[runtimes] Fix link order of system librarires on Apple platforms (llvm#66940)

On Apple platforms, we always support the -nostdlib++ flag. Hence, it is
not necessary to manually link against system libraries. In fact, doing
so causes us to link against libSystem explicitly, which messes up with
the order of libraries we should use. Indeed:

   Before patch, using the system unwinder (LIBCXXABI_USE_LLVM_UNWINDER = OFF)
   ===========================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         /usr/lib/libSystem.B.dylib
         @rpath/libc++abi.1.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

   After patch, using the system unwinder (LIBCXXABI_USE_LLVM_UNWINDER = OFF)
   ===========================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

   Before patch, with the LLVM unwinder (LIBCXXABI_USE_LLVM_UNWINDER = ON)
   =======================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         /usr/lib/libSystem.B.dylib
         @rpath/libc++abi.1.dylib
         @rpath/libunwind.1.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         /usr/lib/libSystem.B.dylib
         @rpath/libunwind.1.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

   After patch, with the LLVM unwinder (LIBCXXABI_USE_LLVM_UNWINDER = ON)
   ======================================================================
   $ otool -L lib/{libc++.1.dylib,libc++abi.1.dylib,libunwind.1.dylib}
   lib/libc++.1.dylib:
         @rpath/libc++.1.dylib
         @rpath/libc++abi.1.dylib
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libc++abi.1.dylib:
         @rpath/libc++abi.1.dylib
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib
   lib/libunwind.1.dylib:
         @rpath/libunwind.1.dylib
         /usr/lib/libSystem.B.dylib

As we can see, libSystem appears before the just-built libraries before
the patch, which causes the libunwind.dylib bundled in libSystem.dylib
to be used instead of the just-built libunwind.dylib.

We didn't notice the issue until recently when I tried to update the
macOS CI builders to macOS 13.5, where it is necessary to use the right
libunwind library (the exact reason still needs to be investigated).

Author: ldionne

libcxx/CMakeLists.txt

@@ -659,24 +659,22 @@ function(cxx_link_system_libraries target)
     target_add_link_flags_if_supported(${target} PRIVATE "--unwindlib=none")
   endif()
 
-  if (LIBCXX_HAS_SYSTEM_LIB)
-    target_link_libraries(${target} PRIVATE System)
-  endif()
-
-  if (LIBCXX_HAS_PTHREAD_LIB)
-    target_link_libraries(${target} PRIVATE pthread)
-  endif()
+  if (NOT APPLE) # On Apple platforms, we always use -nostdlib++ so we don't need to re-add other libraries
+    if (LIBCXX_HAS_PTHREAD_LIB)
+      target_link_libraries(${target} PRIVATE pthread)
+    endif()
 
-  if (LIBCXX_HAS_C_LIB)
-    target_link_libraries(${target} PRIVATE c)
-  endif()
+    if (LIBCXX_HAS_C_LIB)
+      target_link_libraries(${target} PRIVATE c)
+    endif()
 
-  if (LIBCXX_HAS_M_LIB)
-    target_link_libraries(${target} PRIVATE m)
-  endif()
+    if (LIBCXX_HAS_M_LIB)
+      target_link_libraries(${target} PRIVATE m)
+    endif()
 
-  if (LIBCXX_HAS_RT_LIB)
-    target_link_libraries(${target} PRIVATE rt)
+    if (LIBCXX_HAS_RT_LIB)
+      target_link_libraries(${target} PRIVATE rt)
+    endif()
   endif()
 
   if (LIBCXX_USE_COMPILER_RT)

libcxx/cmake/config-ix.cmake

@@ -110,10 +110,8 @@ if(WIN32 AND NOT MINGW)
   set(LIBCXX_HAS_PTHREAD_LIB NO)
   set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
-  set(LIBCXX_HAS_SYSTEM_LIB NO)
   set(LIBCXX_HAS_ATOMIC_LIB NO)
 elseif(APPLE)
-  check_library_exists(System write "" LIBCXX_HAS_SYSTEM_LIB)
   set(LIBCXX_HAS_PTHREAD_LIB NO)
   set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
@@ -122,12 +120,10 @@ elseif(FUCHSIA)
   set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_PTHREAD_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
-  set(LIBCXX_HAS_SYSTEM_LIB NO)
   check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
 else()
   check_library_exists(pthread pthread_create "" LIBCXX_HAS_PTHREAD_LIB)
   check_library_exists(m ccos "" LIBCXX_HAS_M_LIB)
   check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)
-  set(LIBCXX_HAS_SYSTEM_LIB NO)
   check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
 endif()

libcxxabi/cmake/config-ix.cmake

@@ -95,11 +95,9 @@ if(FUCHSIA)
   set(LIBCXXABI_HAS_PTHREAD_LIB NO)
   check_library_exists(c __cxa_thread_atexit_impl ""
     LIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL)
-  set(LIBCXXABI_HAS_SYSTEM_LIB NO)
 else()
   check_library_exists(dl dladdr "" LIBCXXABI_HAS_DL_LIB)
   check_library_exists(pthread pthread_once "" LIBCXXABI_HAS_PTHREAD_LIB)
   check_library_exists(c __cxa_thread_atexit_impl ""
     LIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL)
-  check_library_exists(System write "" LIBCXXABI_HAS_SYSTEM_LIB)
 endif()

libcxxabi/src/CMakeLists.txt

@@ -67,9 +67,7 @@ if (LIBCXXABI_ENABLE_FORGIVING_DYNAMIC_CAST)
   add_definitions(-D_LIBCXXABI_FORGIVING_DYNAMIC_CAST)
 endif()
 
-if (APPLE)
-  add_library_flags_if(LIBCXXABI_HAS_SYSTEM_LIB System)
-else()
+if (NOT APPLE) # On Apple platforms, we always use -nostdlib++ so we don't need to re-add other libraries
   if (LIBCXXABI_ENABLE_THREADS)
     add_library_flags_if(LIBCXXABI_HAS_PTHREAD_LIB pthread)
   endif()

libunwind/src/CMakeLists.txt

@@ -65,16 +65,23 @@ set(LIBUNWIND_SOURCES
     ${LIBUNWIND_ASM_SOURCES})
 
 # Generate library list.
-add_library_flags_if(LIBUNWIND_HAS_C_LIB c)
 if (LIBUNWIND_USE_COMPILER_RT)
   add_library_flags("${LIBUNWIND_BUILTINS_LIBRARY}")
 else()
   add_library_flags_if(LIBUNWIND_HAS_GCC_S_LIB gcc_s)
   add_library_flags_if(LIBUNWIND_HAS_GCC_LIB gcc)
 endif()
-add_library_flags_if(LIBUNWIND_HAS_DL_LIB dl)
+if (NOT APPLE) # On Apple platforms, we don't need to link explicitly against system libraries
+  add_library_flags_if(LIBUNWIND_HAS_C_LIB c)
+  add_library_flags_if(LIBUNWIND_HAS_DL_LIB dl)
+
+  if (LIBUNWIND_ENABLE_THREADS)
+    add_library_flags_if(LIBUNWIND_HAS_PTHREAD_LIB pthread)
+    add_compile_flags_if(LIBUNWIND_WEAK_PTHREAD_LIB -DLIBUNWIND_USE_WEAK_PTHREAD=1)
+  endif()
+endif()
+
 if (LIBUNWIND_ENABLE_THREADS)
-  add_library_flags_if(LIBUNWIND_HAS_PTHREAD_LIB pthread)
   add_compile_flags_if(LIBUNWIND_WEAK_PTHREAD_LIB -DLIBUNWIND_USE_WEAK_PTHREAD=1)
 endif()