Documentation/kbuild/llvm.rst

   1 .. _kbuild_llvm:
   2
   3 ==============================
   4 Building Linux with Clang/LLVM
   5 ==============================
   6
   7 This document covers how to build the Linux kernel with Clang and LLVM
   8 utilities.
   9
  10 About
  11 -----
  12
  13 The Linux kernel has always traditionally been compiled with GNU toolchains
  14 such as GCC and binutils. Ongoing work has allowed for `Clang
  15 <https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
  16 used 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
  20 collection 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
  22 supports C and the GNU C extensions required by the kernel, and is pronounced
  23 "klang," not "see-lang."
  24
  25 Clang
  26 -----
  27
  28 The 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
  35 Cross Compiling
  36 ---------------
  37
  38 A single Clang compiler binary will typically contain all supported backends,
  39 which 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
  45 example: ::
  46
  47         clang --target=aarch64-linux-gnu foo.c
  48
  49 LLVM Utilities
  50 --------------
  51
  52 LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
  53 to enable them. ::
  54
  55         make LLVM=1
  56
  57 They 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 READELF=llvm-readelf \
  61           HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
  62
  63 Currently, the integrated assembler is disabled by default. You can pass
  64 ``LLVM_IAS=1`` to enable it.
  65
  66 Getting Help
  67 ------------
  68
  69 - `Website <https://clangbuiltlinux.github.io/>`_
  70 - `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com>
  71 - `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
  72 - IRC: #clangbuiltlinux on chat.freenode.net
  73 - `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
  74 - `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
  75 - `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
  76
  77 .. _getting_llvm:
  78
  79 Getting LLVM
  80 -------------
  81
  82 - https://releases.llvm.org/download.html
  83 - https://github.com/llvm/llvm-project
  84 - https://llvm.org/docs/GettingStarted.html
  85 - https://llvm.org/docs/CMake.html
  86 - https://apt.llvm.org/
  87 - https://www.archlinux.org/packages/extra/x86_64/llvm/
  88 - https://github.com/ClangBuiltLinux/tc-build
  89 - https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
  90 - https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/