Investigate support for WebGPU Immediates (push constants)

[mvaligursky]

Feature

Investigate Immediates (a.k.a. push/root constants), shipped in Chrome 149/150.

Immediates let you pass small, frequently-changing data directly to a shader without creating a GPU buffer or bind group. The values are injected straight into the pass encoder, avoiding a memory write and a bind-group lookup per update.

API surface:

• WGSL — declare via the immediate address space:

  var<immediate> color: vec4f;

• JS — set before a draw call:

  passEncoder.setImmediates(/*rangeOffset=*/0, new Float32Array([1, 0, 0, 1]));
  passEncoder.draw(3);

• Pipeline layout — layout: 'auto' infers immediateSize from the WGSL module; explicit layouts declare it.
• Capability detection — navigator.gpu.wgslLanguageFeatures.has('immediate_address_space').

Benefit

• Reduce CPU overhead for per-draw data that changes every draw call (object IDs, small per-draw transforms/params) by skipping uniform-buffer writes and bind-group rebinds.
• A potential fast path for the kind of redundant state churn discussed in Avoid Redundant State Setting in WebGPU backend? #7656 / WebGL renders much faster than WebGPU #7880, where WebGPU's per-draw binding cost lags WebGL2.

Notes

• Chrome 149/150 only at the moment; WebGPU-only with no WebGL2 equivalent, so it requires capability detection and a graceful fallback to uniform buffers.
• Strictly for small values — large arrays, complex lighting structures, and big matrices should stay in uniform/storage buffers (push-constant size budget is small).
• Requires plumbing the immediate address space through shader generation and the WebGPU pipeline-layout/draw path.
• Ref: https://developer.chrome.com/blog/new-in-webgpu-149-150#immediates