Referencing arrays in JS

[CarstenHoyer]

I have looked at:
#105
#263
WebAssembly/design#1231

But I cannot make this thing work as I expect it to.

Given the below code, I would expect the console.logs to output [1, 2, 3, 4), [100, 2, 3, 4) then [200, 2, 3, 4). But what I see is [1, 2, 3, 4), [100, 2, 3, 4) then [100, 2, 3, 4). That indicates that my processF32Array is wrong. I tried different ideas, but none of them seems to reflect any changes on the JS side.

I could use a little help.

JS

const module = await loader.instantiateStreaming(fetch("untouched.wasm"), myImports);
const { processF32Array } = module
const myArray = [1,2,3,4];
const myArrayId = module.__retain(module.__allocArray(module.FLOAT32ARRAY_ID, myArray))

const myArrayView = module.__getArrayView(myArrayId);
console.log(myArrayView)

myArrayView[0] = 100;
console.log(myArrayView)

processF32Array(myArrayId);
console.log(myArrayView)

WASM:

export const FLOAT32ARRAY_ID = idof<Float32Array>()

export function processF32Array(ptr: i32): void {
  const val: f32 = 200
  store<f32>((ptr) << 2, val)
}

[MaxGraey]

Try this one:

export function processF32Array(ptr: i32): void {
  const val: f32 = 200
  store<f32>(ptr + (0 << 2), val)
}

[CarstenHoyer]

Thanks. Tried it. It stays the same [100, 2, 3, 4].

[CarstenHoyer]

Actually, if i make a slight change, I can see the array values get reset, so that indicates that it is not the same array I am working on in JS/WASM.

JS:

const module = await loader.instantiateStreaming(fetch("untouched.wasm"), myImports);
const { processF32Array } = module
const myArray = [1,2,3,4];
const myArrayId = module.__retain(module.__allocArray(module.FLOAT32ARRAY_ID, myArray))
let myArrayView = module.__getArrayView(myArrayId); // NOT A CONSTANT
console.log(myArrayView)
myArrayView[0] = 100;
console.log(myArrayView)
processF32Array(myArrayId);
myArrayView = module.__getArrayView(myArrayId); // GET THE VIEW AGAIN
console.log(myArrayView)

Logs:
Float32Array(4) [1, 2, 3, 4]
Float32Array(4) [100, 2, 3, 4]
Float32Array(4) [1, 2, 3, 4]

[MaxGraey]

You should invalidate your myArrayView after processF32Array. It could be done via calling myArrayView = module.__getArrayView(myArrayId) again after processF32Array

[CarstenHoyer]

Ah, exactly what I just tried above :)

[CarstenHoyer]

I wonder if this is the part i misunderstood?

WASM

export const FLOAT32ARRAY_ID = idof<Float32Array>()

[CarstenHoyer]

No that part looks correct.

[CarstenHoyer]

I am not getting any kind of updates. Below I would expect the console.log to be [200]

JS

const myImports = {
      env: {
        abort(_msg, _file, line, column) {
          console.error("abort called at main.ts:" + line + ":" + column);
        },
        memory: new WebAssembly.Memory({ initial: 256 })
      },
      imports: { }
    }

    const module = await loader.instantiateStreaming(fetch("optimized.wasm"), myImports);
    const { processF32Array } = module
    const arr = new Float32Array([1]);
    const id = module.__retain(module.__allocArray(module.FLOAT32ARRAY_ID, arr))
    
    processF32Array(id);
    const arr1 = module.__getArray(id)
    console.log(arr1)

WASM

export function processF32Array(ptr: i32): void {
  const val: f32 = 200
  store<f32>(ptr + (0 << 2), val)
}

SCRIPT

asc assembly/index.ts -b build/optimized.wasm -t build/optimized.wat -d build/optimized.d.ts --use Math=JSMath --runtime full -O3 --importMemory --sourceMap --validate --measure

[CarstenHoyer]

On the WASM side, it looks like I can do this:

WASM

export function processF32Array(arr: Float32Array): void {
  arr[0] = 200
}

Now to figure out what to do on the JS side.

[MaxGraey]

Yeah, I forgot, you can't access direct to typed array by pointer, it has backing buffer, so you should use:

export function processF32Array(arr: Float32Array, val: f32): void {
  var index = 0;
  store<f32>(arr.dataStart + (index << alignof<f32>), val);
}

[MaxGraey]

I guess before doing such low level things better read this part of documentation. Hope this help you

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.