cbuffer-load-in-loop

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/cbuffer-load-in-loop.md contains the canonical detection logic + GPU reasoning.

TL;DR

Cbuffer data resides in a dedicated constant buffer cache — a small, high-bandwidth, read-only cache separate from L1/L2 texture and UAV caches. On AMD RDNA and RDNA 2/3, the constant cache (also called the scalar cache or K-cache) is accessed via the scalar register file (SGPRs). The hardware is architecturally designed for the case where every lane in a wave reads the same cbuffer value simultaneously, which it does for any truly uniform constant: one scalar load fills an SGPR, and that SGPR value is broadcast to all 32 or 64 lanes without consuming per-lane VGPR space. In practice, the cbuffer value is loaded into an SGPR once per wave (or per draw call in the driver's implementation) and cached there — no repeated cache requests occur.

What the rule fires on

Reads from a cbuffer (or ConstantBuffer<T>) field — or any arithmetic expression whose operands are exclusively cbuffer fields and numeric literals — inside a loop body, when the expression is loop-invariant: it does not depend on the loop induction variable or any value defined inside the loop. The rule fires on repeated use of the same cbuffer field or constant-folded cbuffer expression within a loop (e.g., Sigma * Sigma, NearZ, FarZ - NearZ) where the field itself is not indexed by the loop counter. It does not fire when the cbuffer read is through a loop-counter-dependent index (e.g., LightArray[i].position), or when the field's value changes inside the loop body (which cannot happen with cbuffer reads, but may happen with local variable aliasing).

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 cbuffer-load-in-loop.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.