.. _kbuild_llvm: ============================== Building Linux with Clang/LLVM ============================== This document covers how to build the Linux kernel with Clang and LLVM utilities. About ----- The Linux kernel has always traditionally been compiled with GNU toolchains such as GCC and binutils. Ongoing work has allowed for `Clang <https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be used as viable substitutes. Distributions such as `Android <https://www.android.com/>`_, `ChromeOS <https://www.chromium.org/chromium-os>`_, and `OpenMandriva <https://www.openmandriva.org/>`_ use Clang built kernels. `LLVM is a collection of toolchain components implemented in terms of C++ objects <https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that supports C and the GNU C extensions required by the kernel, and is pronounced "klang," not "see-lang." Clang ----- The compiler used can be swapped out via ``CC=`` command line argument to ``make``. ``CC=`` should be set when selecting a config and during a build. :: make CC=clang defconfig make CC=clang Cross Compiling --------------- A single Clang compiler binary will typically contain all supported backends, which can help simplify cross compiling. :: ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For example: :: clang --target=aarch64-linux-gnu foo.c LLVM Utilities -------------- LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1`` to enable them. :: make LLVM=1 They can be enabled individually. The full list of the parameters: :: make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \ READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \ HOSTLD=ld.lld Currently, the integrated assembler is disabled by default. You can pass ``LLVM_IAS=1`` to enable it. Getting Help ------------ - `Website <https://clangbuiltlinux.github.io/>`_ - `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com> - `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_ - IRC: #clangbuiltlinux on chat.freenode.net - `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux - `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_ - `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_ .. _getting_llvm: Getting LLVM ------------- - https://releases.llvm.org/download.html - https://github.com/llvm/llvm-project - https://llvm.org/docs/GettingStarted.html - https://llvm.org/docs/CMake.html - https://apt.llvm.org/ - https://www.archlinux.org/packages/extra/x86_64/llvm/ - https://github.com/ClangBuiltLinux/tc-build - https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source - https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/