sv-depth-vs-conservative-depth
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/sv-depth-vs-conservative-depth.md contains the canonical detection logic + GPU reasoning.
TL;DR
Early depth-stencil (early-Z, early-S) is a hidden-surface optimisation that runs the depth/stencil test before the pixel shader executes. On NVIDIA Turing/Ada Lovelace and AMD RDNA 2/3, the rasterizer hands eligible fragments to the PixelHash unit, and the GPU uses the rasterised depth to cull occluded pixels before allocating a wave. Intel Xe-HPG (Arc Alchemist, Battlemage) implements the same early-Z stage. When SV_Depth is written from PS without any ordering hint, the hardware cannot use the rasterised depth for the test — the actual depth might be anywhere — so it disables early-Z and runs the full PS to compute the depth, then tests. For a draw that would have been 50% occluded, this doubles the PS workload.
What the rule fires on
A pixel-shader entry that writes SV_Depth (a per-pixel depth override) without using one of the conservative-depth variants SV_DepthGreaterEqual or SV_DepthLessEqual when the value being written is monotonically related to the rasterised depth. The rule uses Slang reflection to identify the depth-output semantic on the PS entry, then walks the assignment expression: writes of the form depth + bias, depth - bias, max(depth, k), or min(depth, k) are obvious conservative-direction patterns. Plain SV_Depth defeats early depth-stencil rejection on every IHV; the conservative variants preserve it under the matching ordering.
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 sv-depth-vs-conservative-depth.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.