vrs-incompatible-output
Status: stub. The full-length analysis is queued for a v1.0.x patch release per ADR 0018, section 5, criterion #6. The companion rule page at docs/rules/vrs-incompatible-output.md contains the canonical detection logic + GPU reasoning.
TL;DR
VRS is a coarse-shading optimisation: NVIDIA Turing and Ada Lovelace expose Tier 1 (per-draw) and Tier 2 (image-based + per-primitive) shading rates that let one PS invocation cover up to a 4x4 pixel region. AMD RDNA 2/3 implements the same surface as Variable Rate Shading at the rasterizer, with hardware that broadcasts the single shaded result across the coarse footprint. Intel Xe-HPG (Arc/Battlemage) added VRS Tier 2 with the same semantics. The whole point is to amortise PS work across multiple raster samples — the wave executes one set of derivatives, one set of sample fetches, and one ALU sequence per coarse fragment.
What the rule fires on
Pixel-shader entry points that write per-sample outputs (SV_Coverage, SV_SampleIndex, an [earlydepthstencil]-marked SV_Depth, or per-sample interpolated inputs marked sample) while a Variable Rate Shading (VRS) shading rate coarser than 1x1 is applied to the draw. The rule uses Slang reflection to enumerate the entry's output semantics and pixel-shader attributes, then flags combinations that the D3D12 VRS specification calls out as forcing the runtime to silently drop the shading-rate request and revert to per-pixel shading. Common offenders: a shader that writes both a coarse colour and SV_Coverage, or a deferred g-buffer pass that uses VRS Tier 2 image-based shading rates while still emitting MSAA-aware per-sample inputs.
See the What it detects section of the rule page for the full pattern definition.
Why it matters
The full GPU-mechanism analysis lives in the Why it matters on a GPU section of the companion rule page.
Examples
The bad / good code snippets are kept canonical on the rule page; see vrs-incompatible-output.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- vrs overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.