quadany-replaceable-with-derivative-uniform-branch
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-replaceable-with-derivative-uniform-branch.md contains the canonical detection logic + GPU reasoning.
TL;DR
QuadAny (SM 6.7) is the canonical guard for keeping helper-lane participation alive across a per-lane branch in PS: when any lane in the quad wants to enter the branch, all four enter, and the helper lanes provide the derivative neighbours that texture sampling and ddx/ddy require. The cost is the wave-shuffle that QuadAny issues — typically 2-4 instructions on NVIDIA Turing/Ada Lovelace, AMD RDNA 2/3, and Intel Xe-HPG.
What the rule fires on
A QuadAny(cond) or QuadAll(cond) guard wrapping an if-branch whose body is derivative-uniform — every operation inside the branch either uses no derivatives or operates on values that are constant across the quad. In that case, the surrounding if (QuadAny(cond)) adds no quad-correctness benefit and the simpler if (cond) is sufficient. The Phase 4 branch-shape detection identifies the wrapper pattern; the data-flow analysis verifies that the body has no derivative-bearing operations on quad-divergent values.
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-replaceable-with-derivative-uniform-branch.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- wave-helper-lane overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.