1.. _kbuild_llvm: 2 3============================== 4Building Linux with Clang/LLVM 5============================== 6 7This document covers how to build the Linux kernel with Clang and LLVM 8utilities. 9 10About 11----- 12 13The Linux kernel has always traditionally been compiled with GNU toolchains 14such as GCC and binutils. Ongoing work has allowed for `Clang 15<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be 16used as viable substitutes. Distributions such as `Android 17<https://www.android.com/>`_, `ChromeOS 18<https://www.chromium.org/chromium-os>`_, and `OpenMandriva 19<https://www.openmandriva.org/>`_ use Clang built kernels. `LLVM is a 20collection of toolchain components implemented in terms of C++ objects 21<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that 22supports C and the GNU C extensions required by the kernel, and is pronounced 23"klang," not "see-lang." 24 25Clang 26----- 27 28The compiler used can be swapped out via ``CC=`` command line argument to ``make``. 29``CC=`` should be set when selecting a config and during a build. :: 30 31 make CC=clang defconfig 32 33 make CC=clang 34 35Cross Compiling 36--------------- 37 38A single Clang compiler binary will typically contain all supported backends, 39which can help simplify cross compiling. :: 40 41 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang 42 43``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead 44``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For 45example: :: 46 47 clang --target=aarch64-linux-gnu foo.c 48 49LLVM Utilities 50-------------- 51 52LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1`` 53to enable them. :: 54 55 make LLVM=1 56 57They can be enabled individually. The full list of the parameters: :: 58 59 make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ 60 OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \ 61 READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \ 62 HOSTLD=ld.lld 63 64Currently, the integrated assembler is disabled by default. You can pass 65``LLVM_IAS=1`` to enable it. 66 67Getting Help 68------------ 69 70- `Website <https://clangbuiltlinux.github.io/>`_ 71- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com> 72- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_ 73- IRC: #clangbuiltlinux on chat.freenode.net 74- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux 75- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_ 76- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_ 77 78.. _getting_llvm: 79 80Getting LLVM 81------------- 82 83- https://releases.llvm.org/download.html 84- https://github.com/llvm/llvm-project 85- https://llvm.org/docs/GettingStarted.html 86- https://llvm.org/docs/CMake.html 87- https://apt.llvm.org/ 88- https://www.archlinux.org/packages/extra/x86_64/llvm/ 89- https://github.com/ClangBuiltLinux/tc-build 90- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source 91- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/ 92