[llvm] [llvm][cmake] Try LLVM libunwind when frame registration is unavailable (PR #112087)

[via llvm-commits]

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@@ -272,6 +272,13 @@ endif()
 check_symbol_exists(__register_frame "${CMAKE_CURRENT_LIST_DIR}/unwind.h" HAVE_REGISTER_FRAME)
 check_symbol_exists(__deregister_frame "${CMAKE_CURRENT_LIST_DIR}/unwind.h" HAVE_DEREGISTER_FRAME)
 check_symbol_exists(__unw_add_dynamic_fde "${CMAKE_CURRENT_LIST_DIR}/unwind.h" HAVE_UNW_ADD_DYNAMIC_FDE)
+if (NOT (HAVE_REGISTER_FRAME OR HAVE_DEREGISTER_FRAME OR HAVE_UNW_ADD_DYNAMIC_FDE))
+  check_library_exists(unwind __unw_add_dynamic_fde "" HAVE_LLVM_LIBUNWIND)
+  if (HAVE_LLVM_LIBUNWIND)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES "unwind")
+    set(HAVE_UNW_ADD_DYNAMIC_FDE 1)
+  endif ()
+endif ()
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ziyao233 wrote:

> > I dont think it's a problem.
> 
> So this _would_ cause all of these tools to explicitly link libunwind?

Yes.

> That seems like a significant change.
> 
> > If __register_frame symbols aren't available without linking to external libraries, we're definitely working with compiler-rt & libcxx & libcxxabi, which already require linking to libunwind.
> 
> That's surprising to me. I thought libcxx / libcxxabi could be mixed with the gcc unwinder, but I've never actually tried (I'm usually developing on Darwin where libunwind is the system unwinder).
> 
> Is it possible to mix libunwind with other (non libcxx) c++ standard libraries?

My statement is partially wrong, sorry.

Here the main problem isn't underlinking, but we don't use `-lunwind` when checking for LLVM libunwind symbols, which are contained in an external library, resulting in misdetection.

A better solution may be

- Detect whether gcc-style unwinding symbols presents by default, if not we assume LLVM libunwind (OrcJIT supports only these two unwinders)
- If we aren't working with gcc unwinder, link libLLVM to libunwind explicitly, for completeness

Linking libunwind only for `clang-repl`, `lli` and `llvm-jitlink` isn't enough, since it's actually LLVM OrcJIT depending on the unwinder, other programs like mesa use LLVM OrcJIT as well.

What do you think about this solution? Thanks for your patience.

https://github.com/llvm/llvm-project/pull/112087