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.

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. 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 OBJSIZE=llvm-size \ READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \ HOSTLD=ld.lld

Currently, the integrated assembler is disabled by default. You can pass LLVM_IAS=1 to enable it.

Getting Help