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