[libunwind] [libunwind][cmake] Compile _Unwind* routines with -fexceptions (PR #121819)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> Maybe this is a hidden bug, but I haven't investigated it too deeply, @mstorsjo may know better.

Sorry, no clue offhand here.

> If you build MinGW LTO libunwind without `-fno-exceptions` or with -`fexceptions`, libunwind references `__gcc_personality_seh0`, which results in undefined symbols.

Perhaps this is a symbol which we need to implicitly mark as needed in the linker? Because before the LTO, I think it only sees a definition of this symbol, but nothing referencing it, so the LTO logic thinks it doesn't need to retain/expose this symbol - while after the LTO, the generated code does add new references to it.

I don't find any precedence for doing this for LTO so far, but we have a couple cases of doing something similar for the regular GC in LLD.

> cmake normally adds `-fno-exceptionson`, but not on the first build, because the toolchain is incomplete which causes cmake to think that the compiler doesn't support `-fno-exceptions`, which is how I found out about the problem.

It does add `-fno-exceptions` even on the first bootstrap build for me. See e.g. https://github.com/mstorsjo/llvm-mingw/actions/runs/12662020869/job/35286224252#step:3:23134:

```
-- The C compiler identification is Clang 20.0.0
-- The CXX compiler identification is Clang 20.0.0
-- The ASM compiler identification is Clang with GNU-like command-line
-- Found assembler: /home/runner/work/llvm-mingw/llvm-mingw/install/llvm-mingw/bin/i686-w64-mingw32-clang
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Detecting CXX compile features
-- Detecting CXX compile features - done
[...]
-- Performing Test CXX_SUPPORTS_FUNWIND_TABLES_FLAG
-- Performing Test CXX_SUPPORTS_FUNWIND_TABLES_FLAG - Success
-- Performing Test CXX_SUPPORTS_FNO_EXCEPTIONS_FLAG
-- Performing Test CXX_SUPPORTS_FNO_EXCEPTIONS_FLAG - Success
```


https://github.com/llvm/llvm-project/pull/121819