[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. 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.
This is also why code with reference to __register_frame successfully builds (but not working correctly) on these systems without explicitly link to libunwind: LLVM requires C++ stdlib, and linking to libcxx implies libunwind.

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