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 <<
54 make LLVM=/path/to/llvm/ <<
55 <<
56 which will use ``/path/to/llvm/clang``, ``/pat <<
57 following may also be used:: <<
58 <<
59 PATH=/path/to/llvm:$PATH make LLVM=1 <<
60 <<
61 If your LLVM tools have a version suffix and y <<
62 explicit version rather than the unsuffixed ex <<
63 can pass the suffix using the ``LLVM`` variabl <<
64 <<
65 make LLVM=-14 <<
66 <<
67 which will use ``clang-14``, ``ld.lld-14``, et <<
68 <<
69 To support combinations of out of tree paths w <<
70 recommend:: <<
71 <<
72 PATH=/path/to/llvm/:$PATH make LLVM=-1 <<
73 <<
74 ``LLVM=0`` is not the same as omitting ``LLVM` <<
75 ``LLVM=1``. If you only wish to use certain LL <<
76 respective make variables. <<
77 <<
78 The same value used for ``LLVM=`` should be se <<
79 if configuring and building via distinct comma <<
80 as an environment variable when running script <<
81 ``make``. <<
82 34
83 Cross Compiling 35 Cross Compiling
84 --------------- 36 ---------------
85 37
86 A single Clang compiler binary (and correspond !! 38 A single Clang compiler binary will typically contain all supported backends,
87 typically contain all supported back ends, whi !! 39 which can help simplify cross compiling. ::
88 compiling especially when ``LLVM=1`` is used. <<
89 ``CROSS_COMPILE`` or target-triple-prefixes be <<
90 <<
91 make LLVM=1 ARCH=arm64 <<
92 40
93 As an example of mixing LLVM and GNU utilities !! 41 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
94 which does not yet have ``ld.lld`` or ``llvm-o <<
95 invoke ``make`` via:: <<
96 42
97 make LLVM=1 ARCH=s390 LD=s390x-linux-g !! 43 ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
98 OBJCOPY=s390x-linux-gnu-objcopy !! 44 ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
>> 45 example: ::
99 46
100 This example will invoke ``s390x-linux-gnu-ld. !! 47 clang --target=aarch64-linux-gnu foo.c
101 ``s390x-linux-gnu-objcopy``, so ensure those a <<
102 48
103 ``CROSS_COMPILE`` is not used to prefix the Cl !! 49 LLVM Utilities
104 corresponding LLVM utilities) as is the case f !! 50 --------------
105 is not set. <<
106 51
107 The LLVM_IAS= argument !! 52 LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
108 ---------------------- !! 53 to enable them. ::
109 54
110 Clang can assemble assembler code. You can pas !! 55 make LLVM=1
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 56
127 ``ccache`` can be used with ``clang`` to impro !! 57 They can be enabled individually. The full list of the parameters: ::
128 KBUILD_BUILD_TIMESTAMP_ should be set to a det <<
129 in order to avoid 100% cache misses, see Repro <<
130 58
131 KBUILD_BUILD_TIMESTAMP='' make LLVM=1 !! 59 make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
>> 60 OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
>> 61 HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
132 62
133 .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild !! 63 Currently, the integrated assembler is disabled by default. You can pass
134 .. _Reproducible_builds: reproducible-builds.h !! 64 ``LLVM_IAS=1`` to enable it.
135 65
136 Supported Architectures 66 Supported Architectures
137 ----------------------- 67 -----------------------
138 68
139 LLVM does not target all of the architectures 69 LLVM does not target all of the architectures that Linux supports and
140 just because a target is supported in LLVM doe 70 just because a target is supported in LLVM does not mean that the kernel
141 will build or work without any issues. Below i 71 will build or work without any issues. Below is a general summary of
142 architectures that currently work with ``CC=cl 72 architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
143 of support corresponds to "S" values in the MA 73 of support corresponds to "S" values in the MAINTAINERS files. If an
144 architecture is not present, it either means t 74 architecture is not present, it either means that LLVM does not target
145 it or there are known issues. Using the latest 75 it or there are known issues. Using the latest stable version of LLVM or
146 even the development tree will generally yield 76 even the development tree will generally yield the best results.
147 An architecture's ``defconfig`` is generally e 77 An architecture's ``defconfig`` is generally expected to work well,
148 certain configurations may have problems that 78 certain configurations may have problems that have not been uncovered
149 yet. Bug reports are always welcome at the iss 79 yet. Bug reports are always welcome at the issue tracker below!
150 80
151 .. list-table:: 81 .. list-table::
152 :widths: 10 10 10 82 :widths: 10 10 10
153 :header-rows: 1 83 :header-rows: 1
154 84
155 * - Architecture 85 * - Architecture
156 - Level of support 86 - Level of support
157 - ``make`` command 87 - ``make`` command
158 * - arm 88 * - arm
159 - Supported 89 - Supported
160 - ``LLVM=1`` 90 - ``LLVM=1``
161 * - arm64 91 * - arm64
162 - Supported 92 - Supported
163 - ``LLVM=1`` 93 - ``LLVM=1``
164 * - hexagon <<
165 - Maintained <<
166 - ``LLVM=1`` <<
167 * - loongarch <<
168 - Maintained <<
169 - ``LLVM=1`` <<
170 * - mips 94 * - mips
171 - Maintained 95 - Maintained
172 - ``LLVM=1`` !! 96 - ``CC=clang``
173 * - powerpc 97 * - powerpc
174 - Maintained 98 - Maintained
175 - ``LLVM=1`` !! 99 - ``CC=clang``
176 * - riscv 100 * - riscv
177 - Supported <<
178 - ``LLVM=1`` <<
179 * - s390 <<
180 - Maintained 101 - Maintained
181 - ``LLVM=1`` (LLVM >= 18.1.0), ``CC=clang !! 102 - ``CC=clang``
182 * - um (User Mode) !! 103 * - s390
183 - Maintained 104 - Maintained
184 - ``LLVM=1`` !! 105 - ``CC=clang``
185 * - x86 106 * - x86
186 - Supported 107 - Supported
187 - ``LLVM=1`` 108 - ``LLVM=1``
188 109
189 Getting Help 110 Getting Help
190 ------------ 111 ------------
191 112
192 - `Website <https://clangbuiltlinux.github.io/ 113 - `Website <https://clangbuiltlinux.github.io/>`_
193 - `Mailing List <https://lore.kernel.org/llvm/> !! 114 - `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 115 - `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
196 - IRC: #clangbuiltlinux on irc.libera.chat !! 116 - IRC: #clangbuiltlinux on chat.freenode.net
197 - `Telegram <https://t.me/ClangBuiltLinux>`_: 117 - `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
198 - `Wiki <https://github.com/ClangBuiltLinux/li 118 - `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
199 - `Beginner Bugs <https://github.com/ClangBuil 119 - `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
200 120
201 .. _getting_llvm: 121 .. _getting_llvm:
202 122
203 Getting LLVM 123 Getting LLVM
204 ------------- 124 -------------
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. <<
213 125
214 - https://releases.llvm.org/download.html 126 - https://releases.llvm.org/download.html
215 - https://github.com/llvm/llvm-project 127 - https://github.com/llvm/llvm-project
216 - https://llvm.org/docs/GettingStarted.html 128 - https://llvm.org/docs/GettingStarted.html
217 - https://llvm.org/docs/CMake.html 129 - https://llvm.org/docs/CMake.html
218 - https://apt.llvm.org/ 130 - https://apt.llvm.org/
219 - https://www.archlinux.org/packages/extra/x86 131 - https://www.archlinux.org/packages/extra/x86_64/llvm/
220 - https://github.com/ClangBuiltLinux/tc-build 132 - https://github.com/ClangBuiltLinux/tc-build
221 - https://github.com/ClangBuiltLinux/linux/wik 133 - https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
222 - https://android.googlesource.com/platform/pr 134 - https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/
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