Building Clang from source

If you would like to build Clang (and lld) from source for testing purposes, it is really easy! You should have cmake and ninja installed from your distribution.

$ git clone git://github.com/llvm/llvm-project
$ cd llvm-project
$ mkdir build
$ cd build
$ cmake -G Ninja \
        -DCMAKE_BUILD_TYPE=Release \
        -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt" \
        -DLLVM_ENABLE_WARNINGS=OFF \
        ../llvm
$ ninja

If you want Clang to compile faster and the time it takes to compile other projects to potentially decrease, you can run the following cmake command instead of the one above, which:

• Uses clang if it is installed to initially build Clang, falling back to gcc if it isn't present.
• Uses -march=native -mtune=native to generate code optimized for your current processor.
• Turns off a lot of unnecessary features like tests, examples, and other things that the kernel doesn't care about.
• Only enables the architectures that we are currently testing (aarch64, arm32, powerpc, x86).
• Uses ld.lld for linking when building with Clang (which has been shown to be MUCH faster than ld.bfd and slightly faster than ld.gold), falling back to ld.gold then ld.bfd if it isn't present.

cmake -Wno-dev \
      -G Ninja \
      -DCLANG_ENABLE_ARCMT=OFF \
      -DCLANG_ENABLE_STATIC_ANALYZER=OFF \
      -DCLANG_PLUGIN_SUPPORT=OFF \
      -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_C_COMPILER="$(command -v clang || command -v gcc)" \
      -DCMAKE_C_FLAGS="-O2 -march=native -mtune=native" \
      -DCMAKE_CXX_COMPILER="$(command -v clang++ || command -v g++)" \
      -DCMAKE_CXX_FLAGS="-O2 -march=native -mtune=native" \
      -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
      -DLLVM_ENABLE_BINDINGS=OFF \
      -DLLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR="" \
      -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt" \
      -DLLVM_CCACHE_BUILD=ON \
      -DLLVM_ENABLE_OCAMLDOC=OFF \
      -DLLVM_ENABLE_TERMINFO=OFF \
      -DLLVM_ENABLE_WARNINGS=OFF \
      -DLLVM_INCLUDE_EXAMPLES=OFF \
      -DLLVM_INCLUDE_TESTS=OFF \
      -DLLVM_INCLUDE_DOCS=OFF \
      -DLLVM_TARGETS_TO_BUILD="AArch64;ARM;PowerPC;X86" \
      -DLLVM_USE_LINKER="$(if command -v clang &>/dev/null; for LD in lld gold bfd; do LD=$(command -v ld.${LD}); [[ -n ${LD} ]] && break; done; echo ${LD}; fi)" \
      ../llvm

Once it is done, you can export PATH=${PWD}/bin:${PATH} to have it available for kernel compiles.