How To Build On ARM

Introduction

This document contains information about building/testing LLVM and 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 How To Cross-Compile Clang/LLVM using Clang/LLVM.

Notes On Building LLVM/Clang on ARM

Here are some notes on building/testing LLVM/Clang on ARM. Note that 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.

1. 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 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.

2. 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:

   $ ../$LLVM_SRC_DIR/configure --with-abi=aapcs-vfp
   

3. 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:

   ../$LLVM_SRC_DIR/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
   

4. 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:

   # 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
   

5. 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.

6. 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.