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

Default Compiler Runtime

Default is to use GNU runtime RUNTIME = "gnu" which consists of libgcc, libstdc++ to provide C/C++ runtime support. However its 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

RUNTIME = "llvm"

RUNTIME variable influences individual runtime elements and can be set explcitly 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 intergrated into toolchain

Default C++ Standard Library Switch

Using RUNTIME bariable will select which C++ runtime is used, however it can be overridden if needed to by modifying LIBCPLUSPLUS variable, usually defaults used by RUNTIME is 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. Its always good to use single runtime on a system, mixing runtimes can cause complications during compilation as well as runtime. However, its upto 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"

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 own value of TARGET_VENDOR, then it's need 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 is should have 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_more_target_vendors 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: git://github.com/openembedded/openembedded-core.git
branch: master
revision: HEAD

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

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