Developer’s Guide

This article provides information that is relevant to people who want to contribute to Emscripten. We welcome contributions from anyone that is interested in helping out!

Tip

The information will be less relevant if you’re just using Emscripten, but may still be of interest.

Getting Binaries

For contributing to core Emscripten code, such as emcc.py, you don’t need to build any binaries as emcc.py is in Python, and the core JS generation is in JavaScript. You do still need binaries for LLVM and Binaryen, which you can get using the emsdk:

emsdk install tot-upstream
emsdk activate tot-upstream

That gets a “tip-of-tree” build of the very latest binaries. You can use those binaries with a checkout of the core Emscripten repository, simply by calling emcc.py from that checkout, and it will use the binaries from the emsdk.

If you do want to contribute to LLVM or Binaryen, or to test modifications to them, you can build them from source.

Repositories and branches of interest

The Emscripten main repository is https://github.com/emscripten-core/emscripten. The main branch is incoming, which exists for historic reasons as we did not always have good pre-commit CI, and so we landed code on incoming first, tested, then merged to master. Today we have extensive pre-commit CI on GitHub, so the incoming branch functions like the master branch in most projects.

Aside from the Emscripten repo, the other codebases of interest are LLVM and Binaryen, which Emscripten invokes, and have their own repos.

Submitting patches

Patches should be submitted as pull requests to the incoming branch.

Note

Before submitting your first patch, add yourself to the AUTHORS file. By doing so, you agree to license your code under the project’s open source licenses (MIT/LLVM).

When submitting patches, please:

  • Make pull requests to incoming, not master.
  • Add an automatic test if you add any new functionality or fix a bug. Search in tests/*.py for related tests, as often the simplest thing is to add to an existing one. If you’re not sure how to test your code, feel free to ask for help.
  • We normally squash and merge PRs, which means the PR turns into a single commit on the target branch. Because of that, it’s ok to have merge commits in the PR itself, as they get removed. Please put a good description for the final commit in the PR description, and we’ll use it when squashing.

Code reviews

One of the core developers will review a pull request before merging it. If several days pass without any comments on your PR, please comment in the PR which will ping them. (If that happens, sorry! Sometimes things get missed.)

Compiler overview

The Emscripten Compiler Frontend (emcc) is a python script that manages the entire compilation process:

  • emcc calls Clang to compile C++ and wasm-ld to link it. It builds and integrates with the Emscripten system libraries, both the compiled ones and the ones implemented in JS.
  • emcc then calls emscripten.py which performs the final transformation to wasm (including invoking wasm-emscripten-finalize from Binaryen) and calls the JS compiler (see src/compiler.js and related files) which emits the JS.
  • If optimizing wasm, emcc will then call wasm-opt, run meta-dce, and other useful things. It will also run JS optimizations on the JS that is emitted alongside the wasm.

Emscripten Test Suite

Emscripten has a comprehensive test suite, which covers virtually all Emscripten functionality. These tests are run on CI automatically when you create a pull request, and they should all pass. If you run into trouble with a test failure you can’t fix, please let the developers know.