Android build document in English (#5029)

* Add English version of Android cross-compiling document

* Add English version of Android cross-compiling document

* Follow comments from Yi-qun and Kavya

doc/howto/cross_compiling/cross_compiling_for_android.md

+# Build PaddlePaddle for Android
+
+There are two approaches to build PaddlePaddle for Android: using Docker and on Linux without Docker. 
+
+## Cross-Compiling Using Docker
+
+Docker-based cross-compiling is the recommended approach because Docker runs on all major operating systems, including Linux, Mac OS X, and Windows.
+
+### Build the Docker Image
+
+The following steps pack all the tools that we need to build PaddlePaddle into a Docker image.
+
+```bash
+$ git clone https://github.com/PaddlePaddle/Paddle.git
+$ cd Paddle
+$ docker build -t paddle:dev-android . -f Dockerfile.android
+```
+
+### Build the Inference Library
+
+We can run the Docker image we just created to build the inference library of PaddlePaddle for Android using the command below:
+
+```bash
+$ docker run -it --rm -v $PWD:/paddle -e "ANDROID_ABI=armeabi-v7a" -e "ANDROID_API=21" paddle:dev-android
+```
+
+The Docker image accepts two arguments `ANDROID_ABI` and `ANDROID_API`:
+
+| Argument        | Optional Values         | Default |
+|-----------------|-------------------------|---------|
+|`ANDROID_ABI`    |`armeabi-v7a, arm64-v8a` | `armeabi-v7a` |
+|`ANDROID_API`    |`>= 21` | `21` |
+
+The ARM-64 architecture (`arm64-v8a`) requires at least level 21 of Android API.
+
+The default entry-point of the Docker image, [`paddle/scripts/docker/build_android.sh`](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/scripts/docker/build_android.sh) generates the [Android cross-compiling standalone toolchain](https://developer.android.com/ndk/guides/standalone_toolchain.html) based on the argument: `ANDROID_ABI` or `ANDROID_API`.  For information about other configuration arguments, please continue reading.
+
+The above command generates and outputs the inference library in `$PWD/install_android` and puts third-party libraries in `$PWD/install_android/third_party`.
+
+## Cross-Compiling on Linux
+
+The Linux-base approach to cross-compile is to run steps in `Dockerfile.android` manually on a Linux x64 computer.
+
+### Setup the Environment
+
+To build for Android's, we need [Android NDK](
+https://developer.android.com/ndk/downloads/index.html):
+
+```bash
+wget -q https://dl.google.com/android/repository/android-ndk-r14b-linux-x86_64.zip
+unzip -q android-ndk-r14b-linux-x86_64.zip
+```
+
+Android NDK includes everything we need to build the [*standalone toolchain*](https://developer.android.com/ndk/guides/standalone_toolchain.html), which in then used to build PaddlePaddle for Android.  (We plan to remove the intermediate stage of building the standalone toolchain in the near future.)
+
+- To build the standalone toolchain for `armeabi-v7a` and Android API level 21:
+
+  ```bash
+  your/path/to/android-ndk-r14b-linux-x86_64/build/tools/make-standalone-toolchain.sh \
+          --arch=arm --platform=android-21 --install-dir=your/path/to/arm_standalone_toolchain
+  ```
+  
+  The generated standalone toolchain will be in `your/path/to/arm_standalone_toolchain`.
+
+- To build the standalone toolchain for `arm64-v8a` and Android API level 21:
+
+  ```bash
+  your/path/to/android-ndk-r14b-linux-x86_64/build/tools/make-standalone-toolchain.sh \
+          --arch=arm64 --platform=android-21 --install-dir=your/path/to/arm64_standalone_toolchain
+  ```
+
+  The generated standalone toolchain will be in `your/path/to/arm64_standalone_toolchain`.
+
+**Please be aware that the minimum level of Android API required by PaddlePaddle is 21.**
+
+### Cross-Compiling Arguments
+
+CMake supports [choosing the toolchain](https://cmake.org/cmake/help/v3.0/manual/cmake-toolchains.7.html#cross-compiling).  PaddlePaddle provides [`android.cmake`](https://github.com/PaddlePaddle/Paddle/blob/develop/cmake/cross_compiling/android.cmake), which configures the Android cross-compiling toolchain for CMake.  `android.cmake` is not required for CMake >= 3.7, which support Android cross-compiling. PaddlePaddle detects the CMake version, for those newer than 3.7, it uses [the official version](https://cmake.org/cmake/help/v3.7/manual/cmake-toolchains.7.html#cross-compiling).
+
+Some other CMake arguments you need to know:
+
+- `CMAKE_SYSTEM_NAME` must be `Android`.  This tells PaddlePaddle's CMake system to cross-compile third-party dependencies.  This also changes some other CMake arguments like `WITH_GPU=OFF`, `WITH_AVX=OFF`, `WITH_PYTHON=OFF`, and `WITH_RDMA=OFF`.
+- `WITH_C_API` must be `ON`, to build the C-based inference library for Android.
+- `WITH_SWIG_PY` must be `OFF` because the Android platform doesn't support SWIG-based API.
+
+Some Android-specific arguments:
+
+- `ANDROID_STANDALONE_TOOLCHAIN`: the absolute path of the Android standalone toolchain, or the path relative to the CMake build directory.  PaddlePaddle's CMake extensions would derive the cross-compiler, sysroot and Android API level from this argument.
+- `ANDROID_TOOLCHAIN`: could be `gcc` or `clang`.  The default value is `clang`.
+  - For CMake >= 3.7, it should anyway be `clang`.  For older versions, it could be `gcc`.
+  - Android's official `clang` requires `glibc` >= 2.15.
+- `ANDROID_ABI`: could be `armeabi-v7a` or `arm64-v8a`.  The default value is `armeabi-v7a`.
+- `ANDROID_NATIVE_API_LEVEL`: could be derived from the value of `ANDROID_STANDALONE_TOOLCHAIN`.
+- `ANROID_ARM_MODE`:
+  - could be `ON` or `OFF`, and defaults to `ON`, when `ANDROID_ABI=armeabi-v7a`;
+  - no need to specify when `ANDROID_ABI=arm64-v8a`.
+- `ANDROID_ARM_NEON`: indicates if to use NEON instructions.
+  - could be `ON` or `OFF`, and defaults to `ON`, when `ANDROID_ABI=armeabi-v7a`;
+  - no need to specify when `ANDROID_ABI=arm64-v8a`.
+
+Other useful arguments:
+
+- `USE_EIGEN_FOR_BLAS`: indicates if using Eigen.  Could be `ON` or `OFF`, defaults to `OFF`.
+- `HOST_C/CXX_COMPILER`: specifies the host compiler, which is used to build the host-specific protoc and target-specific OpenBLAS.  It defaults to the value of the environment variable `CC`, or `cc`.
+
+Some frequent configurations for your reference:
+
+```bash
+cmake -DCMAKE_SYSTEM_NAME=Android \
+      -DANDROID_STANDALONE_TOOLCHAIN=your/path/to/arm_standalone_toolchain \
+      -DANDROID_ABI=armeabi-v7a \
+      -DANDROID_ARM_NEON=ON \
+      -DANDROID_ARM_MODE=ON \
+      -DUSE_EIGEN_FOR_BLAS=ON \
+      -DCMAKE_INSTALL_PREFIX=your/path/to/install \
+      -DWITH_C_API=ON \
+      -DWITH_SWIG_PY=OFF \
+      ..
+```
+
+```
+cmake -DCMAKE_SYSTEM_NAME=Android \
+      -DANDROID_STANDALONE_TOOLCHAIN=your/path/to/arm64_standalone_toolchain \
+      -DANDROID_ABI=arm64-v8a \
+      -DUSE_EIGEN_FOR_BLAS=OFF \
+      -DCMAKE_INSTALL_PREFIX=your/path/to/install \
+      -DWITH_C_API=ON \
+      -DWITH_SWIG_PY=OFF \
+      ..
+```
+
+
+There are some other arguments you might want to configure.
+
+- `CMAKE_BUILD_TYPE=MinSizeRel` minimizes the size of library.
+- `CMAKE_BUILD_TYPE-Release` optimizes the runtime performance.
+
+Our own tip for performance optimization to use clang and Eigen or OpenBLAS:
+- `CMAKE_BUILD_TYPE=Release`
+- `ANDROID_TOOLCHAIN=clang`
+- `USE_EIGEN_BLAS=ON` for `armeabi-v7a`, or `USE_EIGEN_FOR_BLAS=OFF` for `arm64-v8a`.
+
+### Build and Install
+
+After running `cmake`, we can run `make; make install` to build and install.
+
+Before building, you might want to remove the `third_party` and `build` directories including pre-built libraries for other architectures.
+
+After building，in the directory `CMAKE_INSTALL_PREFIX`, you will find three sub-directories:
+
+- `include`: the header file of the inference library,
+- `lib`: the inference library built for various Android ABIs,
+- `third_party`: dependent third-party libraries built for Android.

doc/howto/cross_compiling/cross_compiling_for_android_cn.md

@@ -26,14 +26,14 @@ Android的Docker开发镜像向用户提供两个可配置的参数：
 |`ANDROID_API`    |`>= 21` | `21` |
 
 - 编译`armeabi-v7a`，`Android API 21`的PaddlePaddle库
-```bash
-$ docker run -it --rm -v $PWD:/paddle -e "ANDROID_ABI=armeabi-v7a" -e "ANDROID_API=21" username/paddle-android:dev
-```
+  ```bash
+  $ docker run -it --rm -v $PWD:/paddle -e "ANDROID_ABI=armeabi-v7a" -e "ANDROID_API=21" username/paddle-android:dev
+  ```
 
 - 编译`arm64-v8a`，`Android API 21`的PaddlePaddle库
-```bash
-$ docker run -it --rm -v $PWD:/paddle -e "ANDROID_ABI=arm64-v8a" -e "ANDROID_API=21" username/paddle-android:dev
-```
+  ```bash
+  $ docker run -it --rm -v $PWD:/paddle -e "ANDROID_ABI=arm64-v8a" -e "ANDROID_API=21" username/paddle-android:dev
+  ```
 
 执行上述`docker run`命令时，容器默认执行[paddle/scripts/docker/build_android.sh](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/scripts/docker/build_android.sh)脚本。该脚本中记录了交叉编译Android版PaddlePaddle库常用的CMake配置，并且会根据`ANDROID_ABI`和`ANDROID_API`自动构建独立工具链、进行编译和安装。由于arm64架构要求Android API不小于21。因此当`ANDROID_ABI=arm64-v8a`，`ANDROID_API<21`时，Docker容器中将默认使用`Android API 21`的编译工具链。用户可以参考下文**配置交叉编译参数**章节，根据个人的需求修改定制Docker容器所执行的脚本。编译安装结束之后，PaddlePaddle的C-API库将被安装到`$PWD/install_android`目录，所依赖的第三方库同时也被安装到`$PWD/install_android/third_party`目录。