[libc++abi] Make libunwind static linking possible in standalone builds

[tambre]

Summary

This is the same way that libc++abi static linking is handled in libc++.

Resolves PR45348.

[compnerd]

I think that @ldionne should chime in on this change. Mechanically, sure, I don't see any problems with the change.

[tambre]

ldionne: ping

[ldionne]

libcxxabi/src/CMakeLists.txt

263

Where is LIBCXXABI_UNWIND_LIBRARY_PATH defined? I can't find that CMake option anywhere.

[tambre]

Add cache variable for LIBCXXABI_UNWIND_LIBRARY_PATH to improve discoverability and clarify usage.

[tambre]

libcxxabi/src/CMakeLists.txt

263

This is for the user to define if they want to. Same way as LIBCXX_CXX_ABI_LIBRARY_PATH. I've added a cache variable for this in the main file to make it easier to discover.

[ldionne]

Could you please chime in on http://lists.llvm.org/pipermail/libcxx-dev/2020-April/000793.html? I'd like to better understand why we even need to support a standalone build -- I thought we were not supporting those anymore since the monorepo.

[tambre]

I'll chime in on that. I just recently migrated the company I work for to LLVM, so I was only aware that building in a non-monorepo layout is unsupported now.

[tambre]

ldionne: Ping. Per the mailing list thread it seems reasonable to support this scenario.

[tambre]

ldionne: ping

[tambre]

ldionne: ping

[tambre]

ldionne: ping

[tambre]

ldionne: ping

[tambre]

ldionne: ping. From what I gather, building projects separately is still supported, but I remain unaware of an option to accomplish what I want without downsides. I'd really rather not keep a single out-of-tree patch for my use.

[tambre]

ldionne: ping

[tambre]

Abandoning as it seems this will never get reviewed.

libcxxabi/CMakeLists.txt

option(LIBCXXABI_ENABLE_EXCEPTIONS
   "Provide support for exceptions in the runtime.
   When disabled, libc++abi does not support stack unwinding and other exceptions-related features." ON)
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LIBCXXABI_ENABLE_PIC "Build Position-Independent Code, even in static library" ON)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
+set(LIBCXXABI_UNWIND_LIBRARY_PATH "" CACHE STRING "Path to the static unwind library (for standalone builds).")
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
 option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXXABI_HAS_EXTERNAL_THREAD_API
   "Build libc++abi with an externalized threading API.
   This option may only be set to ON when LIBCXXABI_ENABLE_THREADS=ON." OFF)
 option(LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY
   "Build libc++abi with an externalized threading library.

libcxxabi/src/CMakeLists.txt

if (LIBCXXABI_ENABLE_STATIC)
   endif()
 
   if (APPLE)
     set(MERGE_ARCHIVES_LIBTOOL "--use-libtool" "--libtool" "${CMAKE_LIBTOOL}")
   endif()
 
   # Merge the libc++abi.a and libunwind.a into one.
   if(LIBCXXABI_USE_LLVM_UNWINDER AND LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY)
+    if(NOT "${LIBCXXABI_UNWIND_LIBRARY_PATH}" STREQUAL "")
+      set(MERGE_ARCHIVES_SEARCH_PATHS "-L${LIBCXXABI_UNWIND_LIBRARY_PATH}")
+    endif()
+
+    if(TARGET unwind_static)
+      set(MERGE_ARCHIVES_UNWIND "$<TARGET_LINKER_FILE:unwind_static>")
+    else()
+      set(MERGE_ARCHIVES_UNWIND "${CMAKE_STATIC_LIBRARY_PREFIX}unwind${CMAKE_STATIC_LIBRARY_SUFFIX}")
+    endif()
+
     add_custom_command(TARGET cxxabi_static POST_BUILD
       COMMAND ${PYTHON_EXECUTABLE} ${LIBCXXABI_LIBCXX_PATH}/utils/merge_archives.py
       ARGS
         -o "$<TARGET_LINKER_FILE:cxxabi_static>"
         --ar "${CMAKE_AR}"
         ${MERGE_ARCHIVES_LIBTOOL}
         "$<TARGET_LINKER_FILE:cxxabi_static>"
-        "$<TARGET_LINKER_FILE:unwind_static>"
+        "${MERGE_ARCHIVES_UNWIND}"
+        "${MERGE_ARCHIVES_SEARCH_PATHS}"
       WORKING_DIRECTORY ${LIBCXXABI_BUILD_DIR}
     )
   endif()
 endif()
 
 # Add a meta-target for both libraries.
 add_custom_target(cxxabi DEPENDS ${LIBCXXABI_BUILD_TARGETS})