Link the Wasm EH runtime under -fwasm-exceptions without requiring C++

[guybedford]

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 [WebAssembly] Move __cpp_exception to libunwind 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

[hoodmane]

Is this really needed? Rust with wasm eh has been working, so if it's needed it should be fixing a regression.

[hoodmane]

Since DEFAULT_TO_CXX was disabled by default

Oh I see. This is pretty confusing because what does it mean for the linker?

[sbc100]

How does this work on other platforms? Does rust use exception thrown/catching without linking libc++abi there? Does rust use clang (not clang++) during linking?

[guybedford]

How does this work on other platforms? Does rust use exception thrown/catching without linking libc++abi there? Does rust use clang (not clang++) during linking?

Yes and yes. rustc hardcodes emcc for linking, and Rust panics only use the language-agnostic _Unwind_* layer (libgcc_s/libunwind), which the C driver links unconditionally on native targets. libc++abi is only the C++ personality/_cxa* layer above that, which Rust replaces with its own personality. This PR gives Wasm the same split: the __cpp_exception tag + Unwind-wasm.c are the base layer (LLVM's wasm EH lowering throws that tag for all languages, despite the name), libc++abi stays C++-only.

Comment made with AI assistance

[sbc100]

I see. I hadn't realized that libunwind was not a C++ specific thing.

I was able to reproduce this using my desktop clang:

$ clang -Wl,--trace-symbol=_Unwind_RaiseException -Wl,-u_Unwind_RaiseException -fuse-ld=lld --unwindlib=libunwind -rtlib=compiler-rt  ~/test/hello.c -v
Debian clang version 21.1.8 (3)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-21/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/13
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/15
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
 "/usr/lib/llvm-21/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-obj -dumpdir a- -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name hello.c -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/usr/local/google/home/sbc/dev/wasm/emscripten -v -fcoverage-compilation-dir=/usr/local/google/home/sbc/dev/wasm/emscripten -resource-dir /usr/lib/llvm-21/lib/clang/21 -internal-isystem /usr/lib/llvm-21/lib/clang/21/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/15/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -ferror-limit 19 -fmessage-length=124 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcolor-diagnostics -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/hello-4585ba.o -x c /usr/local/google/home/sbc/test/hello.c
clang -cc1 version 21.1.8 based upon LLVM 21.1.8 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/15/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/llvm-21/lib/clang/21/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 "/usr/lib/llvm-21/bin/ld.lld" --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -pie -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /lib/x86_64-linux-gnu/Scrt1.o /lib/x86_64-linux-gnu/crti.o /usr/lib/llvm-21/lib/clang/21/lib/linux/clang_rt.crtbegin-x86_64.o -L/usr/lib/gcc/x86_64-linux-gnu/15 -L/usr/lib/gcc/x86_64-linux-gnu/15/../../../../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib64 -L/lib -L/usr/lib --trace-symbol=_Unwind_RaiseException -u_Unwind_RaiseException /tmp/hello-4585ba.o /usr/lib/llvm-21/lib/clang/21/lib/linux/libclang_rt.builtins-x86_64.a --as-needed -lunwind --no-as-needed -lc /usr/lib/llvm-21/lib/clang/21/lib/linux/libclang_rt.builtins-x86_64.a --as-needed -lunwind --no-as-needed /usr/lib/llvm-21/lib/clang/21/lib/linux/clang_rt.crtend-x86_64.o /lib/x86_64-linux-gnu/crtn.o
<internal>: reference to _Unwind_RaiseException
/lib/x86_64-linux-gnu/libunwind.so: shared definition of _Unwind_RaiseException
<internal>: reference to _Unwind_RaiseException

[sbc100]

I think this full comment is no longer needed. libunwind should always be linked into all programs one or another. I should have known that before.

How about just:

`libunwind` is implement in JS, and not nativly, when Emscripten EH is used.

[sbc100]

Add PR number here.

[aheejin]

Does this mean we need LINK_AS_CXX in case of Emscripten EH? This sounds weird. If LINK_AS_CXX should not be required for linking linbunwind in Wasm EH, shouldn't that be true also for Emscripten EH too?

[aheejin]

Can we use @with_all_eh_sjlj instead of this?

[sbc100]

IIUC that does not include the empty case (i.e. no exception handling)?