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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 | .. _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 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. Supported Architectures ----------------------- LLVM does not target all of the architectures that Linux supports and just because a target is supported in LLVM does not mean that the kernel will build or work without any issues. Below is a general summary of architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level of support corresponds to "S" values in the MAINTAINERS files. If an architecture is not present, it either means that LLVM does not target it or there are known issues. Using the latest stable version of LLVM or even the development tree will generally yield the best results. An architecture's ``defconfig`` is generally expected to work well, certain configurations may have problems that have not been uncovered yet. Bug reports are always welcome at the issue tracker below! .. list-table:: :widths: 10 10 10 :header-rows: 1 * - Architecture - Level of support - ``make`` command * - arm - Supported - ``LLVM=1`` * - arm64 - Supported - ``LLVM=1`` * - mips - Maintained - ``CC=clang`` * - powerpc - Maintained - ``CC=clang`` * - riscv - Maintained - ``CC=clang`` * - s390 - Maintained - ``CC=clang`` * - x86 - Supported - ``LLVM=1`` 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/ |