Introduction

These are the step by step instructions on Cross-Compiling Arm NN under an x86_64 system to target an Arm64 Ubuntu Linux system. This build flow has been tested with Ubuntu 18.04 and 20.04 and it depends on the same version of Ubuntu or Debian being installed on both the build host and target machines. The instructions assume you are using a bash shell and show how to build the Arm NN core library, Protobuf, Tflite, Flatbuffer and Compute Libraries. Start by creating a directory to contain all components:

mkdir $HOME/armnn-devenv

cd $HOME/armnn-devenv

Cross-compiling ToolChain

Install the standard cross-compilation libraries for arm64:
sudo apt install crossbuild-essential-arm64

Install Cmake

Cmake 3.19rc3 is required to build TF Lite Delegate.

sudo apt-get install libssl-dev
wget https://github.com/Kitware/CMake/releases/download/v3.19.0-rc3/cmake-3.19.0-rc3.tar.gz
tar -zxvf cmake-3.19.0-rc3.tar.gz
cd cmake-3.19.0-rc3
./bootstrap --prefix=$HOME/armnn-devenv/cmake/install
make all install
cd..

Build and install Google's Protobuf library

We support protobuf version 3.12.0

Get protobuf from here: https://github.com/protocolbuffers/protobuf: (Requires Git if not previously installed: sudo apt install git)
git clone -b v3.12.0 https://github.com/google/protobuf.git protobuf
cd protobuf
git submodule update --init --recursive
./autogen.sh
Build a native (x86_64) version of the protobuf libraries and compiler (protoc): (Requires cUrl, autoconf, llibtool, and other build dependencies if not previously installed: sudo apt install curl autoconf libtool build-essential g++)
mkdir x86_64_build
cd x86_64_build
../configure --prefix=$HOME/armnn-devenv/google/x86_64_pb_install
make install -j16
cd ..
Build the arm64 version of the protobuf libraries:
mkdir arm64_build
cd arm64_build
CC=aarch64-linux-gnu-gcc \
CXX=aarch64-linux-gnu-g++ \
../configure --host=aarch64-linux \
--prefix=$HOME/armnn-devenv/google/arm64_pb_install \
--with-protoc=$HOME/armnn-devenv/google/x86_64_pb_install/bin/protoc
make install -j16
cd ..

Download Arm NN

Clone Arm NN:
cd $HOME/armnn-devenv
git clone https://github.com/ARM-software/armnn.git
Checkout Arm NN branch:
cd armnn
git checkout <branch_name>
git pull
For example, if you want to check out the 21.11 release branch:
git checkout branches/armnn_21_11
git pull

Build Arm Compute Library

Clone Arm Compute Library:

cd $HOME/armnn-devenv

git clone https://github.com/ARM-software/ComputeLibrary.git

Checkout Arm Compute Library release tag:
cd ComputeLibrary
git checkout <tag_name>
Arm NN and Arm Compute Library are developed closely together. If you would like to use the Arm NN 21.11 release you will need the 21.11 release of ACL too. For example, if you want to checkout the 21.11 release tag:
git checkout v21.11
Arm NN provides a script that downloads the version of Arm Compute Library that Arm NN was tested with:
git checkout $(../armnn/scripts/get_compute_library.sh -p)
Build the Arm Compute Library: (Requires SCons if not previously installed: sudo apt install scons)
scons arch=arm64-v8a neon=1 opencl=1 embed_kernels=1 extra_cxx_flags="-fPIC" -j4

Build Flatbuffer

Building Flatbuffer version 1.12.0
cd $HOME/armnn-devenv
wget -O flatbuffers-1.12.0.tar.gz https://github.com/google/flatbuffers/archive/v1.12.0.tar.gz
tar xf flatbuffers-1.12.0.tar.gz
cd flatbuffers-1.12.0
rm -f CMakeCache.txt
mkdir build
cd build
cmake .. -DFLATBUFFERS_BUILD_FLATC=1 \
-DCMAKE_INSTALL_PREFIX:PATH=$HOME/armnn-devenv/flatbuffers \
-DFLATBUFFERS_BUILD_TESTS=0
make all install
Build arm64 version of flatbuffer
cd ..
mkdir build-arm64
cd build-arm64
# Add -fPIC to allow us to use the libraries in shared objects.
CXXFLAGS="-fPIC" cmake .. -DCMAKE_C_COMPILER=/usr/bin/aarch64-linux-gnu-gcc \
-DCMAKE_CXX_COMPILER=/usr/bin/aarch64-linux-gnu-g++ \
-DFLATBUFFERS_BUILD_FLATC=1 \
-DCMAKE_INSTALL_PREFIX:PATH=$HOME/armnn-devenv/flatbuffers-arm64 \
-DFLATBUFFERS_BUILD_TESTS=0
make all install

Build Onnx

Building Onnx
cd $HOME/armnn-devenv
git clone https://github.com/onnx/onnx.git
cd onnx
git fetch https://github.com/onnx/onnx.git 553df22c67bee5f0fe6599cff60f1afc6748c635 && git checkout FETCH_HEAD
LD_LIBRARY_PATH=$HOME/armnn-devenv/google/x86_64_pb_install/lib:$LD_LIBRARY_PATH \
$HOME/armnn-devenv/google/x86_64_pb_install/bin/protoc \
onnx/onnx.proto --proto_path=. --proto_path=../google/x86_64_pb_install/include --cpp_out $HOME/armnn-devenv/onnx

Build TfLite

Arm NN provides a script, armnn/scripts/get_tensorflow.sh, that can be used to check out the version of TensorFlow that Arm NN was tested with:
cd $HOME/armnn-devenv
git clone https://github.com/tensorflow/tensorflow.git
cd tensorflow/
git checkout $(../armnn/scripts/get_tensorflow.sh -p) # Checks out the latest tested version of TF
cd ..
You will need to download gcc-arm-8.3-2019.03 toolchain and continue building TF Lite as following:
curl -LO https://storage.googleapis.com/mirror.tensorflow.org/developer.arm.com/media/Files/downloads/gnu-a/8.3-2019.03/binrel/gcc-arm-8.3-2019.03-x86_64-aarch64-linux-gnu.tar.xz
mkdir tflite-toolchains
tar xvf gcc-arm-8.3-2019.03-x86_64-aarch64-linux-gnu.tar.xz -C tflite-toolchains
mkdir -p tflite/build
cd tflite/build
ARMCC_PREFIX=$HOME/armnn-devenv/tflite-toolchains/gcc-arm-8.3-2019.03-x86_64-aarch64-linux-gnu/bin/aarch64-linux-gnu- \
ARMCC_FLAGS="-funsafe-math-optimizations" \
cmake -DCMAKE_C_COMPILER=${ARMCC_PREFIX}gcc \
-DCMAKE_CXX_COMPILER=${ARMCC_PREFIX}g++ \
-DCMAKE_C_FLAGS="${ARMCC_FLAGS}" -DCMAKE_CXX_FLAGS="${ARMCC_FLAGS}" \
-DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DCMAKE_SYSTEM_NAME=Linux \
-DTFLITE_ENABLE_XNNPACK=OFF \
-DCMAKE_SYSTEM_PROCESSOR=aarch64 \
$HOME/armnn-devenv/tensorflow/tensorflow/lite/ \
cmake --build .

## Generate TF Lite Schema

cd $HOME/armnn-devenv/tflite
cp $HOME/armnn-devenv/tensorflow/tensorflow/lite/schema/schema.fbs .
../flatbuffers-1.12.0/build/flatc -c --gen-object-api --reflect-types --reflect-names schema.fbs

Build Arm NN

Compile Arm NN for arm64:
cd $HOME/armnn-devenv/armnn
mkdir build
cd build
Use CMake to configure your build environment, update the following script and run it from the armnn/build directory to set up the Arm NN build:
#!/bin/bash
CXX=aarch64-linux-gnu-g++ CC=aarch64-linux-gnu-gcc cmake .. \
-DARMCOMPUTE_ROOT=$HOME/armnn-devenv/ComputeLibrary \
-DARMCOMPUTE_BUILD_DIR=$HOME/armnn-devenv/ComputeLibrary/build/ \
-DARMCOMPUTENEON=1 -DARMCOMPUTECL=1 -DARMNNREF=1 \
-DONNX_GENERATED_SOURCES=$HOME/armnn-devenv/onnx \
-DBUILD_ONNX_PARSER=1 \
-DBUILD_TF_LITE_PARSER=1 \
-DTENSORFLOW_ROOT=$HOME/armnn-devenv/tensorflow \
-DTF_LITE_SCHEMA_INCLUDE_PATH=$WORKING_DIR/tflite \
-DFLATBUFFERS_ROOT=$HOME/armnn-devenv/flatbuffers-arm64 \
-DFLATC_DIR=$HOME/armnn-devenv/flatbuffers-1.12.0/build \
-DPROTOBUF_ROOT=$HOME/armnn-devenv/google/x86_64_pb_install \
-DPROTOBUF_LIBRARY_DEBUG=$HOME/armnn-devenv/google/arm64_pb_install/lib/libprotobuf.so.23.0.0 \
-DPROTOBUF_LIBRARY_RELEASE=$HOME/armnn-devenv/google/arm64_pb_install/lib/libprotobuf.so.23.0.0
If you want to include standalone sample dynamic backend tests, add the argument to enable the tests and the dynamic backend path to the CMake command:
-DSAMPLE_DYNAMIC_BACKEND=1 \
-DDYNAMIC_BACKEND_PATHS=$SAMPLE_DYNAMIC_BACKEND_PATH
If you want to build Arm NN TF Lite Delegate, add the arguments:
-DTFLITE_LIB_ROOT=$HOME/armnn-devenv/tflite/build \
-DBUILD_ARMNN_TFLITE_DELEGATE=1
Run the build
make -j32

Build Standalone Sample Dynamic Backend

The sample dynamic backend is located in armnn/src/dynamic/sample
cd $HOME/armnn-devenv/armnn/src/dynamic/sample
mkdir build
cd build
Use CMake to configure your build environment, update the following script and run it from the armnn/src/dynamic/sample/build directory to set up the Arm NN build:
#!/bin/bash
CXX=aarch64-linux-gnu-g++ CC=aarch64-linux-gnu-gcc cmake .. \
-DCMAKE_CXX_FLAGS=--std=c++14 \
-DARMNN_PATH=$HOME/armnn-devenv/armnn/build/libarmnn.so
Run the build
make

Run Unit Tests

Copy the build folder to an arm64 linux machine
Copy the libprotobuf.so.23.0.0 library file to the build folder
If you enable the standalone sample dynamic tests, also copy libArm_SampleDynamic_backend.so library file to the folder specified as $SAMPLE_DYNAMIC_BACKEND_PATH when you build Arm NN
cd to the build folder on your arm64 machine and set your LD_LIBRARY_PATH to its current location:

cd build/

Create a symbolic link to libprotobuf.so.23.0.0:

ln -s libprotobuf.so.23.0.0 ./libprotobuf.so.23

Run the UnitTests:

LD_LIBRARY_PATH=./:$LD_LIBRARY_PATH ./UnitTests
[doctest] doctest version is "2.4.6"
[doctest] run with "--help" for options
===============================================================================
[doctest] test cases:   4817 |   4817 passed | 0 failed | 0 skipped
[doctest] assertions: 807634 | 807634 passed | 0 failed |
[doctest] Status: SUCCESS!

Run the Delegate UnitTests:

LD_LIBRARY_PATH=./:$LD_LIBRARY_PATH ./delegate/DelegateUnitTests

Troubleshooting and Errors:

Missing xxd

When Cross-compiling Arm NN if you are getting error
[ 62%] Generating SchemaText.cpp
/bin/sh: 1: xxd: not found
make[2]: *** [armnn/CMakeFiles/UnitTests.dir/build.make:63: armnn/SchemaText.cpp] Error 127
make[1]: *** [CMakeFiles/Makefile2:674: armnn/CMakeFiles/UnitTests.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
Missing xxd, this can be fixed by installing it:
sudo apt-get install xxd

Missing libz.so.1

When compiling armNN:
/usr/lib/gcc-cross/aarch64-linux-gnu/5/../../../../aarch64-linux-gnu/bin/ld: warning: libz.so.1, needed by /home/<username>/armNN/usr/lib64/libprotobuf.so.23.0.0, not found (try using -rpath or -rpath-link)
Missing arm64 libraries for libz.so.1, these can be added by adding a second architecture to dpkg and explicitly installing them:
sudo dpkg --add-architecture arm64
sudo apt-get install zlib1g:arm64
sudo apt-get update
sudo ldconfig
If apt-get update returns 404 errors for arm64 repos refer to section 5 below.
Alternatively the missing arm64 version of libz.so.1 can be downloaded and installed from a .deb package here: https://launchpad.net/ubuntu/wily/arm64/zlib1g/1:1.2.8.dfsg-2ubuntu4
sudo dpkg -i zlib1g_1.2.8.dfsg-2ubuntu4_arm64.deb

Unable to install arm64 packages after adding arm64 architecture

Using sudo apt-get update should add all of the required repos for arm64 but if it does not or you are getting 404 errors the following instructions can be used to add the repos manually:
From stackoverflow: https://askubuntu.com/questions/430705/how-to-use-apt-get-to-download-multi-arch-library/430718
Open /etc/apt/sources.list with your preferred text editor.
Mark all the current (default) repos as [arch=<current_os_arch>], e.g.
deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ xenial main restricted
Then add the following:
deb [arch=arm64] http://ports.ubuntu.com/ xenial main restricted
deb [arch=arm64] http://ports.ubuntu.com/ xenial-updates main restricted
deb [arch=arm64] http://ports.ubuntu.com/ xenial universe
deb [arch=arm64] http://ports.ubuntu.com/ xenial-updates universe
deb [arch=arm64] http://ports.ubuntu.com/ xenial multiverse
deb [arch=arm64] http://ports.ubuntu.com/ xenial-updates multiverse
deb [arch=arm64] http://ports.ubuntu.com/ xenial-backports main restricted universe multiverse
Update and install again:
sudo apt-get install zlib1g:arm64
sudo apt-get update
sudo ldconfig

Undefined references to google::protobuf:: functions

Missing or out of date protobuf compilation libraries. Use the command 'protoc –version' to check which version of protobuf is available (version 3.12.0 is required). Follow the instructions above to install protobuf 3.12.0

Errors on strict-aliasing rules when compiling the Compute Library

When compiling the Compute Library there are multiple errors on strict-aliasing rules:
cc1plus: error: unrecognized command line option ‘-Wno-implicit-fallthrough’ [-Werror]
Add Werror=0 to the scons command:
scons arch=arm64-v8a neon=1 opencl=1 embed_kernels=1 extra_cxx_flags="-fPIC" -j8 Werror=0