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 and LLVM utilities to be used as viable substitutes. Distributions such as Android, ChromeOS, OpenMandriva, and Chimera Linux use Clang built kernels. Google's and Meta's datacenter fleets also run kernels built with Clang.

LLVM is a collection of toolchain components implemented in terms of C++ objects. 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.

make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-

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. They can be enabled individually. The full list of supported make variables:

make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld

To simplify the above command, Kbuild supports the LLVM variable:

make LLVM=1

If your LLVM tools are not available in your PATH, you can supply their location using the LLVM variable with a trailing slash:

make LLVM=/path/to/llvm/

which will use /path/to/llvm/clang, /path/to/llvm/ld.lld, etc.

If your LLVM tools have a version suffix and you want to test with that explicit version rather than the unsuffixed executables like LLVM=1, you can pass the suffix using the LLVM variable:

make LLVM=-14

which will use clang-14, ld.lld-14, etc.

LLVM=0 is not the same as omitting LLVM altogether, it will behave like LLVM=1. If you only wish to use certain LLVM utilities, use their respective make variables.

The integrated assembler is enabled by default. You can pass LLVM_IAS=0 to disable it.

Omitting CROSS_COMPILE

As explained above, CROSS_COMPILE is used to set --target=<triple>.

If CROSS_COMPILE is not specified, the --target=<triple> is inferred from ARCH.

That means if you use only LLVM tools, CROSS_COMPILE becomes unnecessary.

For example, to cross-compile the arm64 kernel:

make ARCH=arm64 LLVM=1

If LLVM_IAS=0 is specified, CROSS_COMPILE is also used to derive --prefix=<path> to search for the GNU assembler and linker.

make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-

Supported Architectures

LLVM does not target all of the architectures that Linux supports and just because a target is supported in LLVM does not mean that the kernel will build or work without any issues. Below is a general summary of architectures that currently work with CC=clang or LLVM=1. Level of support corresponds to "S" values in the MAINTAINERS files. If an architecture is not present, it either means that LLVM does not target it or there are known issues. Using the latest stable version of LLVM or even the development tree will generally yield the best results. An architecture's defconfig is generally expected to work well, certain configurations may have problems that have not been uncovered yet. Bug reports are always welcome at the issue tracker below!

Architecture

Level of support

make command

arm

Supported

LLVM=1

arm64

Supported

LLVM=1

hexagon

Maintained

LLVM=1

mips

Maintained

LLVM=1

powerpc

Maintained

CC=clang

riscv

Maintained

LLVM=1

s390

Maintained

CC=clang

um (User Mode)

Maintained

LLVM=1

x86

Supported

LLVM=1

Getting Help

Getting LLVM

We provide prebuilt stable versions of LLVM on kernel.org. Below are links that may be useful for building LLVM from source or procuring it through a distribution's package manager.