D863 - An instanced primitive draw runs a workgroup per instance

Status: decided Date: 2026-10-14 known_by: published

A draw of more than one instance runs its primitive shader once per instance, each instance a mesh workgroup of its own: the dispatch is a workgroup per draw across and one per instance down, and an instance finds itself in WorkgroupId.y. A seeded primitive shader (D730) receives it in v8, the instance id radeonsi's primitive shaders read after the vertex id and two user VGPRs (si_shader_args.c 363-371), in every vertex thread. The draw's instance count travels in its words (DRAW_DATA_INSTANCES_WORD), and a batch (D718) takes only draws of one count, since one dispatch runs every draw in it that many times.

Why: the geometry engine launches a draw's instances as separate subgroups, each with the same vertices and its own instance id, so a workgroup per instance is the host's form of the same thing. PPSA21564 draws a full-screen pass per slice of an array this way, its layer taken from v8 (D859), and refused before any of its frames could be drawn.

Rejected:

• Repeating the draw once per instance on the host: a pipeline bind and a dispatch per instance where one dispatch does.
• Instances as workgroups along x beside the draws: a batch's draws read their words by x (D718).