Subgroup Operations

[Motivation]: We should have subgroup operations.
+
<Standard Library>: Common across all three APIs, see #667, #78, and WSL#5.
+
<Performance>: Proven contribution to general purpose algorithms to make them run multiple times faster.
+
<Viable>: There is a safe subset of subgroup operations.
+>
[Extension]
+>
[Shuffle Operations]
+>
[Quad Operations]
+>
[Explicit Operations]
[Extension]: Subgroup operations should be exposed as an extension.
+
<Target>: Subgroup operations are not available on all WebGPU target hardware.
[Host Interface]: Subgroup size control and properties should be exposed to the host API.
-
<Pipeline Properties>: Exact subgroup size can't be queried in DirectX 12.
-
<Size Control>: Only possible for Vulkan with VK_EXT_subgroup_size_control.
[Compute Only]: Subgroup operations should only work in compute kernels.
+
<Hardware>: Restricting to compute only increases market (e.g., Adreno).
+
<Definition>: Operations are better defined for compute. And this makes concern of helper invocations irrelevant.
+>
<Viable>
[Shuffle Operations]: There should be shuffle and relative shuffle operations.
-
<DirectX 12>: DirectX 12 doesn't have shuffle operations.
-
<Narrow Support>: Not having shuffle operations increases support (e.g., ARM).
[Quad Operations]: Quad operations should be their own extension.
-
<Varying Support>: Not ubiquitously supported (e.g., PowerVR).
-
<Quadgroup Extension>: If quad operations are made into their own extension, both its potential market becomes larger and subgroup operations' market grows.
[Explicit Operations]: Operations that take in active mask or lane index should exist.
-
<Undefined Behavior>: Explicit operations such as broadcasting with index depends on successful query of active lanes which can't be guaranteed with targeted underlying APIs.
-
<Avoiding Helps>: Implicitly active operations are useful enough while making concerns of divergence or reconvergence irrelevant.
+>
<Viable>
[Non-Uniform]: We should support the non-uniform subgroup model.
+
<Ubiquitous>: All WebGPU target APIs use non-uniform model.
-
<Ambiguous Divergence>: when invocations in a subgroup are executing "together", how long is that guaranteed?
-
<Ambiguous Reconvergence>: once invocations diverge, what are the guarantees about where you reconverge?
+
Vulkan has weak guarantees, D3D12 and Metal don't have anything.
-
we can just say invocations never reconverge, which matches AMD ISA and CUDA models.
-
<Ambiguous Forward Progress>: how blocks affect progress of other blocks, and invocations within blocks affect progress on other invocations?
+
D3D, Metal, and Vulkan are silent on both of these.
-
<Ambiguous Helpers>: do helper invocations participate in subgroup operations?