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Building LLVM Clang from Source

This is a TL;DR compiled from the documentation on building LLVM with CMake.

Clone the repository. Use --branch to directly check out the version to build.

$ git clone --branch llvmorg-16.0.6 https://github.com/llvm/llvm-project.git
$ cd llvm-project

Configure a release build. Make sure to change the setting for -DLLVM_TARGETS_TO_BUILD= to match your architecture. E.g., for macOS on M1, set to AArch64. If you witness the build exhausting your main memory, lower the number of -DLLVM_PARALLEL_LINK_JOBS.

$ cmake -S llvm -B build \
    -G Ninja \
    -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_USE_LINKER=lld \
    -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;lld" \
    -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
    -DLLVM_PARALLEL_COMPILE_JOBS=$(( $(getconf _NPROCESSORS_ONLN) - 1)) \
    -DLLVM_PARALLEL_LINK_JOBS=4 \
    -DLLVM_TARGETS_TO_BUILD="X86" \
    -DLLVM_RUNTIME_TARGETS="x86_64-unknown-linux-gnu"

Make sure that the CMake output includes the following lines:

-- clang project is enabled
-- clang-tools-extra project is enabled
-- compiler-rt project is enabled
...
-- lld project is enabled

You are now ready to build LLVM, Clang, and the other enabled projects. This will take a while. Even on a high-end PC with an AMD 7800X3D and 32 GiB of DDR5 memory and NVME storage it took 15 minutes.

$ cmake --build build

When you are done building, we are ready to install. Here, we will install Clang and other projects to /usr/local, as we do not want to overwrite existing installations, that also may be managed by your systems package manager (e.g. Pacman or Homebrew). Also, usually /usr/local/bin precedes /usr/bin in $PATH, and so our freshly built and installed Clang will be used by default. If you do not want to install Clang system-wide and/or you do not want to overrule the Clang installation in /usr/bin, you can also install to ${HOME}/.local/.

$ cd build
$ sudo cmake -DCMAKE_INSTALL_PREFIX=/usr/local -P cmake_install.cmake