1===================================================================
2How To Build On ARM
3===================================================================
4
5Introduction
6============
7
8This document contains information about building/testing LLVM and
9Clang on an ARM machine.
10
11This document is *NOT* tailored to help you cross-compile LLVM/Clang
12to ARM on another architecture, for example an x86_64 machine. To find
13out more about cross-compiling, please check :doc:`HowToCrossCompileLLVM`.
14
15Notes On Building LLVM/Clang on ARM
16=====================================
17Here are some notes on building/testing LLVM/Clang on ARM. Note that
18ARM encompasses a wide variety of CPUs; this advice is primarily based
19on the ARMv6 and ARMv7 architectures and may be inapplicable to older chips.
20
21#. The most popular Linaro/Ubuntu OS's for ARM boards, e.g., the
22   Pandaboard, have become hard-float platforms. There are a number of
23   choices when using CMake. Autoconf usage is deprecated as of 3.8.
24
25   Building LLVM/Clang in ``Relese`` mode is preferred since it consumes
26   a lot less memory. Otherwise, the building process will very likely
27   fail due to insufficient memory. It's also a lot quicker to only build
28   the relevant back-ends (ARM and AArch64), since it's very unlikely that
29   you'll use an ARM board to cross-compile to other arches. If you're
30   running Compiler-RT tests, also include the x86 back-end, or some tests
31   will fail.
32
33   .. code-block:: bash
34
35     cmake $LLVM_SRC_DIR -DCMAKE_BUILD_TYPE=Release \
36                         -DLLVM_TARGETS_TO_BUILD="ARM;X86;AArch64"
37
38   Other options you can use are:
39
40   .. code-block:: bash
41
42     Use Ninja instead of Make: "-G Ninja"
43     Build with assertions on: "-DLLVM_ENABLE_ASSERTIONS=True"
44     Force Python2: "-DPYTHON_EXECUTABLE=/usr/bin/python2"
45     Local (non-sudo) install path: "-DCMAKE_INSTALL_PREFIX=$HOME/llvm/instal"
46     CPU flags: "DCMAKE_C_FLAGS=-mcpu=cortex-a15" (same for CXX_FLAGS)
47
48   After that, just typing ``make -jN`` or ``ninja`` will build everything.
49   ``make -jN check-all`` or ``ninja check-all`` will run all compiler tests. For
50   running the test suite, please refer to :doc:`TestingGuide`.
51
52#. If you are building LLVM/Clang on an ARM board with 1G of memory or less,
53   please use ``gold`` rather then GNU ``ld``. In any case it is probably a good
54   idea to set up a swap partition, too.
55
56   .. code-block:: bash
57
58     $ sudo ln -sf /usr/bin/ld /usr/bin/ld.gold
59
60#. ARM development boards can be unstable and you may experience that cores
61   are disappearing, caches being flushed on every big.LITTLE switch, and
62   other similar issues.  To help ease the effect of this, set the Linux
63   scheduler to "performance" on **all** cores using this little script:
64
65   .. code-block:: bash
66
67      # The code below requires the package 'cpufrequtils' to be installed.
68      for ((cpu=0; cpu<`grep -c proc /proc/cpuinfo`; cpu++)); do
69          sudo cpufreq-set -c $cpu -g performance
70      done
71
72   Remember to turn that off after the build, or you may risk burning your
73   CPU. Most modern kernels don't need that, so only use it if you have
74   problems.
75
76#. Running the build on SD cards is ok, but they are more prone to failures
77   than good quality USB sticks, and those are more prone to failures than
78   external hard-drives (those are also a lot faster). So, at least, you
79   should consider to buy a fast USB stick.  On systems with a fast eMMC,
80   that's a good option too.
81
82#. Make sure you have a decent power supply (dozens of dollars worth) that can
83   provide *at least* 4 amperes, this is especially important if you use USB
84   devices with your board. Externally powered USB/SATA harddrives are even
85   better than having a good power supply.
86