|
| 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. These can be invoked as |
| 51 | +additional parameters to `make`. |
| 52 | + |
| 53 | +make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip\\ |
| 54 | + OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-objsize\\ |
| 55 | + READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar\\ |
| 56 | + HOSTLD=ld.lld |
| 57 | + |
| 58 | +Getting Help |
| 59 | +------------ |
| 60 | + |
| 61 | +- `Website<https://clangbuiltlinux.github.io/>`_ |
| 62 | +- `Mailing List<https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com> |
| 63 | +- `Issue Tracker<https://github.com/ClangBuiltLinux/linux/issues>`_ |
| 64 | +- IRC: #clangbuiltlinux on chat.freenode.net |
| 65 | +- `Telegram<https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux |
| 66 | +- `Wiki<https://github.com/ClangBuiltLinux/linux/wiki>`_ |
| 67 | +- `Beginner Bugs<https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_ |
| 68 | + |
| 69 | +Getting LLVM |
| 70 | +------------- |
| 71 | + |
| 72 | +- http://releases.llvm.org/download.html |
| 73 | +- https://github.com/llvm/llvm-project |
| 74 | +- https://llvm.org/docs/GettingStarted.html |
| 75 | +- https://llvm.org/docs/CMake.html |
| 76 | +- https://apt.llvm.org/ |
| 77 | +- https://www.archlinux.org/packages/extra/x86_64/llvm/ |
| 78 | +- https://github.com/ClangBuiltLinux/tc-build |
| 79 | +- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source |
| 80 | +- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/ |