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+# How to Cross-Compile ArmNN on x86_64 for arm64
+
+*  [Introduction](#introduction)
+*  [Build and install Google's Protobuf library](#buildProtobuf)
+*  [Build Caffe for x86_64](#buildCaffe)
+*  [Cross-compiling ToolChain](#installCCT)
+*  [Build Boost library for arm64](#installBaarch)
+*  [Build Compute Library](#buildCL)
+*  [Build Compute Library](#buildCL)
+*  [Build ArmNN](#buildANN)
+*  [Run Unit Tests](#unittests)
+*  [Troubleshooting and Errors](#troubleshooting)
+
+
+#### <a name="introduction">Introduction</a>
+These are the step by step instructions on Cross-Compiling ArmNN under an x86_64 system to target an Arm64 system. This build flow has been tested with Ubuntu 16.04.
+The instructions show how to build the ArmNN core library and the Boost, Protobuf, Caffe and Compute Libraries necessary for compilation.
+
+#### <a name="buildProtobuf">Build and install Google's Protobuf library</a>
+
+* Get protobuf-all-3.5.1.tar.gz from here: https://github.com/google/protobuf/releases
+* Extract:
+    ```bash
+    tar -zxvf protobuf-all-3.5.1.tar.gz
+    cd protobuf-3.5.1
+    ```
+* 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 ..
+    ```
+
+#### <a name="buildCaffe">Build Caffe for x86_64</a>
+* Ubuntu 16.04 installation. These steps are taken from the full Caffe installation documentation at: http://caffe.berkeleyvision.org/install_apt.html
+* Install dependencies:
+    ```bash
+    sudo apt-get install libleveldb-dev libsnappy-dev libopencv-dev libhdf5-serial-dev
+    sudo apt-get install --no-install-recommends libboost-all-dev
+    sudo apt-get install libgflags-dev libgoogle-glog-dev liblmdb-dev
+    sudo apt-get install libopenblas-dev
+    sudo apt-get install libatlas-base-dev
+    ```
+* Download Caffe-Master from: https://github.com/BVLC/caffe
+    ```bash
+    git clone https://github.com/BVLC/caffe.git
+    cd caffe
+    cp Makefile.config.example Makefile.config
+    ```
+* Adjust Makefile.config (for example, if using Anaconda Python, or if cuDNN is desired):
+    ```
+    CPU only version -
+    CPU_ONLY := 1
+    Add hdf5 and protobuf include and library directories (Replace $HOME with your actual /home/username dir)
+        INCLUDE_DIRS := $(PYTHON_INCLUDE) /usr/local/include /usr/include/hdf5/serial/ $HOME/armnn-devenv/google/x86_64_pb_install/include/
+        LIBRARY_DIRS := $(PYTHON_LIB) /usr/local/lib /usr/lib /usr/lib/x86_64-linux-gnu/hdf5/serial/ $HOME/armnn-devenv/google/x86_64_pb_install/lib/
+    g++ need to be version 5
+    CUSTOM_CXX := g++-5
+    ```
+* Setup environment:
+    ```bash
+    export PATH=$HOME/armnn-devenv/google/x86_64_pb_install/bin/:$PATH
+    export LD_LIBRARY_PATH=$HOME/armnn-devenv/google/x86_64_pb_install/lib/:LD_LIBRARY_PATH
+    ```
+* Compilation with Make:
+    ```bash
+    make all
+    make test
+    make runtest
+    ```
+    These should all run without errors
+* caffe.pb.h and caffe.pb.cc will be needed when building ArmNN's Caffe Parser
+
+#### <a name="installCCT">Cross-compiling ToolChain</a>
+* Install the standard cross-compilation libraries for arm64:
+   ```
+   sudo apt install crossbuild-essential-arm64
+   ```
+#### <a name="installBaarch">Build Boost library for arm64</a>
+* Build Boost library for arm64
+    Download Boost version 1.64 from http://www.boost.org/doc/libs/1_64_0/more/getting_started/unix-variants.html  
+    Version 1.66 is not supported.
+    ```bash
+    tar -zxvf boost_1_64_0.tar.gz
+    cd boost_1_64_0
+    echo "using gcc : arm : aarch64-linux-gnu-g++ ;" > user_config.jam
+    ./bootstrap.sh --prefix=$HOME/armnn-devenv/boost_arm64_install
+    ./b2 install toolset=gcc-arm link=static cxxflags=-fPIC --with-filesystem --with-test --with-log --with-program_options -j32 --user-config=user_config.jam
+    ```
+
+#### <a name="buildCL">Build Compute Library</a>
+* Building the Arm Compute Library:
+    ```bash
+    git clone https://github.com/ARM-software/ComputeLibrary.git
+    cd ComputeLibrary/
+    scons arch=arm64-v8a neon=1 opencl=1 embed_kernels=1 extra_cxx_flags="-fPIC" -j8 internal_only=0
+    ```
+
+#### <a name="buildANN">Build ArmNN</a>
+* Compile ArmNN for arm64:
+    ```bash
+    git clone https://github.com/ARM-software/armnn.git
+    cd armnn
+    mkdir build
+    cd build
+    ```
+
+* Use CMake to configure your build environment, update the following script and run it from the armnn/build directory to set up the armNN build:
+    ```bash
+    #!/bin/bash
+    CXX=aarch64-linux-gnu-g++ \
+    CC=aarch64-linux-gnu-gcc \
+    cmake .. \
+    -DARMCOMPUTE_ROOT=$HOME/armnn-devenv/ComputeLibrary \
+    -DARMCOMPUTE_BUILD_DIR=$HOME/armnn-devenv/ComputeLibrary/build/ \
+    -DBOOST_ROOT=$HOME/armnn-devenv/boost_arm64_install/ \
+    -DARMCOMPUTENEON=1  -DARMCOMPUTECL=1 \
+    -DCAFFE_GENERATED_SOURCES=$HOME/armnn-devenv/caffe/build/src \
+    -DBUILD_CAFFE_PARSER=1 \
+    -DPROTOBUF_ROOT=$HOME/armnn-devenv/google/x86_64_pb_install/ \
+    -DPROTOBUF_LIBRARY_DEBUG=$HOME/armnn-devenv/google/arm64_pb_install/lib/libprotobuf.so.15.0.1 \
+    -DPROTOBUF_LIBRARY_RELEASE=$HOME/armnn-devenv/google/arm64_pb_install/lib/libprotobuf.so.15.0.1
+    ```
+* Run the build
+    ```bash
+    make -j32
+    ```
+
+#### <a name="unittests">Run Unit Tests</a>
+* Copy the build folder to an arm64 linux machine
+* Copy the libprotobuf.so.15.0.1 library file to the build folder
+* cd to the build folder on your arm64 machine and set your LD_LIBRARY_PATH to its current location:
+    ```
+    cd build/
+    export LD_LIBRARY_PATH=`pwd`
+    ```
+* Run the UnitTests:
+    ```
+    ./UnitTests
+    Running 567 test cases...
+
+    *** No errors detected
+    ```
+#### <a name="troubleshooting">Troubleshooting and Errors:</a>
+#### Error adding symbols: File in wrong format
+* When building armNN:
+    ```
+    /usr/local/lib/libboost_log.a: error adding symbols: File in wrong format
+    collect2: error: ld returned 1 exit status
+    CMakeFiles/armnn.dir/build.make:4028: recipe for target 'libarmnn.so' failed
+    make[2]: *** [libarmnn.so] Error 1
+    CMakeFiles/Makefile2:105: recipe for target 'CMakeFiles/armnn.dir/all' failed
+    make[1]: *** [CMakeFiles/armnn.dir/all] Error 2
+    Makefile:127: recipe for target 'all' failed
+    make: *** [all] Error 2
+    ```
+* Boost libraries are not compiled for the correct architecture, try recompiling for arm64
+##
+#### Virtual memory exhausted
+* When compiling the boost libraries:
+    ```bash
+    virtual memory exhausted: Cannot allocate memory
+    ```
+* Not enough memory available to compile. Increase the amount of RAM or swap space available.
+
+##
+#### Unrecognized command line option '-m64'
+* When compiling the boost libraries:
+    ```bash
+    aarch64-linux-gnu-g++: error: unrecognized command line option ‘-m64’
+    ```
+* Clean the boost library directory before trying to build with a different architecture:
+    ```bash
+    sudo ./b2 clean
+    ```
+* It should show the following for arm64:
+    ```bash
+    - 32-bit                   : no
+    - 64-bit                   : yes
+    - arm                      : yes
+    ```
+
+##
+#### Missing libz.so.1
+* When compiling armNN:
+    ```bash
+    /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.15.0.0, 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 explicitely installing them:
+    ```bash
+    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
+  ```bash
+  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.
+    ```bash
+    deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ xenial main restricted
+    ```
+* Then add the following:
+    ```bash
+    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:
+    ```bash
+    sudo apt-get install zlib1g:arm64
+    sudo apt-get update
+    sudo ldconfig
+    ```
+##
+#### Undefined references to google::protobuf:: functions
+* When compiling armNN there are multiple errors of the following type:
+    ```
+    libarmnnCaffeParser.so: undefined reference to `google::protobuf:*
+    ```
+* Missing or out of date protobuf compilation libraries.  
+    Use the command 'protoc --version' to check which version of protobuf is available (version 3.5.1 is required).  
+    Follow the instructions above to install protobuf 3.5.1  
+    Note this will require you to recompile Caffe for x86_64
+
+##
+#### 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 internal_only=0 Werror=0
+    ```
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