Linux/Documentation/kbuild/llvm.rst

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1 .. _kbuild_llvm: 1 .. _kbuild_llvm: 2 2 3 ============================== 3 ============================== 4 Building Linux with Clang/LLVM 4 Building Linux with Clang/LLVM 5 ============================== 5 ============================== 6 6 7 This document covers how to build the Linux ke 7 This document covers how to build the Linux kernel with Clang and LLVM 8 utilities. 8 utilities. 9 9 10 About 10 About 11 ----- 11 ----- 12 12 13 The Linux kernel has always traditionally been 13 The Linux kernel has always traditionally been compiled with GNU toolchains 14 such as GCC and binutils. Ongoing work has all 14 such as GCC and binutils. Ongoing work has allowed for `Clang 15 <https://clang.llvm.org/>`_ and `LLVM <https:/ 15 <https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be 16 used as viable substitutes. Distributions such 16 used as viable substitutes. Distributions such as `Android 17 <https://www.android.com/>`_, `ChromeOS 17 <https://www.android.com/>`_, `ChromeOS 18 <https://www.chromium.org/chromium-os>`_, `Ope !! 18 <https://www.chromium.org/chromium-os>`_, and `OpenMandriva 19 <https://www.openmandriva.org/>`_, and `Chimer !! 19 <https://www.openmandriva.org/>`_ use Clang built kernels. `LLVM is a 20 <https://chimera-linux.org/>`_ use Clang built !! 20 collection of toolchain components implemented in terms of C++ objects 21 datacenter fleets also run kernels built with !! 21 <https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that 22 !! 22 supports C and the GNU C extensions required by the kernel, and is pronounced 23 `LLVM is a collection of toolchain components !! 23 "klang," not "see-lang." 24 objects <https://www.aosabook.org/en/llvm.html !! 24 25 that supports C and the GNU C extensions requi !! 25 Clang 26 pronounced "klang," not "see-lang." !! 26 ----- 27 27 28 Building with LLVM !! 28 The compiler used can be swapped out via ``CC=`` command line argument to ``make``. 29 ------------------ !! 29 ``CC=`` should be set when selecting a config and during a build. :: 30 30 31 Invoke ``make`` via:: !! 31 make CC=clang defconfig 32 32 33 make LLVM=1 !! 33 make CC=clang 34 << 35 to compile for the host target. For cross comp << 36 << 37 make LLVM=1 ARCH=arm64 << 38 << 39 The LLVM= argument << 40 ------------------ << 41 << 42 LLVM has substitutes for GNU binutils utilitie << 43 individually. The full list of supported make << 44 << 45 make CC=clang LD=ld.lld AR=llvm-ar NM= << 46 OBJCOPY=llvm-objcopy OBJDUMP=llvm-ob << 47 HOSTCC=clang HOSTCXX=clang++ HOSTAR= << 48 << 49 ``LLVM=1`` expands to the above. << 50 << 51 If your LLVM tools are not available in your P << 52 location using the LLVM variable with a traili << 53 34 54 make LLVM=/path/to/llvm/ !! 35 Cross Compiling 55 !! 36 --------------- 56 which will use ``/path/to/llvm/clang``, ``/pat << 57 following may also be used:: << 58 37 59 PATH=/path/to/llvm:$PATH make LLVM=1 !! 38 A single Clang compiler binary will typically contain all supported backends, >> 39 which can help simplify cross compiling. :: 60 40 61 If your LLVM tools have a version suffix and y !! 41 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang 62 explicit version rather than the unsuffixed ex << 63 can pass the suffix using the ``LLVM`` variabl << 64 42 65 make LLVM=-14 !! 43 ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead >> 44 ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For >> 45 example: :: 66 46 67 which will use ``clang-14``, ``ld.lld-14``, et !! 47 clang --target=aarch64-linux-gnu foo.c 68 48 69 To support combinations of out of tree paths w !! 49 LLVM Utilities 70 recommend:: !! 50 -------------- 71 51 72 PATH=/path/to/llvm/:$PATH make LLVM=-1 !! 52 LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1`` >> 53 to enable them. :: 73 54 74 ``LLVM=0`` is not the same as omitting ``LLVM` !! 55 make LLVM=1 75 ``LLVM=1``. If you only wish to use certain LL << 76 respective make variables. << 77 56 78 The same value used for ``LLVM=`` should be se !! 57 They can be enabled individually. The full list of the parameters: :: 79 if configuring and building via distinct comma << 80 as an environment variable when running script << 81 ``make``. << 82 58 83 Cross Compiling !! 59 make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ 84 --------------- !! 60 OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \ >> 61 HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld 85 62 86 A single Clang compiler binary (and correspond !! 63 Currently, the integrated assembler is disabled by default. You can pass 87 typically contain all supported back ends, whi !! 64 ``LLVM_IAS=1`` to enable it. 88 compiling especially when ``LLVM=1`` is used. << 89 ``CROSS_COMPILE`` or target-triple-prefixes be << 90 << 91 make LLVM=1 ARCH=arm64 << 92 << 93 As an example of mixing LLVM and GNU utilities << 94 which does not yet have ``ld.lld`` or ``llvm-o << 95 invoke ``make`` via:: << 96 << 97 make LLVM=1 ARCH=s390 LD=s390x-linux-g << 98 OBJCOPY=s390x-linux-gnu-objcopy << 99 << 100 This example will invoke ``s390x-linux-gnu-ld. << 101 ``s390x-linux-gnu-objcopy``, so ensure those a << 102 << 103 ``CROSS_COMPILE`` is not used to prefix the Cl << 104 corresponding LLVM utilities) as is the case f << 105 is not set. << 106 << 107 The LLVM_IAS= argument << 108 ---------------------- << 109 << 110 Clang can assemble assembler code. You can pas << 111 behavior and have Clang invoke the correspondi << 112 instead. Example:: << 113 << 114 make LLVM=1 LLVM_IAS=0 << 115 << 116 ``CROSS_COMPILE`` is necessary when cross comp << 117 is used in order to set ``--prefix=`` for the << 118 corresponding non-integrated assembler (typica << 119 system assembler when targeting another archit << 120 << 121 make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_ << 122 << 123 << 124 Ccache << 125 ------ << 126 << 127 ``ccache`` can be used with ``clang`` to impro << 128 KBUILD_BUILD_TIMESTAMP_ should be set to a det << 129 in order to avoid 100% cache misses, see Repro << 130 << 131 KBUILD_BUILD_TIMESTAMP='' make LLVM=1 << 132 << 133 .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild << 134 .. _Reproducible_builds: reproducible-builds.h << 135 << 136 Supported Architectures << 137 ----------------------- << 138 << 139 LLVM does not target all of the architectures << 140 just because a target is supported in LLVM doe << 141 will build or work without any issues. Below i << 142 architectures that currently work with ``CC=cl << 143 of support corresponds to "S" values in the MA << 144 architecture is not present, it either means t << 145 it or there are known issues. Using the latest << 146 even the development tree will generally yield << 147 An architecture's ``defconfig`` is generally e << 148 certain configurations may have problems that << 149 yet. Bug reports are always welcome at the iss << 150 << 151 .. list-table:: << 152 :widths: 10 10 10 << 153 :header-rows: 1 << 154 << 155 * - Architecture << 156 - Level of support << 157 - ``make`` command << 158 * - arm << 159 - Supported << 160 - ``LLVM=1`` << 161 * - arm64 << 162 - Supported << 163 - ``LLVM=1`` << 164 * - hexagon << 165 - Maintained << 166 - ``LLVM=1`` << 167 * - loongarch << 168 - Maintained << 169 - ``LLVM=1`` << 170 * - mips << 171 - Maintained << 172 - ``LLVM=1`` << 173 * - powerpc << 174 - Maintained << 175 - ``LLVM=1`` << 176 * - riscv << 177 - Supported << 178 - ``LLVM=1`` << 179 * - s390 << 180 - Maintained << 181 - ``LLVM=1`` (LLVM >= 18.1.0), ``CC=clang << 182 * - um (User Mode) << 183 - Maintained << 184 - ``LLVM=1`` << 185 * - x86 << 186 - Supported << 187 - ``LLVM=1`` << 188 65 189 Getting Help 66 Getting Help 190 ------------ 67 ------------ 191 68 192 - `Website <https://clangbuiltlinux.github.io/ 69 - `Website <https://clangbuiltlinux.github.io/>`_ 193 - `Mailing List <https://lore.kernel.org/llvm/> !! 70 - `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com">https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com> 194 - `Old Mailing List Archives <https://groups.g << 195 - `Issue Tracker <https://github.com/ClangBuil 71 - `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_ 196 - IRC: #clangbuiltlinux on irc.libera.chat !! 72 - IRC: #clangbuiltlinux on chat.freenode.net 197 - `Telegram <https://t.me/ClangBuiltLinux>`_: 73 - `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux 198 - `Wiki <https://github.com/ClangBuiltLinux/li 74 - `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_ 199 - `Beginner Bugs <https://github.com/ClangBuil 75 - `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_ 200 76 201 .. _getting_llvm: 77 .. _getting_llvm: 202 78 203 Getting LLVM 79 Getting LLVM 204 ------------- 80 ------------- 205 << 206 We provide prebuilt stable versions of LLVM on << 207 <https://kernel.org/pub/tools/llvm/>`_. These << 208 data for building Linux kernels, which should << 209 relative to other distributions of LLVM. << 210 << 211 Below are links that may be useful for buildin << 212 it through a distribution's package manager. << 213 81 214 - https://releases.llvm.org/download.html 82 - https://releases.llvm.org/download.html 215 - https://github.com/llvm/llvm-project 83 - https://github.com/llvm/llvm-project 216 - https://llvm.org/docs/GettingStarted.html 84 - https://llvm.org/docs/GettingStarted.html 217 - https://llvm.org/docs/CMake.html 85 - https://llvm.org/docs/CMake.html 218 - https://apt.llvm.org/ 86 - https://apt.llvm.org/ 219 - https://www.archlinux.org/packages/extra/x86 87 - https://www.archlinux.org/packages/extra/x86_64/llvm/ 220 - https://github.com/ClangBuiltLinux/tc-build 88 - https://github.com/ClangBuiltLinux/tc-build 221 - https://github.com/ClangBuiltLinux/linux/wik 89 - https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source 222 - https://android.googlesource.com/platform/pr 90 - https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/

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