============
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
=====================================
#. 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
+ is preferred 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.
-#. 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:
+#. If you want to run ``make check-all`` after building LLVM/Clang, to avoid
+ false alarms (e.g., ARCMT failure) please use at least the following
+ configuration:
.. code-block:: bash
$ ../$LLVM_SRC_DIR/configure --with-abi=aapcs-vfp
-#. The most popular linaro/ubuntu OS's for ARM boards, eg, the
+#. The most popular Linaro/Ubuntu OS's for ARM boards, e.g., the
Pandaboard, have become hard-float platforms. The following set
of configuration options appears to be a good choice for this
platform:
.. 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
+ ./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 --enable-optimized --enable-assertions
+
+#. 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
+
+ # 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
+
+#. 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.