Using Clang with OpenEmbedded/Yocto Project

Introduction

Clang is C/C++/ObjC  frontend for LLVM compiler suite,  supporting several architectures now. We have added support for clang into OpenEmbedded using a standalone layer

https://github.com/kraj/meta-clang

This layer requires OpenEmbedded-core layer as dependency

Configuring meta-clang

git clone git://github.com/openembedded/openembedded-core
cd openembedded-core
git clone git://github.com/openembedded/bitbake
git clone git://github.com/kraj/meta-clang

Setup

. ./oe-init-build-env

Edit conf/bblayer.conf to add meta-clang

# LAYER_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
LCONF_VERSION = "6"

BBPATH = "${TOPDIR}"
BBFILES ?= ""

BBLAYERS ?= " \
 /a/wheezy/home/kraj/work/openembedded-core/meta-clang \
 /a/wheezy/home/kraj/work/openembedded-core/meta \
 "

BBLAYERS_NON_REMOVABLE ?= " \
 /a/wheezy/home/kraj/work/openembedded-core/meta \
 "

 

Selecting Default system Compiler

When meta-clang is added to layers it switches the default cross compiler for the system to be clang naturally. This selection is made by a variable which is assigned to choose clang if nothing else is selected

TOOLCHAIN ??= "clang"

One could start using GCC as default system compiler by setting TOOLCHAIN variable to ‘gcc’

TOOLCHAIN = "gcc"

This selection is done in configuration metadata files like conf/local.conf

Per-recipe ( package ) Compiler selection

When using clang, not all components are compilable yet, hence we have to sometimes choose gcc for certain package recipe. In order to do it, you have to write a .bbappend in meta-clang, as an example below bbappend shows how we select gcc for building glibc

$ cat recipes-excluded/nonclangable/glibc_%.bbappend
TOOLCHAIN = "gcc"

Current Status

We can build/boot core-image-minimal for qemux86 and raspberryPi machines, of course that includes few components which are still compiled using gcc. But at this point, we are ready to port the packages to compile with clang

Compiling Kernel with Clang

For kernel we still use gcc, to start using clang for kernel we would require to override KERNEL_CC variable

kernel-arch.bbclass:KERNEL_CC = "${CCACHE}${HOST_PREFIX}gcc ${HOST_CC_KERNEL_ARCH} -fuse-ld=bfd"

To use clang

kernel-arch.bbclass:KERNEL_CC = "${CCACHE}${HOST_PREFIX}clang ${HOST_CC_KERNEL_ARCH} -fuse-ld=bfd"

This can also be done in kernel recipe too.

KERNEL_CC_forcevariable = "${CCACHE}${HOST_PREFIX}clang ${HOST_CC_KERNEL_ARCH} -fuse-ld=bfd"

Compiler runtime

We currently use gcc runtime by default with clang as well, meta-clang does have recipes for compiler-rt and libc++ which can be used to replace libgcc and libstdc++, at application level this selection can be made to depend on compiler-rt and libc++

SDK

We can generate OpenEmbedded SDK which will have both gcc and clang based cross compilers

bitbake -cpopulate_sdk core-image-minimal

Installing the SDK should be done something like this Installing the Toolchain¶

To use clang in SDK one should use CLANGCC and CLANGCXX environment variables to reference the clang compilers