1[](https://github.com/kraj/meta-clang/actions/workflows/yoe.yml?query=workflow%3AYoe+branch%3Amaster) 2 3# meta-clang (C/C++ frontend and LLVM compiler backend) 4 5This layer provides [clang/llvm](http://clang.llvm.org/) as alternative to system 6C/C++ compiler for OpenEmbedded/Yocto Project based distributions. This can cohabit 7with GNU compiler and can be used for specific recipes or full system compiler. 8 9# Getting Started 10 11```shell 12git clone git://github.com/openembedded/openembedded-core.git 13cd openembedded-core 14git clone git://github.com/openembedded/bitbake.git 15git clone git://github.com/kraj/meta-clang.git 16 17$ . ./oe-init-build-env 18``` 19If using poky ( Yocto project reference Distribution ) 20 21```shell 22git clone https://git.yoctoproject.org/git/poky 23cd poky 24git clone git://github.com/kraj/meta-clang.git 25 26$ . ./oe-init-build-env 27``` 28 29Add meta-clang overlay 30``` 31bitbake-layers add-layer ../meta-clang 32``` 33 34Check `conf/bblayers.conf` to see that meta-clang is added to layer mix e.g. 35 36# Default Compiler 37 38Note that by default gcc will remain the system compiler, however if you wish 39clang to be the default compiler then set 40 41```shell 42TOOLCHAIN ?= "clang" 43``` 44 45in `local.conf`, this would now switch default cross-compiler to be clang 46you can select clang per recipe too by writing bbappends for them containing 47 48```shell 49TOOLCHAIN = "clang" 50``` 51also look at `conf/nonclangable.conf` for list of recipes which do not yet fully 52build with clang. 53 54# Default Compiler Runtime 55 56Default is to use GNU runtime `RUNTIME = "gnu"` which consists of libgcc, libstdc++ to provide C/C++ 57runtime support. However it's possible to use LLVM runtime to replace it where 58compile-rt, llvm libunwind, and libc++ are used to provide C/C++ runtime, while 59GNU runtime works with both GCC and Clang, LLVM runtime is only tested with Clang 60compiler, switching to use LLVM runtime is done via a config metadata knob 61 62```shell 63RUNTIME = "llvm" 64``` 65 66RUNTIME variable influences individual runtime elements and can be set explicitly as well 67e.g. `LIBCPLUSPLUS` `COMPILER_RT` and `UNWINDLIB`. 68 69Please note that this will still use crt files from GNU compiler always, while llvm now 70do provide crt files, they have not been yet integrated into the toolchain. 71 72# Default C++ Standard Library Switch 73 74Using RUNTIME variable will select which C++ runtime is used, however it can be overridden 75if needed to by modifying `LIBCPLUSPLUS` variable, usually defaults used by `RUNTIME` are 76best fit. e.g. below we select LLVM C++ as default C++ runtime. 77 78```shell 79LIBCPLUSPLUS = "-stdlib=libc++" 80``` 81 82in `local.conf`. 83You can select libstdc++ per package too by writing bbappends for them containing 84 85```shell 86LIBCPLUSPLUS:toolchain-clang:pn-<recipe> = "-stdlibc=libc++" 87``` 88Defaults are chosen to be GNU for maximum compatibility with existing GNU systems. It's always 89good to use single runtime on a system, mixing runtimes can cause complications during 90compilation as well as runtime. However, it's up to distribution policies to decide which runtime 91to use. 92 93# Adding clang in generated SDK toolchain 94 95Clang based cross compiler is not included into the generated SDK using `bitbake meta-toolchain` or 96`bitbake -cpopulate_sdk <image>` if clang is expected to be part of SDK, add `CLANGSDK = "1"` 97in `local.conf` 98 99```shell 100CLANGSDK = "1" 101``` 102 103# Building 104 105Below we build for qemuarm machine as an example 106 107```shell 108$ MACHINE=qemuarm bitbake core-image-full-cmdline 109``` 110# Running 111 112```shell 113$ runqemu nographic 114``` 115 116# Limitations 117 118Few components do not build with clang, if you have a component to add to that list 119simply add it to `conf/nonclangable.inc` e.g. 120 121```shell 122TOOLCHAIN:pn-<recipe> = "gcc" 123``` 124 125and OE will start using gcc to cross compile that recipe. 126 127If a component does not build with libc++, you can add it to `conf/nonclangable.inc` e.g. 128 129```shell 130CXX:remove:pn-<recipe>:toolchain-clang = " -stdlib=libc++ " 131``` 132 133# compiler-rt failing in do_configure with custom TARGET_VENDOR 134 135If your DISTRO sets own value of TARGET_VENDOR, then it's need to be added in 136CLANG_EXTRA_OE_VENDORS, it should be done automatically, but if compiler-rt fails 137like bellow, then check the end of work-shared/llvm-project-source-12.0.0-r0/temp/log.do_patch 138is should have line like: 139NOTE: Adding support following TARGET_VENDOR values: foo in 140 /OE/build/oe-core/tmp-glibc/work-shared/llvm-project-source-12.0.0-r0/git/llvm/lib/Support/Triple.cpp and 141 /OE/build/oe-core/tmp-glibc/work-shared/llvm-project-source-12.0.0-r0/git/clang/lib/Driver/ToolChains/Gnu.cpp 142and check these files if //CLANG_EXTRA_OE_VENDORS* strings were replaced correctly. 143Read add_more_target_vendors function in recipes-devtools/clang/llvm-project-source.inc for more details. 144 145http://errors.yoctoproject.org/Errors/Details/574365/ 146```shell 147-- 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 148-- Detecting C compiler ABI info 149-- Detecting C compiler ABI info - failed 150-- 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 151-- 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 152CMake 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): 153 The C compiler 154 155 "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" 156 157 is not able to compile a simple test program. 158 159 It fails with the following output: 160 161 Change Dir: TOPDIR/tmp-glibc/work/core2-64-foo-linux/compiler-rt/12.0.0-r0/build/CMakeFiles/CMakeTmp 162 163 Run Build Command(s):ninja cmTC_928f4 && [1/2] Building C object CMakeFiles/cmTC_928f4.dir/testCCompiler.c.o 164 [2/2] Linking C executable cmTC_928f4 165``` 166 167# Dependencies 168 169```shell 170URI: git://github.com/openembedded/openembedded-core.git 171branch: master 172revision: HEAD 173 174URI: git://github.com/openembedded/bitbake.git 175branch: master 176revision: HEAD 177``` 178 179# Contributing 180 181You are encouraged to follow Github Pull request workflow 182to share changes and following commit message guidelines are recommended: [OE patch guidelines](https://www.openembedded.org/wiki/Commit_Patch_Message_Guidelines). 183 184Layer Maintainer: [Khem Raj](<mailto:raj.khem@gmail.com>) 185
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