`-flto` + `-fwasm-exceptions` flags together break the compiler

[whitequark]

Here's a minimal reproducer:

$ echo 'int main(){}' >test.c
$ wasi-sdk-33.0-x86_64-linux/bin/clang --target=wasm32-wasip1 --sysroot wasi-sdk-33.0-x86_64-linux/share/wasi-sysroot -fwasm-exceptions -mllvm -wasm-use-legacy-eh=false -flto -lunwind test.c
wasm-ld: error: symbol type mismatch: __cpp_exception
>>> defined as WASM_SYMBOL_TYPE_FUNCTION in .../wasi-sdk-33.0-x86_64-linux/share/wasi-sysroot/lib/wasm32-wasip1/eh/llvm-lto/22.1.0-wasi-sdk/libunwind.a(Unwind-wasm.c.o)
>>> defined as WASM_SYMBOL_TYPE_TAG in a.out.lto.o
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Here's the error I'm getting from CMake during compiler detection:

-- Check for working C compiler: /usr/bin/ccache
-- Check for working C compiler: /usr/bin/ccache - broken
CMake Error at /usr/share/cmake-3.31/Modules/CMakeTestCCompiler.cmake:67 (message):
  The C compiler

    "/usr/bin/ccache"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: '/home/whitequark/Projects/yowasp/nextpnr/prjtrellis-build/CMakeFiles/CMakeScratch/TryCompile-axg3T6'
    
    Run Build Command(s): /usr/bin/cmake -E env VERBOSE=1 /usr/bin/gmake -f Makefile cmTC_31eb1/fast
    /usr/bin/gmake  -f CMakeFiles/cmTC_31eb1.dir/build.make CMakeFiles/cmTC_31eb1.dir/build
    gmake[1]: Entering directory '/home/whitequark/Projects/yowasp/nextpnr/prjtrellis-build/CMakeFiles/CMakeScratch/TryCompile-axg3T6'
    Building C object CMakeFiles/cmTC_31eb1.dir/testCCompiler.c.obj
    /usr/bin/ccache /home/whitequark/Projects/yowasp/nextpnr/wasi-sdk-33.0-x86_64-linux/bin/clang --target=wasm32-wasip1   --sysroot /home/whitequark/Projects/yowasp/nextpnr/wasi-sdk-33.0-x86_64-linux/share/wasi-sysroot -fwasm-exceptions -mllvm -wasm-use-legacy-eh=false -flto  -MD -MT CMakeFiles/cmTC_31eb1.dir/testCCompiler.c.obj -MF CMakeFiles/cmTC_31eb1.dir/testCCompiler.c.obj.d -o CMakeFiles/cmTC_31eb1.dir/testCCompiler.c.obj -c /home/whitequark/Projects/yowasp/nextpnr/prjtrellis-build/CMakeFiles/CMakeScratch/TryCompile-axg3T6/testCCompiler.c
    Linking C executable cmTC_31eb1
    /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_31eb1.dir/link.txt --verbose=1
    clang: warning: argument unused during compilation: '-mllvm -wasm-use-legacy-eh=false' [-Wunused-command-line-argument]
    wasm-ld: error: symbol type mismatch: __cpp_exception
    >>> defined as WASM_SYMBOL_TYPE_FUNCTION in /home/whitequark/Projects/yowasp/nextpnr/wasi-sdk-33.0-x86_64-linux/share/wasi-sysroot/lib/wasm32-wasip1/eh/llvm-lto/22.1.0-wasi-sdk/libunwind.a(Unwind-wasm.c.o)
    >>> defined as WASM_SYMBOL_TYPE_TAG in cmTC_31eb1.lto.o
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    /usr/bin/ccache /home/whitequark/Projects/yowasp/nextpnr/wasi-sdk-33.0-x86_64-linux/bin/clang --target=wasm32-wasip1 --sysroot /home/whitequark/Projects/yowasp/nextpnr/wasi-sdk-33.0-x86_64-linux/share/wasi-sysroot -fwasm-exceptions -mllvm -wasm-use-legacy-eh=false -flto  -lunwind -Wl,--strip-all  CMakeFiles/cmTC_31eb1.dir/testCCompiler.c.obj -o cmTC_31eb1
    gmake[1]: *** [CMakeFiles/cmTC_31eb1.dir/build.make:103: cmTC_31eb1] Error 1
    gmake[1]: Leaving directory '/home/whitequark/Projects/yowasp/nextpnr/prjtrellis-build/CMakeFiles/CMakeScratch/TryCompile-axg3T6'
    gmake: *** [Makefile:134: cmTC_31eb1/fast] Error 2

[alexcrichton]

I've posted this upstream as llvm/llvm-project#195311 as I think it's an incompatibility with LTO and exceptions right now, or at least the way exceptions are defined here.

I'll add this to the notes in CppExceptions.md in this repository, but I'll also go ahead and publish wasi-sdk-33 since I think this is an orthogonal issue that needs to be fixed upstream first.

[whitequark]

but I'll also go ahead and publish wasi-sdk-33 since I think this is an orthogonal issue that needs to be fixed upstream first.

Yeah I'm happy with that, SDK 33 is working great for me and passes all YoWASP tests.