anyhit-heavy-work
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/anyhit-heavy-work.md contains the canonical detection logic + GPU reasoning.
TL;DR
The any-hit shader runs every time the BVH traversal finds a primitive that the ray's bounding test admits as a candidate hit. For a single ray, this can mean dozens or hundreds of any-hit invocations as the traversal walks past intersecting geometry that may or may not be the closest. On AMD RDNA 2/3 with hardware RT, the BVH traversal unit issues an any-hit shader call per candidate primitive in a hardware-managed loop; the per-invocation cost dominates ray throughput when foliage, hair, or alpha-tested geometry is in the BVH. NVIDIA Turing, Ada, and Blackwell similarly invoke any-hit per candidate; the SER (Shader Execution Reordering) hardware on Ada and Blackwell can group similar any-hit invocations across rays for better SIMD efficiency, but the per-call work still scales with candidate count. On Intel Xe-HPG, the RT block dispatches any-hit on every triangle the ray enters, with the same multiplicative cost.
What the rule fires on
Any-hit shaders (entry points marked [shader("anyhit")]) that perform work beyond the supported lightweight tasks: alpha-mask sampling and IgnoreHit()/AcceptHitAndEndSearch() decisions. The rule flags any-hit bodies containing for / while loops, more than one Texture2D::Sample* call, lighting math (dot products against light directions, BRDF evaluation), TraceRay recursion, or substantial scratch-VGPR usage. The same logic should appear in the closest-hit shader where the cost is paid only once per ray.
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 anyhit-heavy-work.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- dxr overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.