D688 - A primitive shader translates to a mesh shader

Status: assumed Date: 2026-09-14

A next-generation-geometry primitive shader, whose one wave declares its vertex and primitive counts with an allocation message and then exports primitives and vertices, translates to a host mesh shader. The allocation message's m0 carries vertices in its low twelve bits and primitives above them.

Why: a mesh shader's output declaration, primitive indices and per-vertex writes correspond one to one with the allocation message and the primitive and vertex exports. Primitive shaders exist to cull, so a real one exports fewer primitives than it was given with a count computed at run time; a vertex-shader translation would silently draw primitives the guest culled.

Rejected:

• A vertex shader dropping the primitive half: correct only for an identity primitive export, silently wrong otherwise.
• Pattern-matching the identity case: fits one shader and breaks on the next that differs by a register.
• Compute writing vertices for a second draw: kept as the fallback where the mesh extension is unavailable.