early-z-disabled-by-conditional-discard
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/early-z-disabled-by-conditional-discard.md contains the canonical detection logic + GPU reasoning.
TL;DR
Modern GPUs perform depth/stencil testing at two possible points in the pipeline: before pixel shading (early-Z) or after pixel shading (late-Z). Early-Z is a major performance win: the depth comparison happens in fixed-function hardware that can reject hundreds of fragments per clock, before any shader work, before texture fetches, before any VGPR is allocated. Late-Z runs after the pixel shader has fully executed and can only update the depth buffer at the very end, after committing colour. The cost difference for a heavily-overdrawn scene can be 5-20x: an early-Z pass on a typical opaque deferred prepass at 1080p discards 80-95% of fragments before they cost anything; the same shader running with late-Z pays the full ALU and bandwidth cost for every fragment.
What the rule fires on
Pixel shaders that contain discard (or its alias clip(...) for negative arguments) reachable from a non-uniform branch, when the entry point is not annotated with [earlydepthstencil]. The rule also flags shaders that write SV_Depth from any non-uniform control-flow path, since this likewise inhibits early-Z. The pattern is the silent, default-on case where a single late discard causes the driver to demote the entire shader to late-Z and pay the full cost of shading every covered fragment regardless of its visibility.
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 early-z-disabled-by-conditional-discard.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- control-flow overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.