Documentation/llvm: add documentation on building w/ Clang/LLVM

    issues

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.. only::  subproject and html

==============================

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. These can be invoked as

additional parameters to `make`.

	make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\

	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-objsize \\

	  READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\

	  HOSTLD=ld.lld

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/

C:	irc://chat.freenode.net/clangbuiltlinux

S:	Supported

K:	\b(?i:clang|llvm)\b

F:	Documentation/kbuild/llvm.rst

CLEANCACHE API

M:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>