[wibagusto]

Correct me if I’m wrong but the memory copying issue is not an issue if you pass an array buffer into WASM from JavaScript. In that scenario there’s no data copying. E.g. similar to how you’d pass the canvas data to WASM for direct manipulation.

[azakai]

No, this is an issue currently, both for network data and canvas data.

All wasm instructions can do is read and write from the wasm Memory that the wasm is initialized with. They can't even refer to separate things like a new ArrayBuffer from JS. So you do need to copy.

Newer wasm additions like reference types allow an ArrayBuffer to be referred to inside wasm, but only as an opaque reference to the entire thing (an externref). There is still no ability to actually read and write from it inside wasm.

The solution to this is BYOB ("bring your own buffer") APIs, which JS is adding. They are experimental atm though. Here is the relevant one here:

https://developer.mozilla.org/en-US/docs/Web/API/ReadableStr...

Note how you pass in a view to the JS API. That can be a view into the wasm memory, letting the browser directly write data into there, and then wasm can operate on it immediately.

[wibagusto]

So the workaround is to initialize WASM with an array buffer which allows direct manipulation of the data in WASM it seems. The canvas bytes can be used in WASM so long as the bytes are passed in at initialization.

In that case, JavaScripts only duty is to pack input data into the byte array and pass control to WASM which writes to an output buffer (also shared byte array) and passes control back to JavaScript to process result avoiding an unnecessary data copy.

You’d have to have some types in JS which can process the data using bit shifts and whatnot though in a lower level way.

[Matthias247]

So how do you fill that arraybuffer from Javascript datatypes (e.g. strings which describe the request)? The answer is "by copying the relevant data" - which is exactly what the bridging code between JS and WASM does. You can only avoid that if the source data is already plain byte arrays and not javascript objects.