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What is this?

This is temporary repo containing cmake toolchain for compiling code using Android NDK to WebAssembly.

Overview

compilation pipeline

Current state

We have a working prototype that lets us compile simple micro-benchmarks, the gzip benchmark from the SPECINT2000 suite, and we have also compiled one of programs from ndk-samples (native-plasma).

Gzip from SPECINT2000

Gzip running on a Pixel 6a via adb shell:

gzip benchmark

Timing comparison

timing comparison

Code size comparison

code size comparison

Pre-requisites

You will need Android NDK and since at the moment of writing this NDK toolchain does not support wasm target, clang from Android OpenSource Project toolchain. Moreover, you need the modified wasm2c from the forked WABT repository.

  1. (Optional) Create a working directory (or directories) to hold your copies of the NDK, the toolchain, and WABT. You can use the same directory for all three, or you can use different locations. The name my_android_wasm is arbitrary and chosen just for illustration here. Also, for brevity we use just this one single directory. (We will later tie things together using environment variables. See below.)

    cd $HOME
mkdir my_android_wasm
cd my_android_wasm

  2. Download NDK here. (At the time of this writing, the current version is r26b.) Then uncompress it, e.g. like so, creating $HOME/my_android_wasm/android-ndk-r26b:

    unzip $HOME/Downloads/android-ndk-r26b-linux.zip

  3. There are currently no prebuilt binaries for clang that contain all the necessary changes, so build the toolchain from sources by following the instructions Build Instructions for the Android Clang/LLVM Toolchain.

    If you did this in your $HOME, you should now have a directory $HOME/llvm-toolchain/out/install. Locate the clang-dev directory for your current development platform therein, e.g., $HOME/llvm-toolchain/out/install/linux-x86/clang-dev.

  4. Set ANDROID_NDK_HOME environment variable to NDK root:

    export ANDROID_NDK_HOME=$HOME/my_android_wasm/android-ndk-r26b

  5. Set ANDROID_CLANG_TOOLCHAIN environment variable to clang toolchain root, which is the clang-dev directory from step 3. above.

    export ANDROID_CLANG_TOOLCHAIN=$HOME/llvm-toolchain/out/install/linux-x86/clang-dev

  6. Check out the worked version of WABT (which includes wasm2c):

    git clone https://github.com/AndroidWasm/wabt

    This creates $HOME/my_android_wasm/wabt.

  7. Build the WABT fork by typing make in the root of the checkout.

    cd wabt
make
cd ..

  8. Set WABT_HOME to the root of the wabt checkout:

    export WABT_HOME=$HOME/my_android_wasm/wabt

  9. Clone this repository and point WASM_NDK to it:

    cd $HOME/my_android_wasm
git clone https://github.com/AndroidWasm/wasm_ndk
export WASM_NDK=$HOME/my_android_wasm/wasm_ndk

How to use the cmake toolchain

Go to cmake project and execute the following commands:

mkdir build
cd build
cmake --toolchain $WASM_NDK/cmake/toolchain/android_wasm.toolchain.cmake ..
cmake --build .

(Optional) How to build android toolchain (in case you need to do some cherry-picks from upstream)

The main document on how to build android llvm toolchain can be found here: https://android.googlesource.com/toolchain/llvm_android/

Before building add upstream remote to android llvm toolchain

cd toolchain/llvm-project
git remote add upstream https://github.com/llvm/llvm-project.git
git fetch upstream

create a local branch (for toolchain/llvm-project)

repo start upstream-patches .

The android toolchain is usually couple of month behind ToT - so these are cherry pick needed to build compilable wasm binaries

git cherry-pick cb5bc756808367d53c870716ce42611a563421e8

Use following command while building to exclude windows toolchain and skip tests

python toolchain/llvm_android/build.py --no-build windows --skip-tests