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/>