D100 - Fidelity levels are a differential oracle

Status: decided Date: 2026-09-26

Three wavefront models exist: Lane (one invocation per lane, no mask), Wavefront (one invocation simulates every lane) and Subgroup (the lane model plus a mask built with a subgroup ballot). Generated programs run at two levels and must agree. A pinned level that cannot run is an error, never a substitution.

Why: disagreement between two levels localises a bug to one shader and one instruction with no hardware reference. The subgroup level is the lane model with and without a mask, so it shares code rather than duplicating it. Levels differ in correctness, so a substitution renders something wrong.

Rejected:

• A fallback ladder: silently drops a level.
• A separate third model: duplicates the lane model.