Add small compute examples illustrating new WGSL primitives for AI

[kenrussell]

@beaufortfrancois requested that some small examples be published here showing how to use the new WGSL primitives aimed at AI/ML workloads: shader-f16, DP4A, and soon, subgroups. Could we consider this?

Not sure what would be the most compelling - perhaps something with some visual output, and running a microbenchmark against the fallback WGSL code, assuming the feature is actually supported?

[kenrussell]

CC @dneto0

[dneto0]

For dp4a:

• They're good for ML image processing: segmentation, object identification. But it's involved to put an ML model in the samples.
• Maybe we can do something really simple, like sobel filter on a grayscale image.

[kainino0x]

dp4a accelerates matrix multiplication right? Even a basic matrix multiplication sample could be enough for a sample. The sample could even just display some text. But sobel or any other simple convolution would make it more compelling.

[kainino0x]

Also it should have a toggle to enable/disable dp4a and hopefully see some performance improvement.

[cmhhelgeson]

Is dp4a available in the current version of WebGPU?

[austinEng]

dp4a is available starting in Chromium M123. So today, that would be Chrome Beta and newer.

[cmhhelgeson]

If possible, I'd like to try my hand at this issue, at least for the next week (sorry about the timeline, day job is gonna day job). Sobel filter is a good place to start I think.

EDIT: 'GPGPU Compute Category' or 'Features Category'?

[cmhhelgeson]

Just want to make sure I understand the assignment, the intended use of dp4a here. Instead of writing, say, this for our sobel filter.... (below is in pseudo-wgsl)

(Pixels loaded from texture using textureLoad and global_invocation_id
let result =  1 * pixel1.r
        + 2 * pixel2.r
        + 1 * pixel 3. r
        - 1 * textureLoad(inputTexture, vec2<u32>(id.x + 1, id.y - 1), 0).r

textureStore(output, id.xy, result)

We should do something like this?

let pixelPack = pack4xU8Clamp(pixel1.r, pixel2.r, pixel3.r, pixel4.r)
let kernelPack = pack4xU8Clamp(1, 2, 1, -1)
let result = dot4U8Packed(pixelPack, kernelPack);
textureStore(output, id.xy, result)

[dneto0]

Also it should have a toggle to enable/disable dp4a and hopefully see some performance improvement.

I suspect a Sobel filter is simple enough that it's limited by memory bandwidth instead of computation.
So I wouldn't get hung up on perf improvement for this sample.

[cmhhelgeson]

Somebody else should take this on, I understand the functionality, but I'm struggling with the quantization of the dp4a result back to something usable.