnwindCursor.hpp:50:
/home/cqwrteur/projects/llvm-project/libunwind/src/RWMutex.hpp:19:10: fatal error: 'pthread.h' file not found
   19 | #include <pthread.h>
      |          ^~~~~~~~~~~
5 errors generated.
ninja: build stopped: subcommand failed.

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: cqwrteur (trcrsired)

error: "Unsupported architecture."

FAILED: src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj
/home/cqwrteur/llvm_toolchains/x86_64-pc-linux-gnu/bin/clang++ --target=wasm32-wasi --sysroot=/home/cqwrteur/toolchains/sysroots/wasm32-wasi/wasm32-wasi -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBUNWIND_IS_BAREMETAL -I/home/cqwrteur/projects/llvm-project/libunwind/include -Wno-array-bounds -Wno-cast-function-type -Wno-uninitialized -fuse-ld=lld -Wno-misleading-indentation -Wno-global-constructors -Wno-unused-command-line-argument -O3 -DNDEBUG -std=c++11 -Werror=return-type -funwind-tables -nostdinc++ -D_DEBUG -UNDEBUG -D_LIBUNWIND_IS_NATIVE_ONLY -Wall -Wextra -Wnewline-eof -Wshadow -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wundef -Wunused-template -Wformat-nonliteral -Wno-user-defined-literals -Wno-covered-switch-default -Wno-suggest-override -Wno-error -pedantic -fno-rtti  -fstrict-aliasing -fno-exceptions -fno-rtti -MD -MT src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj -MF src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj.d -o src/CMakeFiles/unwind_static_objects.dir/libunwind.cpp.obj -c /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:12:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/include/libunwind.h:16:
/home/cqwrteur/projects/llvm-project/libunwind/include/__libunwind_config.h:184:4: error: "Unsupported architecture."
  184 | #  error "Unsupported architecture."
      |    ^
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:12:
/home/cqwrteur/projects/llvm-project/libunwind/include/libunwind.h:71:17: error: use of undeclared identifier '_LIBUNWIND_CONTEXT_SIZE'
   71 |   uint64_t data[_LIBUNWIND_CONTEXT_SIZE];
      |                 ^
/home/cqwrteur/projects/llvm-project/libunwind/include/libunwind.h:76:17: error: use of undeclared identifier '_LIBUNWIND_CURSOR_SIZE'
   76 |   uint64_t data[_LIBUNWIND_CURSOR_SIZE];
      |                 ^
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:30:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/AddressSpace.hpp:23:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/EHHeaderParser.hpp:17:
/home/cqwrteur/projects/llvm-project/libunwind/src/DwarfParser.hpp:69:26: error: use of undeclared identifier '_LIBUNWIND_HIGHEST_DWARF_REGISTER'
   69 |     kMaxRegisterNumber = _LIBUNWIND_HIGHEST_DWARF_REGISTER
      |                          ^
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/libunwind.cpp:31:
In file included from /home/cqwrteur/projects/llvm-project/libunwind/src/UnwindCursor.hpp:50:
/home/cqwrteur/projects/llvm-project/libunwind/src/RWMutex.hpp:19:10: fatal error: 'pthread.h' file not found
   19 | #include <pthread.h>
      |          ^~~~~~~~~~~
5 errors generated.
ninja: build stopped: subcommand failed.
</details>

[dschuff]

I don't think libunwind can be built for wasm, at least not in its "normal" form. IIUC it depends on being able to directly save and restore the values of the stack pointer, and that's just not possible on the wasm architecture (the underlying stack and return addresses are hidden from the wasm code).
If you wanted to build some unwinding functionality it would need to be on top of the wasm exception handling support builtin to the architecture. As an example of how you might do this, you can take a look at the implementation in libcxxabi (e6cbba7). libcxxabi is built on top of the libunwind API on most architectures, but on wasm it uses the EH support in the VM.

[aheejin]

We only use libunwind's API structure and not really the implementation part like libunwind.cpp; it deals with the lower level stuff than wasm user library can handle, as @dschuff said. Effectively the only file we use is https://github.com/llvm/llvm-project/blob/main/libunwind/src/Unwind-wasm.c, which consists of some interface functions that interact with libc++abi.

To prevent this kind of confusion from happening, I guess it would be better to consider wrapping libunwind.cpp with #ifndef __USING_WASM_EXCEPTION__. I'll submit a PR for that.

[aheejin]

#73196

[trcrsired]

_LIBUNWIND_CONTEXT_SIZE

This macro is not defined. You are not clear on how to use this tbh

[arjunr2]

It looks like the issue isn't resolved yet -- https://github.com/llvm/llvm-project/blob/main/libunwind/include/__libunwind_config.h is still included in the build for libunwind.cpp here: https://github.com/llvm/llvm-project/blob/main/libunwind/src/libunwind.cpp#L12, throwing the same error as posed in the question.

Is the intention to remove libunwind.cpp completely from the build?

[aheejin]

Can you try #92192 and let me know if that works?

Is the intention to remove libunwind.cpp completely from the build?

We don't need that file very much actually, but it contains some common debug-printing routines, so we decided to include it while excluding the bulk of the non-debugging contents. Now I see why that caused compilation problems.

[arjunr2]

It works now, but the CMake still pulls in the assembly file UnwindRegistersSave.S and UnwindRegistersRestore.S. It would probably help to add a CMake option to disable including them in the library or stub them out with the wasm define

[aheejin]

Excluded the .S files too.