[libcxx] Add an explicit option to build against system-libcxxabi

[mgorny]

Summary

Add an explicit LIBCXX_CXX_ABI=system-libcxxabi option for linking to
system-installed libc++abi. This fixes the ability to link against one
when building libcxx via the runtimes build, as otherwise the build
system insists on linking into in-tree targets.

[ldionne]

libcxx/cmake/Modules/HandleLibCXXABI.cmake

113

I think this doesn't work because cxxabi_shared and cxxabi_static might not be known by the time we get here. What is the behavior you'd like to see here? Are you attempting to link against the system libc++abi.so?

[mgorny]

libcxx/cmake/Modules/HandleLibCXXABI.cmake

113

Yes, exactly that.

[ldionne]

libcxx/cmake/Modules/HandleLibCXXABI.cmake

113

I think what we want instead is to have another choice for the ABI library, something like system-libcxxabi. When that ABI library is selected, we'd add -lc++abi and the right include path.

[mgorny]

libcxx/cmake/Modules/HandleLibCXXABI.cmake

113

Ok, I'll update the patch in a few hours.

[mgorny]

Add another LIBCXX_CXX_ABI variant instead, as requested.

[mgorny]

Ping. I'd like to try to backport the fixes to 14.x, so would be nice to get it sorted out before it goes out.

[ldionne]

Sorry, I went OOO. I've actually been working on a more holistic refactoring of this, but I think this patch is fine as a first step. I'll get my refactoring out soon.

[ldionne]

My proposed change is up as https://reviews.llvm.org/D120727.

Let's land this one and cherry-pick to release/14.x. Then, I can rebase my D120727 on it.

[mgorny]

Could you push it for me, please? My PC died yesterday and I won't have it fixed in the next few days.

[ldionne]

Done, also cherry-picked onto release/14.x

libcxx/cmake/Modules/HandleLibCXXABI.cmake

 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxabi")
   if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_SOURCE_DIR}/../libcxxabi/include")
   endif()
 
   if(LIBCXX_STANDALONE_BUILD AND NOT (LIBCXX_CXX_ABI_INTREE OR HAVE_LIBCXXABI))
     set(shared c++abi)
     set(static c++abi)
   else()
     set(shared cxxabi_shared)
     set(static cxxabi_static)
   endif()
 
   setup_abi_lib(
     "-DLIBCXX_BUILDING_LIBCXXABI"
     "${shared}" "${static}" "cxxabi.h;__cxxabi_config.h" "")
+elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "system-libcxxabi")
+  setup_abi_lib(
+    "-DLIBCXX_BUILDING_LIBCXXABI"
+    "c++abi" "c++abi" "cxxabi.h;__cxxabi_config.h" "")
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxrt")
   if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "/usr/include/c++/v1")
   endif()
   # libcxxrt does not provide aligned new and delete operators
   set(LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS ON)
   setup_abi_lib(
     "-DLIBCXXRT"

libcxx/docs/BuildingLibcxx.rst

 
 .. _ABI Library Specific Options:
 
 ABI Library Specific Options
 ----------------------------
 
 .. option:: LIBCXX_CXX_ABI:STRING
 
-  **Values**: ``none``, ``libcxxabi``, ``libcxxrt``, ``libstdc++``, ``libsupc++``.
+  **Values**: ``none``, ``libcxxabi``, ``system-libcxxabi``, ``libcxxrt``, ``libstdc++``, ``libsupc++``.
 
   Select the ABI library to build libc++ against.
 
 .. option:: LIBCXX_CXX_ABI_INCLUDE_PATHS:PATHS
 
   Provide additional search paths for the ABI library headers.
 
 .. option:: LIBCXX_CXX_ABI_LIBRARY_PATH:PATH

libcxx/utils/libcxx/test/config.py

def configure_link_flags_abi_library(self):
                     self.cxx.link_flags += ['-lc++abi']
                 else:
                     if self.abi_library_root:
                         libname = self.make_static_lib_name('c++abi')
                         abs_path = os.path.join(self.abi_library_root, libname)
                         self.cxx.link_flags += [abs_path]
                     else:
                         self.cxx.link_flags += ['-lc++abi']
+        elif cxx_abi == 'system-libcxxabi':
+            self.cxx.link_flags += ['-lc++abi']
         elif cxx_abi == 'libcxxrt':
             self.cxx.link_flags += ['-lcxxrt']
         elif cxx_abi == 'vcruntime':
             debug_suffix = 'd' if self.debug_build else ''
             # This matches the set of libraries linked in the toplevel
             # libcxx CMakeLists.txt if building targeting msvc.
             self.cxx.link_flags += ['-l%s%s' % (lib, debug_suffix) for lib in
                                     ['vcruntime', 'ucrt', 'msvcrt', 'msvcprt']]