[llvm-dev] cmake 3.22 breaks libc++ build

[Roland Schatz via llvm-dev]

Hi,

I'm currently debugging a problem in our Github Actions build. We're 
doing a custom build of libc++ and libc++abi, and this is failing since 
the cmake version in the base image was updated to version 3.22.

The issue can be reproduced by following the instructions at 
https://libcxx.llvm.org/BuildingLibcxx.html.

The setup I'm using is this:
* Linux
* cmake version 3.22
* I've tried llvm-project sources 12.0.0, 13.0.0 and the tip of main
* system compiler is clang 12.0.1 (but I don't think that matters here)

The exact commands I'm running are (the last one is failing [1]):
cd llvm-project
mkdir build
cmake -G Ninja -S runtimes -B build 
"-DLLVM_ENABLE_RUNTIMES=libcxx;libcxxabi" -DCMAKE_INSTALL_PREFIX=install
ninja -C build cxx cxxabi
ninja -C build install-cxxabi
ninja -C build install-cxx


What seems to be happening is that cmake tries to patch the RPATH of 
libc++.so. But libc++.so is not a symlink to a shared object, it's a 
linker script.
I have bisected that to a particular cmake change [2].


One thing I noticed is that the build works just fine when I'm doing the 
build of libc++ and libc++abi separately:

cmake -S libcxxabi ...
// build, install, ...
cmake -S libcxx -DLIBCXX_CXX_ABI=libcxxabi -DLIBCXX_CXX_ABI_LIBRARY_PATH=path/to/install/from/prev/step ...


That works on Linux, but breaks the build on Darwin, it seems to be 
missing the re-exports of libc++abi symbols in libc++ then.
Also that's the "deprecated" way of doing things, so not sure if it's 
good to pursue that path.


To be honest, I'm not sure I fully understand the problem, or how these 
different ways of building even make a difference.

Am I doing something wrong? Is this a bug in LLVM's cmake scripts? Or a 
bug in cmake?
Any hints what I can try to fix this?


Thanks!
Roland


[1] failing build output:

...
-- Installing: /home/roland/test/cmake/llvm-project/install/include/c++/v1/wctype.h
-- Installing: /home/roland/test/cmake/llvm-project/install/include/c++/v1/__config_site
[4/4] cd /home/roland/test/cmake/llvm-project/build/libcxx/src && /nix/store/n7j...ENT=cxx -P /home/roland/test/cmake/llvm-project/build/libcxx/cmake_install.cmake
FAILED: libcxx/src/CMakeFiles/install-cxx /home/roland/test/cmake/llvm-project/build/libcxx/src/CMakeFiles/install-cxx
cd /home/roland/test/cmake/llvm-project/build/libcxx/src && /nix/store/n7jz18i0cspdkfd1y0w1mg5rqvs15kdr-cmake-3.22.0/bin/cmake -DCMAKE_INSTALL_COMPONENT=cxx -P /home/roland/test/cmake/llvm-project/build/libcxx/cmake_install.cmake
-- Install configuration: ""
-- Installing: /home/roland/test/cmake/llvm-project/install/lib/libc++.so.1.0
-- Installing: /home/roland/test/cmake/llvm-project/install/lib/libc++.so.1
-- Set runtime path of "/home/roland/test/cmake/llvm-project/install/lib/libc++.so.1.0" to ""
-- Installing: /home/roland/test/cmake/llvm-project/install/lib/libc++.so
CMake Error at cmake_install.cmake:88 (file):
   file RPATH_CHANGE could not write new RPATH:



   to the file:

     /home/roland/test/cmake/llvm-project/install/lib/libc++.so

Call Stack (most recent call first):
   /home/roland/test/cmake/llvm-project/build/libcxx/cmake_install.cmake:56 (include)


ninja: build stopped: subcommand failed.

[2] 
https://github.com/Kitware/CMake/commit/2e1149874d34b63cc16c7330ce1ef5ef779e5140
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