New emscripten features

[buu700]

I've recently added two features to emscripten that would be useful here, particularly concerning WebAssembly support.

(They aren't specific to wasm, but were needed to implement the pattern I chose to use in upgrading mceliece.js, ntru.js, rlwe.js, sidh.js, sphincs.js, and xkcd-passphrase two weeks ago.)

• Module['onRuntimeInitializeFailed'] emscripten-core/emscripten#5357 (just merged): adds a Module.onAbort callback

  • This is needed for detecting failure of asynchronous wasm compilation (WebAssembly.instantiate) and falling back to asm.js, since just checking for the availability of wasm functionality isn't sufficient.

• Add SINGLE_FILE option to embed all subresources into emitted JS emscripten-core/emscripten#5296 (on track to be merged soon): adds an -s SINGLE_FILE=1 flag that embeds all subresources as base64 strings (mem init file, wasm binary, etc.)

  • With this, the wasm version of libsodium.js can remain as a single JS file just like the asm.js version. (It also replaces the previous --memory-init-file 0 implementation with this more efficient encoding.)

I haven't followed the work that's gone into implementing WebAssembly in libsodium.js up until now, but as far as how I implemented WebAssembly in those libraries, the pattern I used looks more or less like this (not exactly, but same basic idea):

${preJS}

try {
    await new Promise((resolve, reject) => {
        var Module = {onRuntimeInitialized: resolve, onAbort: reject};
        ${wasmBuild}
    });
}
catch (_) {
    ${asmjsBuild} // compiled with -Oz, "use asm" stripped to prevent redundant AOT compilation
}

${postJS}

(As a side benefit, with asm.js becoming a second-class citizen, it's also now safe to run the whole output through uglifyjs mangling and compression.)

The other change I had to make to account for wasm compilation being asynchronous was to convert every field and method into a Promise. For libsodium.js, it could alternatively make more sense to keep the existing API surface synchronous and just expose an "is ready" Promise and/or callback.

[jedisct1]

Support for WebAssembly has been added. It's ugly and hacky, so all improvements would be very welcome!

I won't be able to work on this any more in a near future.

[buu700]

Awesome! In that case, I'll just test it out with that new SINGLE_FILE emscripten flag and submit a PR if there are no issues.