-.. _how_to_build_on_arm:
-
===================================================================
How To Build On ARM
===================================================================
============
This document contains information about building/testing LLVM and
-Clang on ARM.
+Clang on an ARM machine.
+
+This document is *NOT* tailored to help you cross-compile LLVM/Clang
+to ARM on another architecture, for example an x86_64 machine. To find
+out more about cross-compiling, please check :doc:`HowToCrossCompileLLVM`.
Notes On Building LLVM/Clang on ARM
=====================================
ARM encompasses a wide variety of CPUs; this advice is primarily based
on the ARMv6 and ARMv7 architectures and may be inapplicable to older chips.
-#. If you are building LLVM/Clang on an ARM board with 1G of memory or less,
- please use ``gold`` rather then GNU ``ld``.
- Building LLVM/Clang with ``--enable-optimized``
- is prefered since it consumes less memory. Otherwise, the building
- process will very likely fail due to insufficient memory. In any
- case it is probably a good idea to set up a swap partition.
+#. The most popular Linaro/Ubuntu OS's for ARM boards, e.g., the
+ Pandaboard, have become hard-float platforms. There are a number of
+ choices when using CMake. Autoconf usage is deprecated as of 3.8.
+
+ Building LLVM/Clang in ``Relese`` mode is preferred since it consumes
+ a lot less memory. Otherwise, the building process will very likely
+ fail due to insufficient memory. It's also a lot quicker to only build
+ the relevant back-ends (ARM and AArch64), since it's very unlikely that
+ you'll use an ARM board to cross-compile to other arches. If you're
+ running Compiler-RT tests, also include the x86 back-end, or some tests
+ will fail.
+
+ .. code-block:: bash
+
+ cmake $LLVM_SRC_DIR -DCMAKE_BUILD_TYPE=Release \
+ -DLLVM_TARGETS_TO_BUILD="ARM;X86;AArch64"
+
+ Other options you can use are:
-#. If you want to run ``make
- check-all`` after building LLVM/Clang, to avoid false alarms (eg, ARCMT
- failure) please use at least the following configuration:
+ .. code-block:: bash
+
+ Use Ninja instead of Make: "-G Ninja"
+ Build with assertions on: "-DLLVM_ENABLE_ASSERTIONS=True"
+ Force Python2: "-DPYTHON_EXECUTABLE=/usr/bin/python2"
+ Local (non-sudo) install path: "-DCMAKE_INSTALL_PREFIX=$HOME/llvm/instal"
+ CPU flags: "DCMAKE_C_FLAGS=-mcpu=cortex-a15" (same for CXX_FLAGS)
+
+ After that, just typing ``make -jN`` or ``ninja`` will build everything.
+ ``make -jN check-all`` or ``ninja check-all`` will run all compiler tests. For
+ running the test suite, please refer to :doc:`TestingGuide`.
+
+#. If you are building LLVM/Clang on an ARM board with 1G of memory or less,
+ please use ``gold`` rather then GNU ``ld``. In any case it is probably a good
+ idea to set up a swap partition, too.
.. code-block:: bash
- $ ../$LLVM_SRC_DIR/configure --with-abi=aapcs-vfp
+ $ sudo ln -sf /usr/bin/ld /usr/bin/ld.gold
-#. The most popular linaro/ubuntu OS's for ARM boards, eg, the
- Pandaboard, have become hard-float platforms. The following set
- of configuration options appears to be a good choice for this
- platform:
+#. ARM development boards can be unstable and you may experience that cores
+ are disappearing, caches being flushed on every big.LITTLE switch, and
+ other similar issues. To help ease the effect of this, set the Linux
+ scheduler to "performance" on **all** cores using this little script:
.. code-block:: bash
- ./configure --build=armv7l-unknown-linux-gnueabihf
- --host=armv7l-unknown-linux-gnueabihf
- --target=armv7l-unknown-linux-gnueabihf --with-cpu=cortex-a9
- --with-float=hard --with-abi=aapcs-vfp --with-fpu=neon
- --enable-targets=arm --disable-optimized --enable-assertions
+ # The code below requires the package 'cpufrequtils' to be installed.
+ for ((cpu=0; cpu<`grep -c proc /proc/cpuinfo`; cpu++)); do
+ sudo cpufreq-set -c $cpu -g performance
+ done
+
+ Remember to turn that off after the build, or you may risk burning your
+ CPU. Most modern kernels don't need that, so only use it if you have
+ problems.
+
+#. Running the build on SD cards is ok, but they are more prone to failures
+ than good quality USB sticks, and those are more prone to failures than
+ external hard-drives (those are also a lot faster). So, at least, you
+ should consider to buy a fast USB stick. On systems with a fast eMMC,
+ that's a good option too.
+
+#. Make sure you have a decent power supply (dozens of dollars worth) that can
+ provide *at least* 4 amperes, this is especially important if you use USB
+ devices with your board. Externally powered USB/SATA harddrives are even
+ better than having a good power supply.
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