Wasm-bindgen: Native JS Float32Array binding to Vec<f32>

[Roba1993]

Hi,

first of all thank you for the great crate!
I actually deep dive into the WebGL2 bindings and in the documentation the following way to get Values to native Float32Array is mentioned:

    let vertices: [f32; 9] = [-0.7, -0.7, 0.0, 0.7, -0.7, 0.0, 0.0, 0.7, 0.0];
    let memory_buffer = wasm_bindgen::memory().dyn_into::<WebAssembly::Memory>().unwrap().buffer();
    let vertices_location = vertices.as_ptr() as u32 / 4;
    let vert_array = js_sys::Float32Array::new(&memory_buffer).subarray(
        vertices_location,
        vertices_location + vertices.len() as u32,
    );

I have the feeling, that the data is cloned twice here?
Additionally this syntax and conversion is really confusing, especially because JS is already handling a native f32.

Is there a way to improve?
If I can get some high level guidance, I would also try to implement it on my own.

Thanks,
Robert

[jsheard]

This came up in https://github.com/rustwasm/wasm-bindgen/pull/965, the buffer creation could be improved but js_sys is missing the necessary 3-parameter typed array constructor. We should be able to do something like

let vert_array = js_sys::Float32Array::new_with_offset_and_length(&memory_buffer, vertices.as_ptr() as u32, vertices.len() as u32);

which creates a view of wasm memory without copying it.

[alexcrichton]

Thanks for the report! The first snippet here is using subarray which avoids copying data, so the vertices array isn't copied in memory and vert_array is just a sub-view into the same data. The syntax is indeed a bit confusing and hopefully won't always be necessary!

I've submitted https://github.com/rustwasm/wasm-bindgen/pull/970 to bind more constructors to make it a bit moer ergonomic in this case.

[Roba1993]

@alexcrichton Thank for this fast update! But I haven't spotted a f32 Array in your code commit. Is this up to come, or did I miss something here?

[alexcrichton]

Oops I left those out by accident! So many arrays to update...

[Roba1993]

Thanks for doing this :)

[Roba1993]

Hey,

I tried the changes in my crate and change to the actual GitHub master branch. But was-bingen fails with the following message:
error: failed to extract wasm-bindgen custom sections | caused by: failed to decode what looked like wasm-bindgen data: missing field

Has anyone experienced the same? I would like to validate the code change from Alex and close this issue.

Thanks,
Roba

[jsheard]

When you're using the git version of wasm-bindgen crates you also need to install the git version of wasm-bindgen-cli, otherwise the version mismatch usually breaks things. Try this:

cargo install wasm-bindgen-cli --force --git https://github.com/rustwasm/wasm-bindgen

[Roba1993]

Okay guys, I go it and the code is now the following:

    let vertices: [f32; 9] = [-0.7, -0.7, 0.0, 0.7, -0.7, 0.0, 0.0, 0.7, 0.0];
    let memory_buffer = wasm_bindgen::memory().dyn_into::<WebAssembly::Memory>().unwrap().buffer();
    let vert_array = js_sys::Float32Array::new_with_byte_offset(&memory_buffer, vertices.as_ptr() as u32);

In my opinion we should make this even easier, like the following:

let vert_array = js_sys::Float32Array::new_from_array(&vertices);

What do you think?

[alexcrichton]

@Roba1993 that was the goal of https://github.com/rustwasm/wasm-bindgen/pull/918 and topic of https://github.com/rustwasm/wasm-bindgen/issues/811 as well, and I think we can probably just do some more work on our end to make that possible!

(although if that's the final issue here we may want to close in favor of https://github.com/rustwasm/wasm-bindgen/issues/811)

[Roba1993]

Hi alex,

I think #811 is the other way around, but yes I'm with you.
I close this and continue to look at the magic which get created here.