Currently the subgroup-id proposal says

Ideally this built-in would be considered subgroup uniform, but our uniformity analysis occurs at the workgroup level.

TODO: Do we want to try and introduce subgroup uniformity? APIs speak little of the guarantees and testing shows that guarantees do not match user expectations.

As currently proposed, subgroup_id is not considered uniform by the uniformity analysis, which understandably will defy shader author expectations. To spell out the options:

Option 1: Keep things as is: This will confuse authors, and cause some (frustrating) conservatism in the analysis.

Option 2: Introduce subgroup uniformity.

• This will increase the complexity of an already extremely subtle part of the spec.
• Even so, subgroup uniformity behaviours of implementations do not match developer expectations, and we have no line of sight to achieving this kind of portability across all platforms. (