Documentation/kbuild/llvm.rst

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