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, and OpenMandriva use Clang built kernels. 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. Kbuild supports LLVM=1 to enable them.

make LLVM=1

They can be enabled individually. The full list of the parameters:

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

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
mips Maintained CC=clang
powerpc Maintained CC=clang
riscv Maintained CC=clang
s390 Maintained CC=clang
x86 Supported LLVM=1

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