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id Emscripten
title Building Hermes with Emscripten

Setting up Emscripten

To setup Emscripten for building Hermes, we recommend using emsdk, which is the same way Emscripten recommends for most circumstances. Follow the directions on the Emscripten website for emsdk to download the SDK.

We recommend using latest-fastcomp to build Hermes, but latest works as well.

emsdk install latest-fastcomp
emsdk activate latest-fastcomp

If you install emsdk at ~/emsdk and activate latest-fastcomp, then you should use this shell variable for the rest of these instructions:

$EmscriptenRoot = ~/emsdk/fastcomp/emscripten

If you use latest instead, replace fastcomp in the above instruction with upstream.

Building Hermes With configure.py

# Configure the build. Here the build is output to a
# directory starting with the prefix "embuild".
python3 ${HermesSourcePath?}/utils/build/configure.py \
    --distribute \
    --wasm \
    --emscripten-platform=fastcomp \
    --emscripten-root="${EmscriptenRoot?}" \
    /tmp/embuild

# Build Hermes. The build directory name will depend on the flags passed to
# configure.py.
cmake --build /tmp/embuild_release_wasm_fastcomp --target hermes
# Execute hermes
node /tmp/embuild_release_wasm_fastcomp/bin/hermes.js --help

In the commands above, replace ${HermesSourcePath?} with the path where you cloned Hermes, and ${EmscriptenRoot?} with the path to your Emscripten install.

Make sure that the --emscripten-platform option matches the directory given to --emscripten-root, and is also the current activated Emscripten toolchain via emsdk activate.

See configure.py --help for more build options.

Build with CMake directly

The configure.py script runs CMake for you with options chosen by the Hermes project. If you want to customize your build, you can take this command as a base.

mkdir embuild && cd embuild
cmake ${HermesSourcePath?} \
        -DCMAKE_TOOLCHAIN_FILE=${EmscriptenRoot?}/cmake/Modules/Platform/Emscripten.cmake \
        -DCMAKE_BUILD_TYPE=Release \
        -DEMSCRIPTEN_FASTCOMP=1 \
        -DCMAKE_EXE_LINKER_FLAGS="-s NODERAWFS=1 -s WASM=1 -s ALLOW_MEMORY_GROWTH=1"

Each option is explained below:

  • CMAKE_BUILD_TYPE: set it to one of CMake's build modes: Debug, Release, MinSizeRel, etc.
  • EMSCRIPTEN_FASTCOMP: set to 1 if using fastcomp, or 0 if using upstream (LLVM)
  • WASM: whether to use asm.js (0), WebAssembly (1), or both (2)
  • NODERAWFS: set to 1 if you will be running Hermes directly with Node. It enables direct access to the filesystem.
  • ALLOW_MEMORY_GROWTH: whether to pre-allocate all memory, or let it grow over time

You can customize the build generator by passing the -G option to CMake, for example -G Ninja.