| 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=]{.title-ref} command line |
| argument to [make]{.title-ref}. [CC=]{.title-ref} 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]{.title-ref} is not used to prefix the Clang compiler |
| binary, instead [CROSS\_COMPILE]{.title-ref} is used to set a command |
| line flag: [--target <triple>]{.title-ref}. For example: |
| |
| > clang --target aarch64-linux-gnu foo.c |
| |
| LLVM Utilities |
| -------------- |
| |
| LLVM has substitutes for GNU binutils utilities. These can be invoked as |
| additional parameters to [make]{.title-ref}. |
| |
| > 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 |
| ------------ |
| |
| - <http://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/> |