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id cross-compilation
title Cross Compilation

This document describes how to build Hermes in a cross compilation setting, e.g. building for Android, WASM (with Emscripten), or any other platforms different than the host development machines.

Android compilation

Edit paths:

  • CMAKE_TOOLCHAIN_FILE should point to whatever android_ndk you need.
  • IMPORT_HOST_COMPILERS should point to whatever SH host build you want to use for InternalBytecode compilation.
  • -B path is to your build directory, or omit it to build in current directory.

Ensure that CMAKE_CXX_FLAGS_RELEASE is -O3 -DNDEBUG if you're in Release mode.

Then you can use adb push to copy any files into /data/local/tmp/ on an Android device and run it there using 'adb shell'.

cmake ~/fbsource/xplat/static_h \
  -B ~/static_h/build_android \
  -DHERMES_FACEBOOK_BUILD=OFF \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_TOOLCHAIN_FILE=/opt/android_ndk/r17fb2/build/cmake/android.toolchain.cmake \
  -DHERMES_UNICODE_LITE=ON \
  -DIMPORT_HOST_COMPILERS=~/static_h/build_release/ImportHostCompilers.cmake \
  -DHERMES_ENABLE_DEBUGGER=OFF \
  -DANDROID_ABI=arm64-v8a -G Ninja

HERMES_UNICODE_LITE=ON selects the self-contained no-op Unicode backend. Without it, an Android build uses the Java backend, which needs a JVM at runtime and so is not usable from a binary pushed with adb push and run under adb shell.

No ICU is needed here, or in any cross-compilation target. Hermes no longer depends on ICU at all. The platform_unicode layer -- case conversion, normalization, collation and date formatting -- is self-contained, and the ICU-backed Intl backend has been removed. -DHERMES_ENABLE_INTL=ON is now supported only on Apple and Android, which have their own native Intl implementations.

Emscripten

For Emscripten, you can find an example from the test-emscripten job from hermes/.circleci/config.yml. Also see more details at Building with Emscripten

Compiling and Running ARM32 Static Hermes from ARM64

On the ARM64 Linux machine you need clang-16 and lld-16.

On the ARM64 Linux machine on which you're compiling:

# Add architecture
dpkg --add-architecture armhf
# Update
sudo apt update
# For compiling
sudo apt install gcc-arm-linux-gnueabihf g++-arm-linux-gnueabihf binutils-arm-linux-gnueabihf libc6:armhf libstdc++6:armhf

If you're executing on an ARM64 machine, set it up as well:

# Add architecture
dpkg --add-architecture armhf
# Update
sudo apt update
# For running
sudo apt install libc6:armhf libstdc++6:armhf

There's a CMake toolchain file in the source directory that will then set up Clang to compile for ARM32. To use it:

CC=clang-16 CXX=clang++-16 cmake ${SRC_DIR?} -GNinja -DCMAKE_TOOLCHAIN_FILE=${SRC_DIR?}/armhf/arm-linux-gnueabihf.toolchain.cmake -DCMAKE_BUILD_TYPE=Release

Then you can run ninja and build 32-bit binaries and libraries. They'll work on the machine on which you're executing.