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WebAssembly and Rust

~ A Web Love Story ~

 love gif  Rustacean

Before Start

  • Basic knowledge of HTML and JavaScript
  • Enthusiasm for going fast on the web
  • Your favourite browser that supports Web Assembly

You can check out about:config (Firefox) or chrome://flags/ (Chrome) and make sure wasm related things are enabled.

WebAssembly (WASM)

WebAssembly or WASM is a low-level bytecode format for in-browser client-side scripting, evolved from JavaScript. Its initial aim is to support compilation from C and C++, though other source languages such as Rust are also supported.

Initial implementation of WebAssembly support in browsers will be based on the featureset of asm.js.

The asm.js is an intermediate programming language designed to allow computer software written in languages such as C to be run as web applications while maintaining performance characteristics considerably better than standard JavaScript, the typical language used for such applications. Consists of a strict subset of JavaScript, into which code written in statically-typed languages with manual memory management (such as C) is translated by a source-to-source compiler such as Emscripten (based on LLVM).

Performance is improved by limiting language features to those amenable to ahead-of-time optimization and other performance improvements.

Code generation based on asm.js

Calculate the length of a string in C:

size_t strlen(char *ptr) {
  char *curr = ptr;
  while (*curr != 0) {
  return (curr - ptr);

Would output the following JS code:

function strlen(ptr) {
  ptr = ptr|0;
  var curr = 0;
  curr = ptr;
  while (MEM8[curr]|0 != 0) {
    curr = (curr + 1)|0;
  return (curr - ptr)|0;

Note: Much of performance gain over normal JavaScript is due to 100% type consistency and virtually no garbage collection (memory is manually managed in a large typed array).

In March 2017, the WebAssembly Community Group reached consensus on the initial (MVP) binary format, JavaScript API, and reference interpreter. It defines a WebAssembly binary format.

See the representation of 3 different views of the same source code:

C (Input Source)

int factorial(int n) {
  if (n == 0)
    return 1;
    return n * factorial(n-1);

Linear Assembly Bytecode (IR)

Data structure used internally by wasm compiler to represent source code.

get_local 0
if i64
    i64.const 1
    get_local 0
    get_local 0
    i64.const 1
    call 0

WASM binary encoding

In hexadecimal:

20 00
04 7E
42 01
20 00
20 00
42 01
10 00

The WASM compiler system internally uses s-expressions - notation for nested list (tree-structured) data, invented for and popularized by the programming language Lisp, which uses them for source code as well as data - for parsing simplicity as well to handle intermediate code.

An example is shown below:

  (type $FUNCSIG$dd (func (param f64) (result f64)))
  (import "global.Math" "exp" (func $exp (param f64) (result f64)))
  (memory 256 256)
  (export "memory" (memory 0))
  (func $doubleExp (param $0 f64) (result f64)
      (call $exp
        (get_local $0)
      (f64.const 2)
  (export "doubleExp" (func $doubleExp))


Emscripten is an LLVM-based project that compiles C and C++ into highly-optimizable JavaScript in asm.js format. This lets you run C and C++ on the web at near-native speed, without plugins.

Thanks to LLVM, Emscripten and asm.js, code runs at near-native speed.

Rust Language

Rust is a systems programming language sponsored by Mozilla Research, which describes it as a "safe, concurrent, practical language," supporting functional and imperative-procedural paradigms. Rust is syntactically similar to C++, but its designers intend it to provide better memory safety while maintaining performance.

Rust is an open source programming language. Its designers have refined the language through the experiences of writing the Servo web browser layout engine and the Rust compiler.

Featuring: zero-cost abstractions, move semantics, guaranteed memory safety, threads without data races, trait-based generics, pattern matching, type inference, minimal runtime and efficient C bindings.

Some code in Rust:

fn main() {
    let greetings = ["Hello", "Hola", "Bonjour",
                     "Ciao", "こんにちは", "안녕하세요",
                     "Cześć", "Olá", "Здравствуйте",
                     "Chào bạn", "您好", "Hallo",

    for (num, greeting) in greetings.iter().enumerate() {
        print!("{} : ", greeting);
        match num {
            0 =>  println!("This code is editable and runnable!"),
            1 =>  println!("¡Este código es editable y ejecutable!"),
            2 =>  println!("Ce code est modifiable et exécutable !"),
            3 =>  println!("Questo codice è modificabile ed eseguibile!"),
            4 =>  println!("このコードは編集して実行出来ます!"),
            5 =>  println!("여기에서 코드를 수정하고 실행할 수 있습니다!"),
            6 =>  println!("Ten kod można edytować oraz uruchomić!"),
            7 =>  println!("Este código é editável e executável!"),
            8 =>  println!("Этот код можно отредактировать и запустить!"),
            9 =>  println!("Bạn có thể edit và run code trực tiếp!"),
            10 => println!("这段代码是可以编辑并且能够运行的!"),
            11 => println!("Dieser Code kann bearbeitet und ausgeführt werden!"),
            12 => println!("Den här koden kan redigeras och köras!"),
            _ =>  {},

Other code written in Rust (example of Array and Slices):

use std::mem;

// This function borrows a slice
fn analyze_slice(slice: &[i32]) {
    println!("first element of the slice: {}", slice[0]);
    println!("the slice has {} elements", slice.len());

fn main() {
    // Fixed-size array (type signature is superfluous)
    let xs: [i32; 5] = [1, 2, 3, 4, 5];

    // All elements can be initialized to the same value
    let ys: [i32; 500] = [0; 500];

    // Indexing starts at 0
    println!("first element of the array: {}", xs[0]);
    println!("second element of the array: {}", xs[1]);

    // `len` returns the size of the array
    println!("array size: {}", xs.len());

    // Arrays are stack allocated
    println!("array occupies {} bytes", mem::size_of_val(&xs));

    // Arrays can be automatically borrowed as slices
    println!("borrow the whole array as a slice");

    // Slices can point to a section of an array
    println!("borrow a section of the array as a slice");
    analyze_slice(&ys[1 .. 4]);

    // Out of bound indexing yields a panic
    println!("{}", xs[5]);

If're not familiar with almost none of this code. I extremally recommend you to stop few minutes on Rust by Example.

Why it Right Now?

JavaScript was created in 1995. It wasn’t designed to be fast, and for the first decade, it wasn’t fast. Then the browsers started getting more competitive.

In 2008, a period that people call the performance wars began. Multiple browsers added just-in-time compilers, also called JITs. As JavaScript was running, the JIT could see patterns and make the code run faster based on those patterns.

The introduction of these JITs led to an inflection point in the performance of JavaScript. Execution of JS was 10x faster.

With this improved performance, JavaScript started being used for things no one ever expected it to be used for, like server-side programming with Node.js. The performance improvement made it feasible to use JavaScript on a whole new class of problems.

We may be at another one of those inflection points now, with WebAssembly.


  1. Setup rustup

Rustup is an official Rust project that allows us to install, manage, and update multiple Rust toolchains.

curl -sSf | sh

After we’ll be prompted to run the following command to update the current shell with the changes:

source $HOME/.cargo/env

Add the wasm32-unknown-emscripten compile target via rustup as well:

rustup target add wasm32-unknown-emscripten

Set up Emscripten via emsdk. We’ll use the incoming version of Emscripten in order to get the best output.

# Make sure to have cmake installed before running this:
# - Ubuntu/Debian: apt install cmake
# - MacOS X : brew install cmake
curl | tar -zxv -C ~/
cd ~/emsdk-portable
./emsdk update
./emsdk install sdk-incoming-64bit
./emsdk activate sdk-incoming-64bit

Emscripten is installed.

The last command will instruct us how to add the binaries to our path for permanent usage, or we can just source ./ for temporary fun.

$ emcc -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 1.37.22
clang version 4.0.0 ( 3659f873b523e5fc89ffa16baab8901fbd084251) ( de9659961c692174fc4651a6ea0720236e4c4739) (emscripten 1.37.22 : 1.37.22)
Target: x86_64-apple-darwin17.2.0
Thread model: posix
InstalledDir: /Users/raphael.amorim/emsdk-portable/clang/fastcomp/build_incoming_64/bin
INFO:root:(Emscripten: Running sanity checks)

Let’s create the project.

cargo init wasm-demo --bin && rustup override set nightly

Put our first code sample into src/

enum Direction { North, South, East, West }

fn is_north(dir: Direction) -> bool {
    match dir {
        Direction::North => true,
        _ => false,

fn main() {
    let points = Direction::South;
    println!("{:?}", points);
    let compass = is_north(points);
    println!("{}", compass);

Run it to see what happens:

$ cargo run
   Compiling wasm-rust v0.1.0 (file:///Users/raphael.amorim/Documents/gcom/webassembly-and-rust/examples/wasm-rust)
   Finished dev [unoptimized + debuginfo] target(s) in 0.66 secs
   Running `target/debug/wasm-rust`

Let's build for browsers:

cargo build --target=wasm32-unknown-emscripten --release

Run tree on target folder:

$ tree target
├── debug
│   ├── build
│   ├── deps
│   │   ├── wasm_rust-34c6ef5f29bee9df
│   │   └── wasm_rust-34c6ef5f29bee9df.dSYM
│   │       └── Contents
│   │           ├── Info.plist
│   │           └── Resources
│   │               └── DWARF
│   │                   └── wasm_rust-34c6ef5f29bee9df
│   ├── examples
│   ├── incremental
│   ├── native
│   ├── wasm-rust
│   ├── wasm-rust.d
│   └── wasm-rust.dSYM -> /Users/raphael.amorim/Documents/gcom/webassembly-and-rust/examples/wasm-rust/target/debug/deps/wasm_rust-34c6ef5f29bee9df.dSYM
├── release
│   ├── build
│   ├── deps
│   ├── examples
│   ├── incremental
│   └── native
└── wasm32-unknown-emscripten
    └── release
        ├── build
        ├── deps
        │   ├── wasm-rust.js
        │   └── wasm_rust.wasm
        ├── examples
        ├── incremental
        ├── native
        ├── wasm-rust.d
        ├── wasm-rust.js
        ├── wasm_rust.d
        └── wasm_rust.wasm

cargo created several files in target/wasm32-unknown-emscripten/release/deps/ for us. Of primary interest are the .wasm and .js files.

Why do we get both a wasm and a js? Wasn’t the whole point of this to not use Javascript? Turns out we need some Javascript glue code to fetch, initialize, and configure it.

Create a site/index.html with the following content:

<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
    // This is read and used by `site.js`
    var Module = {
      wasmBinaryFile: "wasm_demo.wasm"
  <script src="site.js"></script>

Let's automatize it with Makefile, set up some way to get the generated files from the target/ folder into the site/ folder (* use tabs):

SHELL := /bin/bash

  cargo build --target=wasm32-unknown-emscripten --release
  mkdir -p site
  find target/wasm32-unknown-emscripten/release/deps -type f -name "*.wasm" | xargs -I {} cp {} site/wasm_demo.wasm
  find target/wasm32-unknown-emscripten/release/deps -type f ! -name "*.asm.js" -name "*.js" | xargs -I {} cp {} site/site.js

Run make and see the tree:

$ tree site
├── index.html
├── site.js
└── wasm_demo.wasm

Let’s test our generated code by running python -m SimpleHTTPServer, browsing to http://localhost:8000/site/, and opening the browser console.

Example Wasm

Wasm and Rust Called from Javascript

Link to example


use std::os::raw::c_char;
use std::ffi::CString;
use std::collections::HashMap;

pub fn get_data() -> *mut c_char {
    let mut data = HashMap::new();
    data.insert("Alice", "send");
    data.insert("Bob", "recieve");
    data.insert("Carol", "intercept");

    let descriptions = data.iter()
        .map(|(p,a)| format!("{} likes to {} messages", p, a))

    CString::new(descriptions.join(", "))

fn main() {
    // Deliberately blank.

In Client-side:

var Module = {
  wasmBinaryFile: "site.wasm",
  onRuntimeInitialized: main,
function main() {
  var getData = Module.cwrap('get_data', 'string', []);



Thanks to hoverbear by examples and Rust/Wasm setup guide.



WebAssembly and Rust: A Web Love Story




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