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Comment by turnsout | Hacker News Reader

WebAssembly "threads" depend on SharedArrayBuffer, which is not available on Safari. Whatever your opinion of Apple or web feature adoption in Safari, this definitely puts a damper on things if you need to support mobile browsers.

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Web workers already exist and there's wasm threading libraries that target just web workers.

https://github.com/w3reality/wasm-mt

It's just that with sharedarraybuffers and atomics we unlock better performance and can match APIs of native OSes. But that's okay, we can still have a fallback for Safari (specially if we can architect the app so the threads only communicate by passing messages) and it shouldn't be a big deal.

Yes, I've been using WebWorkers for 10 years. My note was intended to give folks a heads-up before they go rewriting their web app in C++ or Rust to take advantage of concurrency via SharedArrayBuffer.

Even on the beta of the next Safari?

According to https://caniuse.com/sharedarraybuffer it's default disabled on Safari due to spectre/meltdown, but can be enabled.

In addition to SharedArrayBuffer, it looks like the atomics themselves are in-progress on Safari[1]. They don't work for me on Safari Technology Preview, FWIW.

As far as I can tell, “threads” from a WebAssembly implementation's perspective is just the combination of SharedArrayBuffer and atomics; support for the actual threading comes from Web Workers which are already a separate standard. Compilers like Emscripten then use the combination of the three to implement threading.

[1] https://webassembly.org/roadmap/

What doesn't work about them?

I use `Atomics.wait` and `Atomics.compareExchange` and maybe a couple other APIs and it works well on Safari Technology Preview (if you enable SharedArrayBuffer).

Ah, I should have clarified that I mean the assembly instructions for atomics, rather than the JavaScript API. I.e. the opcodes listed here: https://github.com/WebAssembly/threads/blob/master/proposals...

Oh right, that makes sense in the context of this thread :)

> As far as I can tell, “threads” from a WebAssembly implementation's perspective is just the combination of SharedArrayBuffer and atomics;

FWIW that's exactly what the linked post describes :)

SharedArrayBuffer was disabled in Safari 11 due to Spectre and Meltdown vulnerabilities. https://bugs.webkit.org/show_bug.cgi?id=212069 re-enabled it behind a flag, and talks about making it public "Once cross-origin isolation is done". However, it links to https://bugs.webkit.org/show_bug.cgi?id=215407 as representing when cross-origin isolation is complete, which as I understand it hasn't seen any activity in the past year.

As far as I can tell, it's not enabled in the beta, but looks like it's available on the Safari Technical Preview on the Mac behind a flag:

https://developer.apple.com/safari/technology-preview/releas...

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