1 .. _kbuild_llvm:
2
3 ==============================
4 Building Linux with Clang/LLVM
5 ==============================
6
7 This document covers how to build the Linux ke
8 utilities.
9
10 About
11 -----
12
13 The Linux kernel has always traditionally been
14 such as GCC and binutils. Ongoing work has all
15 <https://clang.llvm.org/>`_ and `LLVM <https:/
16 used as viable substitutes. Distributions such
17 <https://www.android.com/>`_, `ChromeOS
18 <https://www.chromium.org/chromium-os>`_, `Ope
19 <https://www.openmandriva.org/>`_, and `Chimer
20 <https://chimera-linux.org/>`_ use Clang built
21 datacenter fleets also run kernels built with
22
23 `LLVM is a collection of toolchain components
24 objects <https://www.aosabook.org/en/llvm.html
25 that supports C and the GNU C extensions requi
26 pronounced "klang," not "see-lang."
27
28 Building with LLVM
29 ------------------
30
31 Invoke ``make`` via::
32
33 make LLVM=1
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
83 Cross Compiling
84 ---------------
85
86 A single Clang compiler binary (and correspond
87 typically contain all supported back ends, whi
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
189 Getting Help
190 ------------
191
192 - `Website <https://clangbuiltlinux.github.io/
193 - `Mailing List <https://lore.kernel.org/llvm/>
194 - `Old Mailing List Archives <https://groups.g
195 - `Issue Tracker <https://github.com/ClangBuil
196 - IRC: #clangbuiltlinux on irc.libera.chat
197 - `Telegram <https://t.me/ClangBuiltLinux>`_:
198 - `Wiki <https://github.com/ClangBuiltLinux/li
199 - `Beginner Bugs <https://github.com/ClangBuil
200
201 .. _getting_llvm:
202
203 Getting LLVM
204 -------------
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
214 - https://releases.llvm.org/download.html
215 - https://github.com/llvm/llvm-project
216 - https://llvm.org/docs/GettingStarted.html
217 - https://llvm.org/docs/CMake.html
218 - https://apt.llvm.org/
219 - https://www.archlinux.org/packages/extra/x86
220 - https://github.com/ClangBuiltLinux/tc-build
221 - https://github.com/ClangBuiltLinux/linux/wik
222 - https://android.googlesource.com/platform/pr
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