Building Linux with Clang/LLVM¶
This document covers how to build the Linux kernel with Clang and LLVM utilities.
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¶
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:
which will use
/path/to/llvm/ld.lld, etc. The
following may also be used:
PATH=/path/to/llvm:$PATH make LLVM=1
If your LLVM tools have a version suffix and you want to test with that
explicit version rather than the unsuffixed executables like
can pass the suffix using the
which will use
To support combinations of out of tree paths with version suffixes, we recommend:
PATH=/path/to/llvm/:$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
if configuring and building via distinct commands.
LLVM= should also be set
as an environment variable when running scripts that will eventually run
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
which does not yet have
llvm-objcopy support, you could
make LLVM=1 ARCH=s390 LD=s390x-linux-gnu-ld.bfd \
This example will invoke
s390x-linux-gnu-ld.bfd as the linker and
s390x-linux-gnu-objcopy, so ensure those are reachable in your
CROSS_COMPILE is not used to prefix the Clang compiler binary (or
corresponding LLVM utilities) as is the case for GNU utilities when
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
make LLVM=1 LLVM_IAS=0
CROSS_COMPILE is necessary when cross compiling and
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 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"
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
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.
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!
Level of support
um (User Mode)
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.