| ============================== |
| 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 OBJSIZE=llvm-size \ |
| 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. |
| |
| 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 |
| ------------- |
| |
| - 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/ |
