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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.

A Two-stage Build#

Hermes now requires a two stage build proecess because the VM now contains Hermes bytecode which needs to be compiled by Hermes

Setting up the workspace#

We will use environment variable $HERMES_WS_DIR to indicate the root of your workspace where the hermes git checkout directory is a subdirectory.

1st Stage: Building the Host Hermes Compiler#

cd "$HERMES_WS_DIR"
# Generate the build system at $HERMES_WS_DIR/build_host_hermesc
hermes/utils/build/configure.py ./build_host_hermesc
# Build the Hermes compiler
cmake --build ./build_host_hermesc --target hermesc

2nd Stage: Building the target Hermes#

The key is that we need to pass a CMake flag -DIMPORT_HERMESC:PATH=$HERMES_WS_DIR/build_host_hermesc/ImportHermesc.cmake during the cross compilation build of Hermes so it can access the host hermesc from the first stage to build the VM.

This process is currently happened in different places for different platforms:

  1. For Android, this happened in hermes/android/build.gradle
  2. For Apple platforms, this happend in hermes/utils/build-apple-framework.sh
  3. 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
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