When WebAssembly was first shipped it was an MVP which, while minimal, has spawned a huge number of exciting projects which work today across all major browsers. Rust has capitalized on the wasm MVP’s success as well with tools like wasm-bindgen and wasm-pack by making the MVP feel less minimal. WebAssembly is yet more ambitious, though! Since inception it’s always been intended to extend the WebAssembly specification with new features and functionality.

One of the features I’m particularly excited about coming down the pike for WebAssembly is the threads proposal. The threading proposal was unfortunately halted in its tracks when the Spectre and Meltdown vulnerabilities were first announced, but it’s starting to pick up steam again! Browsers will start shipping SharedArrayBuffer in the near future, and threads for wasm won’t be far behind.

Features like threads for wasm can have a huge impact on Rust and how it’s used on the web, and we want to be sure that Rust is ready and well suited for wasm threads as soon as they’re available! I’ve recently started to try to get more involved in the WebAssembly Community Group and this seemed like the perfect opportunity to test out support for Rust while also hopefully providing feedback for the proposal itself if necessary!

If you’re itching for a happy ending you can jump ahead to the end where (spoilers) there’s a demo showing off Rust, WebAssembly, and threads all in the browser.

Note: As a note to future readers, this post describes and works with many features which, at the time of its writing, were unstable. Not everything here may be accurate in the distant future and examples may no longer work. We’ll try to keep things updated but if you’re reading this far from when it was written take it with a grain of salt!

The WebAssembly threads proposal

While one might naively expect that the idea of “WebAssembly threads” is something like “add pthreads” or “add std::thread” to wasm, the current proposal for threads in WebAssembly is actually quite different! Instead of providing a full library experience the threads proposal is instead specifying the fundamental building blocks upon which you can build a threading library.

Atomic Instructions

The first aspect of the threads proposal that you might notice is the addition of atomic instructions. In Rust parlance this means that AtomicUsize and friends will actually compile to atomic operations, whereas today they’re simply lowered to single-threaded equivalents (as there’s no threads!). While essential, these aren’t too too thrilling just yet until you hit wait and notify.

Atomic modifications allow us to perform some level of synchronization, but full synchronization often requires actual blocking of a thread until another is finished. This is where the i32.atomic.wait and atomic.notify instructions come into play. First we can block a thread (atomically) using i32.atomic.wait, and then another thread can execute atomic.notify to wake up a thread blocked on the same address. I believe this is similar to futexes on Linux, although I’ve never used them myself!

With just this one addition we can now start to see how primitives can be formed, and sure enough the proposal has an example mutex implementation which is also how the Rust Mutex type is implemented as well.

Ok that’s great and all, but how do we spawn more than one thread?

Parallelism through Web Workers

One of WebAssembly’s greatest strengths is that it extends the web platform instead of trying to replace it. Although wasm modules themselves can largely only manipulate numbers directly, they can import any arbitrary function which gives wasm full access to the web platform, DOM and all. From day one WebAssembly is all about reusing and enhancing the web platform experience, avoiding the need to reinvent the wheel for new functionality.

The threads proposal for wasm is no exception to this pattern! The web already has support for multithreaded web applications through Web Workers, and this is exactly what’s used to introduce multithreaded execution to WebAssembly.

Web Workers, however, provide very limited ability to share resources between threads. Communication and synchronization is done through message passing (postMessage), but you can only send values that support structured cloning. In Rust-ic terms, very few types in JS are Send and you always Clone when sending an object to another thread.

Our goal is to share resources though! It turns out that one of the types which supports structured cloning is WebAssembly.Module. Execution of wasm on the web today requires usage of the wasm JS API, where you create a WebAssembly.Module that is akin to the text and data sections of an executable (compiled code) and then you create a WebAssembly.Instance from that module, which is where you actually get things like a heap and a stack. Already today we can pass a WebAssembly.Module between threads, and despite having to Clone it between threads it looks roughly like this in most engines:

pub struct Module {
    contents: Arc<ModuleContents>,

// ...

which means that the clone operation is quite cheap!

Sharing our code is only half the story, though. Many languages (including Rust) also rely on shared memory as a primitive to build all sorts of concurrency paradigms like message passing or mutexes.

Shared Memory

Continuing the theme of “no fundamental new features just for wasm”, shared memory is fundamentally built on an already stable (or rather, becoming stable) JS API: SharedArrayBuffer. A SharedArrayBuffer is like an ArrayBuffer except it’s, well, shared! This manifests itself through the structured cloning algorithm where you can think of it as similar to a WebAssembly.Module, internally containing an Arc that’s cheap to clone.

Using SharedArrayBuffer JS can already share memory between workers and the main thread, making it cheap to compute a large chunk of data and send it to another thread. (Or at least removes the need to copy data between threads).

WebAssembly modules today are optionally associated with at most one instance of “linear memory”. In non-wasm parlance, you can put a stick of RAM into a wasm module. This WebAssembly.Memory is today always backed by an ArrayBuffer, but you’ll soon be able to flag a memory as “shared” which means it’s backed by SharedArrayBuffer instead. This subsequently means that the structured clone of a WebAssembly.Memory backed by a SharedArrayBuffer will refer to the same memory!

At this point the pieces are definitely starting to come together. We can already share modules (code) between threads, and soon we’ll be able to share memory between threads as well! With these new abilities we can instantiate a WebAssembly.Module on multiple web workers quickly and efficiently which all have access to the same memory.

Initializing Memory Once

One fun aspect of WebAssembly modules is that memory is automatically initialized for you. Let’s say, for example, you’ve got a Rust program that looks like:

pub extern fn get_data() -> *const u8 {
    "the data".as_ptr()

If we call that from JS, and if we read the pointer returned, we’ll actually see the data! Who actually wrote those bytes to linear memory, though? Each wasm module can have data segments which specify a blob of bytes that is located at an offset in memory. Upon instantiation of a module, the wasm runtime will copy each of these data segments to the offset specified into linear memory.

But wait, that’s not a good idea if we instantiate our module on multiple threads! Let’s say we have code that looks like:

pub extern fn get_ticket() -> usize {
    static TICKET: AtomicUsize = AtomicUsize::new(1);
    TICKET.fetch_add(1, SeqCst)

Here we’ll have a data segment consisting of 1usize, which is three zero bytes and then a one byte. Each time we instantiate our module we’re resetting this counter back to 1 by overwriting the previous value! What we instead want to happen is the first thread initializes memory and all other threads should just use what’s already there.

To handle this problem, we turn to the bulk memory operations proposal. While the bulk memory operations proposal largely started as a native method of doing memcpy and memset, it’s now also picked up the ability for “passive memory segments” which solves the exact problem we’re having here.

Each data segment can be flagged as “passive” which means it’s not automatically copied into memory on instantiation. Instead a module must manually initialize memory via the memory.init instruction. Using memory.init we can copy memory from any data segment into any location in memory.

With memory.init we at least have the ability to solve the multi-initialization problem, but it’s not entirely clear how we’ll leverage this yet in the toolchain. More on this later!

Existing WebAssembly Features and Threads

That about wraps up the new features proposed in the WebAssembly threads (and bulk memory) proposal. Before we go into how this is all actually going to be used, though, it’s worth taking a quick look at some of the existing features of WebAssembly and what they mean in a world with threads.

The first interesting aspect (which we’ll leverage later on) is the start function. WebAssembly modules can flag a function as automatically executed whenever the module is instantiated. This start function can do things like static initialization or maybe even the main function of a wasm-pretending-to-be-an-executable, but it’s not currently exposed or used in Rust. The semantics of the start function don’t change at all in the threads proposal, but it means that it’s no longer one time initialization! Instead the start function is still run per-instance, and since we’re creating multiple instances on multiple web workers, the start function becomes more like a “thread init” than a “global init”. More on this later!

Next let’s take a look at global variables. Note that these are not Rust static variables (like TICKET above) which are compiled as being located in linear memory. Rust actually provides no ability to create, get, or set custom global variables today, so this is largely an unexposed feature of WebAssembly in Rust. In WebAssembly, however, a global is what its name implies, a global variable for the instance which can be get, set, and even exported to JS! A global is more like a virtual register than linear memory because it can only contain a fixed set of types.

Interestingly, though, globals are per instance. This means that in a world of multiple instances, they’re not actually globals but rather thread locals! Each of our wasm instances will have its own set of globals that can’t be accessed by other instances, giving us the foundation for thread-local data. More on this later as well!

Tables in WebAssembly likely also have fun use cases in a threaded wasm world, but I’m not so sure what those are myself and so for now we’ll largely ignore them. Other than that, that should cover most wasm features and how they relate to threads!

Using Threads in Rust

Now that we’ve gone over the highlights of WebAssembly threads proposal, you might feel how I felt when I first read it. Sure all those features sound great, but how is this going to be safely and ergonomically exposed at the language level? Some problems are nicely self-contained like the implementation of Mutex in Rust, but there’s a host of other problems which aren’t quite as self contained like:

  • First up, stacks! LLVM, Rust’s code generator, assumes that it not only gets to use the native wasm stack (which is per instance and hence “thread local”) but also a linear memory stack as well. This means we need a stack pointer into linear memory (which LLVM already conveniently places in a global) which is unique per thread, and someone’s gonna have to allocate those stacks for each thread.

  • Next up, thread local data. We’ve got the foundations of thread locals with global variables, but as mentioned before Rust (and LLVM or LLD) doesn’t actually provide the ability to manipulate or work with custom global variables. How are we going to implement the standard library’s thread_local! macro in Rust?

  • We talked a bit about memory initialization earlier and how we don’t want to reinitialize and wipe out memory, but who’s actually doing that? Presumably all our data segments need to be passive but who’s executing memory.init safely?

  • How are we actually going to spawn threads? Whose responsibility is it to actually create a web worker? Similarly, by what mechanism is the WebAssembly.Module and WebAssembly.Memory getting transferred between workers and instantiated in the right place?

  • When using tools like wasm-bindgen, how does the shim JS make its way to all the workers which have an instance of wasm? This wrapper JS is needed to make calling Rust ergonomic, and we don’t want to bless the main thread too much!

Unfortunately we don’t have answers to all these questions today. These questions are also somewhat intertwined together when we don’t want them to be!

Rust’s vision for WebAssembly on the web is one of interoperability. You should be able to use Rust and WebAssembly without the rest of your application even needing to know. Furthermore, a crate deep in your dependency graph may depend on JS functionality (like an NPM package or web-sys) and you also shouldn’t need to know about that!

It’s not clear if we can maintain this vision for threads on the web platform right now. This is where I’d love to brainstorm with others and/or get help and thoughts about this. The threads proposal isn’t stable after all, and there’s in theory lots of runway for us to figure out something to help us out!

I’d hate to leave you with a cliffhanger like this, though! While not all of the above questions have great answers today, I’ve been working on at least a functioning solution to many of the issues in wasm-bindgen specifically. Let’s take a look at that and see if we can actually demo threads and WebAssembly today!

Threads and wasm-bindgen

The wasm-bindgen tool is composed of two halves. One half is a procedural macro, the #[wasm_bindgen] attribute, which is expanded and runs at compile time. This generates shims in your Rust code and otherwise preps the final binary for the second half, the wasm-bindgen CLI. The CLI tool wasm-bindgen is uniquely positioned to do all sorts of crazy transformations on the WebAssembly module (and it already does!).

The WebAssembly binary format is well specified and surprisingly easy to manipulate. The wasm-bindgen CLI tool is currently using the excellent parity-wasm crate for parsing WebAssembly, which makes it a breeze for wasm-bindgen to do fancy transformations. (and more news on this soon, an even breezier solution is in the works too!)

With the CLI tool and parity-wasm we’re freed from the “shackles” of LLVM (aka it’s easier to experiment in tooling than in LLVM itself) and have access to the full feature set of WebAssembly. Let’s knock out some of those above questions with this newfound power.

Injecting thread-local globals

Although LLVM/LLD don’t currently have the ability to emit custom global variables, we do in wasm-bindgen! This is an easy method to implement thread-local storage, so let’s have wasm-bindgen inject two globals:

  1. First, a thread ID. Thread IDs can be useful for a number of applications, but we’re specifically interested in the ReentrantMutex of the standard library right now, which needs to know which thread is which to know when reentrant locks are used.

  2. Next, a TCB slot. A TCB is a “thread control block” and is typically used to store an allocated structure in threading runtimes. This allocated structure is an entry point for lots of other runtime-related functionality, but for now we’ll primarily use this as storage for user-defined thread-local values. Or, in other words, this is how we’ll implement thread_local!.

It’s easy enough to add two global variables of type i32 to the wasm module, but we need to manage them too! Someone still has to actually allocate the thread ID, and we also need to be able to access it.

For this let’s take another trick of out wasm-bindgen’s playbook, rewriting function calls. We define that a function imported like this:

#[link(wasm_import_module = "__wbindgen_thread_xform__")]
extern {
    fn __wbindgen_thread_id() -> u32;

is actually magically turned into get_global $thread_id. The call instruction actually has a 1:1 replacement with get_global, so the rewriting here is super simple! We can use similar “intrinsics” like __wbindgen_tcb_get and __wbindgen_tcb_set for getting/setting the TCB as well.

Next up, let’s figure out how to initialize this thread ID global.

Where to start?

We saw earlier that WebAssembly provides a start function which is automatically invoked whenever a module is instantiated, and in a multithreaded world this is per-thread initialization. That’s actually exactly what we want for thread ID initialization as well as other aspects!

Using wasm-bindgen we can solve a good number of the above problems with an injected start function. We can even call the previous start function when we’re done to preserve semantic equivalence! Our injected function will perform these steps:

  1. Atomically increment an injected global thread ID counter. We reserve 4 bytes of space in linear memory for this in wasm-bindgen, and this address will keep track of all threads that have ever been. The result of this atomic addition can then be stored in our thread ID global, meaning we’ve just allocated and initialized our thread ID!

  2. We know we’re the first thread (the main thread) if our thread ID is zero. This is a great time to initialize memory, so wasm-bindgen can flag all our data segments as passive, and if our ID is 0 we can call memory.init.

  3. If our thread ID isn’t 0, then we know that we’re a spawned thread. LLVM already arranged for there to be a global for our stack pointer, but its initial value is only valid for the main thread. To keep going we need to set this up. To allocate a stack we can leverage the convenient memory.grow instruction, a quick-and-dirty way to allocate memory without using the standard library’s actual memory allocator (which to invoke would in turn need a stack). Once we’ve got a stack we can update our stack pointer global, and we should be good to go!

  4. Finally, if a previous start function was available, we can delegate to it at this point and call it.

Ok we’re making some good progress! By assuming that exactly the same WebAssembly.Module is used on all threads, an injected start function can cover a lot of ground to making threads and was easy to use.

Managing WebAssembly.Memory

By default all Rust-compiled wasm binaries will export the memory that they define. This means that the instantiation of a wasm module will automatically create an instance of WebAssembly.Memory and make it available for use. This is incompatible with threads, however, where we want all modules to use the same instance of memory!

Instead we’ll need to arrange for memory to be imported not exported. That can be a bit of a pain to set up and work with, though, so wasm-bindgen can continue to take care of instantiation in the JS bindings so users don’t have to worry about it.

Note that for now LLVM/LLD also don’t currently implement a memory object that’s flagged as shared, so as a minor detail wasm-bindgen can take care of this too.

Sharing JS shims, spawning Workers

This is where the story with wasm-bindgen takes a bit of a nose-dive into “this particular strategy no longer seems long-term viable” territory. The last few things we need to take care of is actually spawning the web workers and somehow getting the WebAssembly.Module and WebAssembly.Memory onto each worker.

I initially attempted to see if we could do this with Webpack as it’d be awesome to have a story for bundlers for large-scale integration. Unfortunately I ran into a few snags like you can’t access the WebAssembly.Module and it also wasn’t clear to me how workers would use a different instantiation path that would onmessage to wait for the module/memory and then instantiate after receiving. Fear not, though, I’m sure we’ll figure out a bundler story for this one way or another!

Next I turned to wasm-bindgen’s --no-modules option to see if something could work. Currently it exports a global (named wasm_bindgen) which is a function that takes the path to the wasm file to instantiate. I extended it to take either this or a WebAssembly.Module instance (along with WebAssembly.Memory). That way when given a path it can create memory and do fetch/instantiate, but with a WebAssembly.Module it can avoid the fetch and use the provided memory to instantiate.

Since --no-modules makes it setting up everything pretty manual anyway it was easy enough to have the main thread work as usual, provide accessors for the module/memory, spin up web workers, and post the module/memory to each worker. Inside the workers we can import the --no-modules generated JS, wait for the message, wait for instantiation, and then start doing some work.

All-in-all this setup made it so something could work. This definitely isn’t a long-term solution as there’s no path to use bundlers or runtimes like node.js yet. We’ll surely flesh out all these details before wasm threads are stable though!

Demo: Raytracing

Phew! That was quite a lot of information and background, but hopefully you’ve got a better idea about what the threads proposal is and some ideas of how we can leverage it in Rust and wasm-bindgen. Let’s get to the good stuff now.

We initially strived to have a cool Mandelbrot Set rendering using Rayon, but unfortunately the limitations with spawning Web Workers meant we couldn’t use Rayon. Coupled with my own short attention span and lack of understanding about Mandelbrot, I switched to raytracing!

Having not worked with raytracing in years I googled around to see if there were any existing Rust raytracers I could try out. My favorite one I found unfortunately requires nightly and last compiled mid-2017, but @bheisler had an awesome tutorial around early-2017, and the associated code still compiles and works today (yay!). After a few inconsequential modifications I was able to use the project as is on the web.

As a side note, this is one of the awesome aspects of Rust and Cargo. It took no time at all to find a raytracer, integrate it, compile it to wasm, and run in a browser.

With raytracing (or at least how this raytracer worked) it’s an embarrassingly parallel task as all pixels of an image are rendered entirely independently of all others. This meant we could rig up a way to fan out work for pixels amongst worker threads pretty easily.

The last piece I thought would be pretty cool for this demo is to see a progressive rendering to see what an image looks like as it’s being rendered. Every so often the main thread will request an update of the worker threads, and they’ll send an ImageData to the main thread which can be rendered to a canvas:

Demo preview

Check out the raytracing demo for yourself! or browse the code online

For this demo remember that this is a lot of unstable and nightly technology. It only works in Firefox (as of this writing) as other browsers don’t implement memory.init instructions yet.

You’ll find a giant JSON blob on the left which is a description of the scene to render. Right now it’s a pretty simplistic raytracer so it only supports planes and spheres, but you can move things around, add spheres, etc. If you’re willing I’d love to get some help to implement more complicated renders as well.

Future Work

While we’ve gotten to the point of making a demo, we’ve still got a good deal of work to do! These are some of the highlights of the remaining tasks.

Main Thread Disallows atomic.wait

The main thread of the browser cannot execute the atomic.wait instruction, it will unconditionally throw an exception if this is executed. This means that, by default, mutexes will not work when contended on the main thread! Additionally, it means that the only way currently to synchronize with the main thread is postMessage in a worker.

This situation is especially exacerbated by the fact that Rust’s global allocator, dlmalloc, is globally synchronized. This means that if your main thread allocates memory, it may occasionally throw an exception if contended! This is actually a bug in the demo above today too!

I’ve opened an issue on the threads proposal repo to discuss this, and hopefully we can figure out a reasonable workaround for the main thread to at least still allocate memory! So far I’ve learned about a proposal for Atomics.waitAsync which is a second mechanism to wake up the main thread. There’s also thoughts about a custom allocator which is largely lock-free but falls back to memory.grow on the main thread during contention. In the meantime though this makes it very difficult for a main thread to use arbitrary libraries on crates.io, as they’d have to be audited for any synchronization.

A “workaround” we may implement for Rust would be to simply spin instead of atomic.wait in the mutex implementation if the thread id is 0. Apart from being a bad way to synchronize, it also bakes in that the first instantiation was always on the main thread, which may not always be true!

Threads Exit isn’t implemented

Right now in the model here for Rust there’s not really a concept of a thread exiting. This means that if a thread does actually exit (aka the worker is gc’d) then it leaks memory allocations like:

  • The thread’s stack (it’s never reclaimed or reused)
  • All data in thread local storage (Rust doesn’t register destructors)

Eventually we’ll need to add the concept of thread exit so we can correctly handle this situation and reclaim resources for reuse later. This may be a case where the WeakRef proposal could help out though by automatically running a thread exit when js objects are gc’d.

Stack overflow is bad again

When laying out linear memory, LLD will by default place static data first and then the main thread’s stack. This has a problem, though, that if the main thread has a stack overflow it’ll silently corrupt all static data! To fix this problem we pass --stack-first to LLD which places, well, the stack first in memory, causing a stack overflow to trap because of an out of bounds memory access.

We unfortunately don’t have this luxury for all worker threads, though. Worker threads suffer the same problem as before where if a stack overflow happens it’ll silently corrupt heap or static data.

One option available to us is to insert a prologue (by LLVM or wasm-bindgen) to check whether we have enough linear stack space before decrementing the stack pointer (trapping if we don’t), but it’s not clear what sort of performance impact this change might have as it affects all functions! Alternative solutions are likely to require new wasm features like unmapping memory to force operations to trap.

You can make sausage too!

If you’re curious about how the sausage is made and/or how you can help out, here’s a list of changes made to build this demo and some helpful repositories!

If you’ve gotten this far then you can probably tell that the threads story in Rust still needs some work! We’d love to have your help and feel free to drop by at #rust-wasm on Mozilla’s IRC, #wg-wasm on Discord, or follow along on GitHub with either wasm-bindgen or wasm-pack.