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2 Cross-compilation using Clang
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5 Introduction
6 ============
8 This document will guide you in choosing the right Clang options
9 for cross-compiling your code to a different architecture. It assumes you
10 already know how to compile the code in question for the host architecture,
11 and that you know how to choose additional include and library paths.
13 However, this document is *not* a "how to" and won't help you setting your
14 build system or Makefiles, nor choosing the right CMake options, etc.
15 Also, it does not cover all the possible options, nor does it contain
16 specific examples for specific architectures. For a concrete example, the
17 `instructions for cross-compiling LLVM itself
18 <https://llvm.org/docs/HowToCrossCompileLLVM.html>`_ may be of interest.
20 After reading this document, you should be familiar with the main issues
21 related to cross-compilation, and what main compiler options Clang provides
22 for performing cross-compilation.
24 Cross compilation issues
25 ========================
27 In GCC world, every host/target combination has its own set of binaries,
28 headers, libraries, etc. So, it's usually simple to download a package
29 with all files in, unzip to a directory and point the build system to
30 that compiler, that will know about its location and find all it needs to
31 when compiling your code.
33 On the other hand, Clang/LLVM is natively a cross-compiler, meaning that
34 one set of programs can compile to all targets by setting the ``-target``
35 option. That makes it a lot easier for programmers wishing to compile to
36 different platforms and architectures, and for compiler developers that
37 only have to maintain one build system, and for OS distributions, that
38 need only one set of main packages.
40 But, as is true to any cross-compiler, and given the complexity of
41 different architectures, OS's and options, it's not always easy finding
42 the headers, libraries or binutils to generate target specific code.
43 So you'll need special options to help Clang understand what target
44 you're compiling to, where your tools are, etc.
46 Another problem is that compilers come with standard libraries only (like
47 ``compiler-rt``, ``libcxx``, ``libgcc``, ``libm``, etc), so you'll have to
48 find and make available to the build system, every other library required
49 to build your software, that is specific to your target. It's not enough to
50 have your host's libraries installed.
52 Finally, not all toolchains are the same, and consequently, not every Clang
53 option will work magically. Some options, like ``--sysroot`` (which
54 effectively changes the logical root for headers and libraries), assume
55 all your binaries and libraries are in the same directory, which may not
56 true when your cross-compiler was installed by the distribution's package
57 management. So, for each specific case, you may use more than one
58 option, and in most cases, you'll end up setting include paths (``-I``) and
59 library paths (``-L``) manually.
61 To sum up, different toolchains can:
62 * be host/target specific or more flexible
63 * be in a single directory, or spread out across your system
64 * have different sets of libraries and headers by default
65 * need special options, which your build system won't be able to figure
66 out by itself
68 General Cross-Compilation Options in Clang
69 ==========================================
71 Target Triple
72 -------------
74 The basic option is to define the target architecture. For that, use
75 ``-target <triple>``. If you don't specify the target, CPU names won't
76 match (since Clang assumes the host triple), and the compilation will
77 go ahead, creating code for the host platform, which will break later
78 on when assembling or linking.
80 The triple has the general format ``<arch><sub>-<vendor>-<sys>-<abi>``, where:
81 * ``arch`` = ``x86_64``, ``i386``, ``arm``, ``thumb``, ``mips``, etc.
82 * ``sub`` = for ex. on ARM: ``v5``, ``v6m``, ``v7a``, ``v7m``, etc.
83 * ``vendor`` = ``pc``, ``apple``, ``nvidia``, ``ibm``, etc.
84 * ``sys`` = ``none``, ``linux``, ``win32``, ``darwin``, ``cuda``, etc.
85 * ``abi`` = ``eabi``, ``gnu``, ``android``, ``macho``, ``elf``, etc.
87 The sub-architecture options are available for their own architectures,
88 of course, so "x86v7a" doesn't make sense. The vendor needs to be
89 specified only if there's a relevant change, for instance between PC
90 and Apple. Most of the time it can be omitted (and Unknown)
91 will be assumed, which sets the defaults for the specified architecture.
92 The system name is generally the OS (linux, darwin), but could be special
93 like the bare-metal "none".
95 When a parameter is not important, it can be omitted, or you can
96 choose ``unknown`` and the defaults will be used. If you choose a parameter
97 that Clang doesn't know, like ``blerg``, it'll ignore and assume
98 ``unknown``, which is not always desired, so be careful.
100 Finally, the ABI option is something that will pick default CPU/FPU,
101 define the specific behaviour of your code (PCS, extensions),
102 and also choose the correct library calls, etc.
104 CPU, FPU, ABI
105 -------------
107 Once your target is specified, it's time to pick the hardware you'll
108 be compiling to. For every architecture, a default set of CPU/FPU/ABI
109 will be chosen, so you'll almost always have to change it via flags.
111 Typical flags include:
112 * ``-mcpu=<cpu-name>``, like x86-64, swift, cortex-a15
113 * ``-mfpu=<fpu-name>``, like SSE3, NEON, controlling the FP unit available
114 * ``-mfloat-abi=<fabi>``, like soft, hard, controlling which registers
115 to use for floating-point
117 The default is normally the common denominator, so that Clang doesn't
118 generate code that breaks. But that also means you won't get the best
119 code for your specific hardware, which may mean orders of magnitude
120 slower than you expect.
122 For example, if your target is ``arm-none-eabi``, the default CPU will
123 be ``arm7tdmi`` using soft float, which is extremely slow on modern cores,
124 whereas if your triple is ``armv7a-none-eabi``, it'll be Cortex-A8 with
125 NEON, but still using soft-float, which is much better, but still not
126 great.
128 Toolchain Options
129 -----------------
131 There are three main options to control access to your cross-compiler:
132 ``--sysroot``, ``-I``, and ``-L``. The two last ones are well known,
133 but they're particularly important for additional libraries
134 and headers that are specific to your target.
136 There are two main ways to have a cross-compiler:
138 #. When you have extracted your cross-compiler from a zip file into
139 a directory, you have to use ``--sysroot=<path>``. The path is the
140 root directory where you have unpacked your file, and Clang will
141 look for the directories ``bin``, ``lib``, ``include`` in there.
143 In this case, your setup should be pretty much done (if no
144 additional headers or libraries are needed), as Clang will find
145 all binaries it needs (assembler, linker, etc) in there.
147 #. When you have installed via a package manager (modern Linux
148 distributions have cross-compiler packages available), make
149 sure the target triple you set is *also* the prefix of your
150 cross-compiler toolchain.
152 In this case, Clang will find the other binaries (assembler,
153 linker), but not always where the target headers and libraries
154 are. People add system-specific clues to Clang often, but as
155 things change, it's more likely that it won't find than the
156 other way around.
158 So, here, you'll be a lot safer if you specify the include/library
159 directories manually (via ``-I`` and ``-L``).
161 Target-Specific Libraries
162 =========================
164 All libraries that you compile as part of your build will be
165 cross-compiled to your target, and your build system will probably
166 find them in the right place. But all dependencies that are
167 normally checked against (like ``libxml`` or ``libz`` etc) will match
168 against the host platform, not the target.
170 So, if the build system is not aware that you want to cross-compile
171 your code, it will get every dependency wrong, and your compilation
172 will fail during build time, not configure time.
174 Also, finding the libraries for your target are not as easy
175 as for your host machine. There aren't many cross-libraries available
176 as packages to most OS's, so you'll have to either cross-compile them
177 from source, or download the package for your target platform,
178 extract the libraries and headers, put them in specific directories
179 and add ``-I`` and ``-L`` pointing to them.
181 Also, some libraries have different dependencies on different targets,
182 so configuration tools to find dependencies in the host can get the
183 list wrong for the target platform. This means that the configuration
184 of your build can get things wrong when setting their own library
185 paths, and you'll have to augment it via additional flags (configure,
186 Make, CMake, etc).
188 Multilibs
189 ---------
191 When you want to cross-compile to more than one configuration, for
192 example hard-float-ARM and soft-float-ARM, you'll have to have multiple
193 copies of your libraries and (possibly) headers.
195 Some Linux distributions have support for Multilib, which handle that
196 for you in an easier way, but if you're not careful and, for instance,
197 forget to specify ``-ccc-gcc-name armv7l-linux-gnueabihf-gcc`` (which
198 uses hard-float), Clang will pick the ``armv7l-linux-gnueabi-ld``
199 (which uses soft-float) and linker errors will happen.
201 The same is true if you're compiling for different ABIs, like ``gnueabi``
202 and ``androideabi``, and might even link and run, but produce run-time
203 errors, which are much harder to track down and fix.