Building Linux with Clang/LLVM

This document covers how to build the Linux kernel with Clang and LLVMutilities.

About

The Linux kernel has always traditionally been compiled with GNU toolchainssuch as GCC and binutils. Ongoing work has allowed forClang andLLVM utilities to beused as viable substitutes. Distributions such asAndroid,ChromeOS, andOpenMandriva use Clang built kernels.LLVM is acollection of toolchain components implemented in terms of C++ objects. Clang is a front-end to LLVM thatsupports 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 viaCC= command line argument tomake.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, insteadCROSS_COMPILE is used to set a command line flag:–target <triple>. Forexample:

clang –target aarch64-linux-gnu foo.c

LLVM Utilities

LLVM has substitutes for GNU binutils utilities. Kbuild supportsLLVM=1to 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 passLLVM_IAS=1 to enable it.

Getting Help