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.”

Building with LLVM

Invoke make via:

make LLVM=1

to compile for the host target. For cross compiling:

make LLVM=1 ARCH=arm64

The LLVM= argument

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

LLVM=1 expands to the above.

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. The following may also be used:

PATH=/path/to/llvm:$PATH make LLVM=1

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.

To support combinations of out of tree paths with version suffixes, we recommend:

PATH=/path/to/llvm/:$PATH make LLVM=-14

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 same value used for LLVM= should be set for each invocation of make if configuring and building via distinct commands. LLVM= should also be set as an environment variable when running scripts that will eventually run make.

Cross Compiling

A single Clang compiler binary (and corresponding LLVM utilities) will typically contain all supported back ends, which can help simplify cross compiling especially when LLVM=1 is used. If you use only LLVM tools, CROSS_COMPILE or target-triple-prefixes become unnecessary. Example:

make LLVM=1 ARCH=arm64

As an example of mixing LLVM and GNU utilities, for a target like ARCH=s390 which does not yet have ld.lld or llvm-objcopy support, you could invoke make via:

make LLVM=1 ARCH=s390 LD=s390x-linux-gnu-ld.bfd \
  OBJCOPY=s390x-linux-gnu-objcopy

This example will invoke s390x-linux-gnu-ld.bfd as the linker and s390x-linux-gnu-objcopy, so ensure those are reachable in your $PATH.

CROSS_COMPILE is not used to prefix the Clang compiler binary (or corresponding LLVM utilities) as is the case for GNU utilities when LLVM=1 is not set.

The LLVM_IAS= argument

Clang can assemble assembler code. You can pass LLVM_IAS=0 to disable this behavior and have Clang invoke the corresponding non-integrated assembler instead. Example:

make LLVM=1 LLVM_IAS=0

CROSS_COMPILE is necessary when cross compiling and LLVM_IAS=0 is used in order to set --prefix= for the compiler to find the corresponding non-integrated assembler (typically, you don’t want to use the system assembler when targeting another architecture). Example:

make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_COMPILE=arm-linux-gnueabi-

Ccache

ccache can be used with clang to improve subsequent builds, (though KBUILD_BUILD_TIMESTAMP should be set to a deterministic value between builds in order to avoid 100% cache misses, see Reproducible_builds for more info):

KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang"

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

loongarch

Maintained

LLVM=1

mips

Maintained

LLVM=1

powerpc

Maintained

LLVM=1

riscv

Supported

LLVM=1

s390

Maintained

LLVM=1 (LLVM >= 18.1.0), CC=clang (LLVM < 18.1.0)

um (User Mode)

Maintained

LLVM=1

x86

Supported

LLVM=1

Getting Help

Getting LLVM

We provide prebuilt stable versions of LLVM on kernel.org. These have been optimized with profile data for building Linux kernels, which should improve kernel build times relative to other distributions of LLVM.

Below are links that may be useful for building LLVM from source or procuring it through a distribution’s package manager.