meta-clang (C/C++ frontend and LLVM compiler backend)

This layer provides clang/llvm as alternative to system C/C++ compiler for OpenEmbedded/Yocto Project based distributions. This can cohabit with GNU compiler and can be used for specific recipes or full system compiler.

Getting Started

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

$ . ./oe-init-build-env

If using poky ( Yocto project reference Distribution )

git clone https://git.yoctoproject.org/git/poky
cd poky
git clone https://github.com/kraj/meta-clang.git

$ . ./oe-init-build-env

Add meta-clang overlay

bitbake-layers add-layer ../meta-clang

Check conf/bblayers.conf to see that meta-clang is added to layer mix e.g.

Default Compiler

Note that by default gcc will remain the system compiler, however if you wish clang to be the default compiler then set

TOOLCHAIN ?= "clang"

in local.conf, this would now switch default cross-compiler to be clang you can select clang per recipe too by writing bbappends for them containing

TOOLCHAIN = "clang"

also look at conf/nonclangable.conf for list of recipes which do not yet fully build with clang.

Providing LLVM

clang recipes can provide llvm and related packages too, it might be worth using single provider for llvm and clang to save some compile time and space, select the knobs to point to clang, default is to use the version provided by core layer.

PREFERRED_PROVIDER_llvm = "clang"
PREFERRED_PROVIDER_llvm-native = "clang-native"
PREFERRED_PROVIDER_nativesdk-llvm = "nativesdk-clang"
PROVIDES:pn-clang = "llvm"
PROVIDES:pn-clang-native = "llvm-native"
PROVIDES:pn-nativesdk-clang = "nativesdk-llvm"

Default Compiler Runtime

Default is to use GNU runtime TC_CXX_RUNTIME = "gnu" which consists of libgcc, libstdc++ to provide C/C++ runtime support. However it's possible to use LLVM runtime to replace it where compile-rt, llvm libunwind, and libc++ are used to provide C/C++ runtime, while GNU runtime works with both GCC and Clang, LLVM runtime is only tested with Clang compiler, switching to use LLVM runtime is done via a config metadata knob

TC_CXX_RUNTIME = "llvm"

TC_CXX_RUNTIME variable influences individual runtime elements and can be set explicitly as well e.g. LIBCPLUSPLUS COMPILER_RT and UNWINDLIB.

Please note that this will still use crt files from GNU compiler always, while llvm now do provide crt files, they have not been yet integrated into the toolchain.

Default C++ Standard Library Switch

Using TC_CXX_RUNTIME variable will select which C++ runtime is used, however it can be overridden if needed to by modifying LIBCPLUSPLUS variable, usually defaults used by TC_CXX_RUNTIME are best fit. e.g. below we select LLVM C++ as default C++ runtime.

LIBCPLUSPLUS = "-stdlib=libc++"

in local.conf. You can select libstdc++ per package too by writing bbappends for them containing

LIBCPLUSPLUS:toolchain-clang:pn-<recipe> = "-stdlibc=libc++"

Defaults are chosen to be GNU for maximum compatibility with existing GNU systems. It's always good to use single runtime on a system, mixing runtimes can cause complications during compilation as well as runtime. However, it's up to distribution policies to decide which runtime to use.

Adding clang in generated SDK toolchain

Clang based cross compiler is not included into the generated SDK using bitbake meta-toolchain or bitbake -cpopulate_sdk <image> if clang is expected to be part of SDK, add CLANGSDK = "1" in local.conf

CLANGSDK = "1"

Kernel build with clang

Newer kernels and Android kernels support clang compilation, and even support LTO, The following takes rockchip's kernel as an example to configure clang compilation. x86, arm and arm64 kernel supported full LLVM toolchain, other arch only support clang. more info https://docs.kernel.org/kbuild/llvm.html

linux-rockchip_%bbappend

TOOLCHAIN:forcevariable = "clang"

DEPENDS:append:toolchain-clang = " clang-cross-${TARGET_ARCH}"
KERNEL_CC:toolchain-clang = "${CCACHE}clang ${HOST_CC_KERNEL_ARCH} -fuse-ld=lld ${DEBUG_PREFIX_MAP} -fdebug-prefix-map=${STAGING_KERNEL_DIR}=${KERNEL_SRC_PATH}"
KERNEL_LD:toolchain-clang = "${CCACHE}ld.lld"
KERNEL_AR:toolchain-clang = "${CCACHE}llvm-ar"

if you want use LLVM integrated assembler for some older kernel, newer vesion is enabled default.

do_compile:prepend:toolchain-clang() {
	export LLVM_IAS=1
}

do_compile_kernelmodules:prepend:toolchain-clang() {
	export LLVM_IAS=1
}

if you want enable LTO, append follow content.

FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
SRC_URI:append:toolchain-clang = "\
	file://lto.cfg \
"

CONFIG_LTO_CLANGis need for some android based kernel, mainline kernel will do auto detect.

lto.cfg

CONFIG_LTO_CLANG=y
CONFIG_LTO=y
CONFIG_LTO_CLANG_THIN=y

Building

Below we build for qemuarm machine as an example

$ MACHINE=qemuarm bitbake core-image-full-cmdline

Running

$ runqemu nographic

Limitations

Few components do not build with clang, if you have a component to add to that list simply add it to conf/nonclangable.inc e.g.

TOOLCHAIN:pn-<recipe> = "gcc"

and OE will start using gcc to cross compile that recipe.

If a component does not build with libc++, you can add it to conf/nonclangable.inc e.g.

CXX:remove:pn-<recipe>:toolchain-clang = " -stdlib=libc++ "

compiler-rt failing in do_configure with custom TARGET_VENDOR

If your DISTRO sets its own value of TARGET_VENDOR, then it needs to be added in CLANG_EXTRA_OE_VENDORS, it should be done automatically, but if compiler-rt fails like bellow, then check the end of work-shared/llvm-project-source-12.0.0-r0/temp/log.do_patch it should have a line like: NOTE: Adding support following TARGET_VENDOR values: foo in /OE/build/oe-core/tmp-glibc/work-shared/llvm-project-source-12.0.0-r0/git/llvm/lib/Support/Triple.cpp and /OE/build/oe-core/tmp-glibc/work-shared/llvm-project-source-12.0.0-r0/git/clang/lib/Driver/ToolChains/Gnu.cpp and check these files if //CLANG_EXTRA_OE_VENDORS* strings were replaced correctly. Read add_distro_vendor function in recipes-devtools/clang/llvm-project-source.inc for more details.

http://errors.yoctoproject.org/Errors/Details/574365/

-- Found assembler: TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/recipe-sysroot-native/usr/bin/x86_64-foo-linux/x86_64-foo-linux-clang
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Check for working C compiler: TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/recipe-sysroot-native/usr/bin/x86_64-foo-linux/x86_64-foo-linux-clang
-- Check for working C compiler: TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/recipe-sysroot-native/usr/bin/x86_64-foo-linux/x86_64-foo-linux-clang - broken
CMake Error at TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/recipe-sysroot-native/usr/share/cmake-3.19/Modules/CMakeTestCCompiler.cmake:66 (message):
  The C compiler

    "TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/recipe-sysroot-native/usr/bin/x86_64-foo-linux/x86_64-foo-linux-clang"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/build/CMakeFiles/CMakeTmp

    Run Build Command(s):ninja cmTC_928f4 && [1/2] Building C object CMakeFiles/cmTC_928f4.dir/testCCompiler.c.o
    [2/2] Linking C executable cmTC_928f4

Dependencies

URI: https://github.com/openembedded/openembedded-core.git
branch: master
revision: HEAD

URI: ghttps://github.com/openembedded/bitbake.git
branch: master
revision: HEAD

Using Devtool and Upstream Development

All LLVM based recipes use single llvm source directory, As a LLVM developer, you might want to work on your own repository to build related recipes, devtool can we useful in establishing such a workflow there is a script provided scripts/devtool-clang.sh which can assist in setting up all the recipes to use custom LLVM repository, in order to setup repository make sure that it has all the needed patches applied or else it will fail to build. Such a tree is already prepared and kept in sync at

https://github.com/kraj/llvm-project

There are branches under oe namespace which carry the needed OE patches

cd $TOPDIR/workspace/sources
git clone https://github.com/kraj/llvm-project -b oe/main llvm-project

Once project is setup and meta-clang is added, run devtool-clang.sh script which will do the needed for setting up external sources for the yocto recipes, now yocto will use custom LLVM tree for its needs.

Contributing

You are encouraged to follow Github Pull request workflow to share changes and following commit message guidelines are recommended: OE patch guidelines.

Layer Maintainer: Khem Raj