# How to use the Android NDK to build Arm NN

- [Introduction](#introduction)
- [Prerequisites](#prerequisites)
- [Download Arm NN](#download-arm-nn)
- [Build Arm Compute Library](#build-arm-compute-library)
- [Build Arm NN](#build-arm-nn)
- [Build Arm NN Support Library](#build-arm-nn-support-library)
- [Build Arm NN Shim](#build-arm-nn-shim)


## Introduction
These are step by step instructions for building the Arm NN shim and support library for NNAPI.
This work is currently in an experimental phase.

## Prerequisites

The following are required to build the support library
* Android NDK r20b
  * Detailed setup can be found in [BuildGuideAndroidNDK.md](../BuildGuideAndroidNDK.md)
* Flatbuffer version 1.12.0
  * Detailed setup can be found in [BuildGuideCrossCompilation.md](../BuildGuideCrossCompilation.md)
* AOSP Source (Android Open Source Project)
  * Download the source from the [official website](https://source.android.com/setup/build/downloading)
  * This guide will use release tag `android12-s1-release`


Set environment variables
```bash
export AOSP_ROOT=<path/to/aosp/root>
export NDK=<path/to/>/android-ndk-r20b
export NDK_TOOLCHAIN_ROOT=$NDK/toolchains/llvm/prebuilt/linux-x86_64
export PATH=$NDK_TOOLCHAIN_ROOT/bin/:$PATH
```

## Download Arm NN
* Clone Arm NN into the AOSP tree:  
  (Requires Git if not previously installed: `sudo apt install git`)

```bash
mkdir -p $AOSP_ROOT/vendor/arm/
cd $AOSP_ROOT/vendor/arm/
git clone https://github.com/ARM-software/armnn.git
```

## Build Arm Compute Library

Arm NN provides a script that downloads the version of Arm Compute Library that Arm NN was tested with:
```bash
./armnn/scripts/get_compute_library.sh
```
* Build the Arm Compute Library:  
  (Requires SCons if not previously installed: `sudo apt install scons`)
```bash
cd $AOSP_ROOT/vendor/arm/clframework

scons arch=arm64-v8a \
toolchain_prefix=aarch64-linux-android- \
compiler_prefix=aarch64-linux-android29- \
neon=1 opencl=1 \
embed_kernels=1 \
build_dir=android-arm64v8a \
extra_cxx_flags="-Wno-parentheses-equality -Wno-missing-braces -fPIC" \
Werror=0 embed_kernels=1 examples=0 \
validation_tests=0 benchmark_tests=0 benchmark_examples=0 os=android -j16
```

## Build Arm NN and Serializer

* Build Arm NN:  
  (Requires CMake if not previously installed: `sudo apt install cmake`)
```bash
mkdir -p $AOSP_ROOT/vendor/arm/armnn/build
cd $AOSP_ROOT/vendor/arm/armnn/build
CXX=aarch64-linux-android29-clang++ \
CC=aarch64-linux-android29-clang \
CXX_FLAGS="-fPIE -fPIC" cmake .. \
-DCMAKE_ANDROID_NDK=$NDK \
-DCMAKE_SYSTEM_NAME=Android \
-DCMAKE_SYSTEM_VERSION=29 \
-DCMAKE_ANDROID_ARCH_ABI=arm64-v8a \
-DCMAKE_EXE_LINKER_FLAGS="-pie -llog -lz" \
-DARMCOMPUTE_ROOT=$AOSP_ROOT/vendor/arm/clframework/ \
-DARMCOMPUTE_BUILD_DIR=$AOSP_ROOT/vendor/arm/clframework/build/android-arm64v8a/ \
-DARMCOMPUTENEON=1 -DARMCOMPUTECL=1 -DARMNNREF=1 \
-DFLATBUFFERS_ROOT=<path/to/flatbuffers/target/android/build> \
-DFLATC_DIR=<path/to/flatbuffers/host/build-x86_64> \
-DBUILD_ARMNN_SERIALIZER=1 -DBUILD_GATORD_MOCK=0 -DBUILD_BASE_PIPE_SERVER=0
```

 * Run the build
```bash
make -j16
```

## Build Arm NN Support Library

Building the support library requires building some AOSP libraries via the NDK. 
It should be possible to create a symlink from $AOSP_ROOT to $AOSP_ROOT/vendor/arm/armnn/sl/aosp.
However this example will instead clone the necessary AOSP repos and apply some minor patches which were required to get it to build.
```bash
cd $AOSP_ROOT/vendor/arm/armnn/shim/sl/
# Call a script which will clone the necessary AOSP repos
./scripts/clone_aosp_libs.sh

# Modify the repos by applying patches and 'switching off' Android.bp/.mk files by renaming them
./scripts/modify_aosp_libs.sh

# Build the Support Library
CMARGS="$CMARGS \
-DCMAKE_TOOLCHAIN_FILE=$NDK/build/cmake/android.toolchain.cmake \
-DANDROID_ABI=arm64-v8a \
-DCMAKE_ANDROID_ARCH_ABI=arm64-v8a \
-DCMAKE_ANDROID_NDK=$NDK \
-DANDROID_PLATFORM=android-29 \
-DARMNN_SOURCE_DIR=$AOSP_ROOT/vendor/arm/armnn/ "

mkdir build
cd build

CXX=aarch64-linux-android29-clang++ \
CC=aarch64-linux-android29-clang \
cmake $CMARGS ../
make
```

## Build Arm NN Shim

By default the Arm NN shim Android.bp is not enabled.
Enable it by editing armnn/shim/Android.bp and setting `enabled: true`

```bash
cd $AOSP_ROOT
source build/envsetup.sh
lunch <device>-eng
cd vendor/arm/armnn/shim
export ARMNN_ANDROID_MK_ENABLE=0
mm
```

The built libraries and manifest file can be found here:
$AOSP_ROOT/out/target/product/<device>/vendor/lib64/libarmnn_support_library.so
$AOSP_ROOT/out/target/product/<device>/vendor/bin/hw/android.hardware.neuralnetworks-shim-service-armnn
$AOSP_ROOT/out/target/product/<device>/vendor/etc/vintf/manifest/android.hardware.neuralnetworks-shim-service-armnn.xml

Currently the Arm NN libraries are shared libraries and therefore will need to be pushed to the device:
$AOSP_ROOT/vendor/arm/armnn/build/libarmnnSerializer.so
$AOSP_ROOT/vendor/arm/armnn/build/libarmnn.so