[all-commits] [llvm/llvm-project] e46de4: [runtimes] Fix link order of system librarires on ...

[Louis Dionne via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e46de4e54f63eb4088ca351253768b5db8cf1985
      https://github.com/llvm/llvm-project/commit/e46de4e54f63eb4088ca351253768b5db8cf1985
  Author: Louis Dionne <ldionne.2 at gmail.com>
  Date:   2023-09-21 (Thu, 21 Sep 2023)

  Changed paths:
    M libcxx/CMakeLists.txt
    M libcxx/cmake/config-ix.cmake
    M libcxxabi/cmake/config-ix.cmake
    M libcxxabi/src/CMakeLists.txt
    M libunwind/src/CMakeLists.txt

  Log Message:
  -----------
  [runtimes] Fix link order of system librarires on Apple platforms (#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).