quadany-quadall-opportunity
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/quadany-quadall-opportunity.md contains the canonical detection logic + GPU reasoning.
TL;DR
Pixel-shader derivatives (ddx, ddy, and the implicit derivatives consumed by Sample) are computed by differencing values across the 2x2 quad of neighbouring pixels. The hardware requires all four quad lanes to be active — even pixels outside the rendered triangle remain active as "helper lanes" specifically to supply derivative samples. When a per-lane if retires some quad lanes (because their condition is false), the derivative inputs from those lanes become undefined: AMD RDNA 2/3 returns implementation-defined values for the derivative tap from a retired lane, NVIDIA Ada produces undefined sampler LOD on the surviving lanes' implicit-derivative samples, and Intel Xe-HPG behaves similarly. The visible artefact is mip-aliasing or seams at the boundary between branch-taken and branch-not-taken regions.
What the rule fires on
A pixel-shader if (cond) whose condition is a per-lane (quad-divergent) boolean and whose body issues at least one derivative-bearing operation — Sample, SampleBias, SampleGrad, ddx, ddy, ddx_fine, ddy_fine, or any function call that transitively invokes one. The opportunity is to wrap the condition in QuadAny(cond), so that whenever any lane in the 2x2 quad takes the branch, all four lanes participate as helpers and the derivative ops have valid neighbour samples. Companion (not duplicate) of the locked ADR 0010 rule quadany-replaceable-with-derivative-uniform-branch — that rule detects the opposite direction (replace QuadAny with a derivative-uniform predicate); this rule detects the forward direction (wrap a plain if in QuadAny).
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 quadany-quadall-opportunity.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.