[clang] DISABLE_LLVM_LINK_LLVM_DYLIB no longer honored after upgrade to LLVM22 when configuring with builtin_llvm=OFF

[ferdymercury]

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• Checked for duplicates

Description

Check duplicate issues.

• Checked for duplicates

Description

With the following snippet:

docker run -it registry.cern.ch/root-ci/ubuntu2604:buildready
apt update
apt install llvm-20 libedit-dev libpolly-20-dev
git clone https://github.com/root-project/root root_src && cd root_src
git checkout v6-40-00-patches
sed -i 's|LLVM REQUIRED|LLVM ${ROOT_LLVM_VERSION_REQUIRED_MAJOR}.1 REQUIRED|g' interpreter/CMakeLists.txt # Backport of a patch from master
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
grep -r LLVM.so *

shows that LLVM is not linked via shared object libraries, as expected.

Instead, in current master:

docker run -it registry.cern.ch/root-ci/ubuntu2604:buildready
apt update
apt install llvm-22 libpolly-22-dev
git clone https://github.com/root-project/root root_src && cd root_src
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
grep -r LLVM.so *

shows a lot of occurrences in grep, which is not what one would expect. Which later leads to the issue described in #12156

Ubuntu26 installs LLVM20 and LLVM22 in the same way, so it's probably not a Debian packaging issue, but rather something with LLVM22 CMakeLists.txt changes. See only minor differences:

diff --brief --recursive  /usr/lib/llvm-20/lib/ /usr/lib/llvm-22/lib/ | grep Only

Only in /usr/lib/llvm-20/lib/: libLLVM-20.so
Only in /usr/lib/llvm-22/lib/: libLLVM-22.so

Only in /usr/lib/llvm-20/lib/: libLLVM.so.20.1
Only in /usr/lib/llvm-22/lib/: libLLVM.so.22.1

Only in /usr/lib/llvm-20/lib/: libLTO.so.20.1
Only in /usr/lib/llvm-22/lib/: libLTO.so.22.1

Only in /usr/lib/llvm-20/lib/: libRemarks.so.20.1
Only in /usr/lib/llvm-22/lib/: libRemarks.so.22.1

Only in /usr/lib/llvm-20/lib/: libclang-cpp.so.20.1
Only in /usr/lib/llvm-22/lib/: libclang-cpp.so.22.1

Only in /usr/lib/llvm-20/lib/cmake: polly
Only in /usr/lib/llvm-20/lib/: libPolly.a
Only in /usr/lib/llvm-20/lib/: libPollyISL.a

Only in /usr/lib/llvm-22/lib/: libLLVMABI.a
Only in /usr/lib/llvm-22/lib/: libLLVMCAS.a
Only in /usr/lib/llvm-22/lib/: libLLVMDTLTO.a
Only in /usr/lib/llvm-22/lib/: libLLVMDWARFCFIChecker.a
Only in /usr/lib/llvm-22/lib/: libLLVMDebugInfoDWARFLowLevel.a
Only in /usr/lib/llvm-22/lib/: libLLVMFrontendDirective.a
Only in /usr/lib/llvm-22/lib/: libLLVMPlugins.a
Only in /usr/lib/llvm-22/lib/: libLLVMSupportLSP.a

Reproducer

docker run -it registry.cern.ch/root-ci/ubuntu2604:buildready
apt update
apt install llvm-22 libpolly-22-dev
git clone https://github.com/root-project/root root_src && cd root_src
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
make clang-tblgen -j$(nproc)

ROOT version

ROOT v6.40.02
Built for linuxx8664gcc on Jun 10 2026, 10:37:47
From tags/v6-40-02@v6-40-02
With c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 std201703
Binary directory: /opt/root/bin

Installation method

Source

Operating system

Ubuntu26 Docker

Additional context

tagging @guitargeek and @vepadulano since they have experience with builtin_llvm=OFF
conda-forge/llvmdev-feedstock#357 (comment)

[guitargeek]

It's a never-ending story, see also:

• Additional patch to Clangs CMake code is required in case LLVM was built with LLVM_LINK_LLVM_DYLIB=ON #18387
• [CMake] Only export the LLVM_LINK_LLVM_DYLIB setting if not yet set llvm/llvm-project#135570

[ferdymercury]

Thanks a lot! I can confirm that your patch #18387 (comment)
solves this issue. (Even if that issue you mean is with builtin_clang=OFF whereas I use builtin_clang=ON, so not exactly duplicate).

What I do not understand is why it works ok with LLVM20 without that patch, and starts failing with LLVM22.