wavereadlaneat-constant-zero-to-readfirst
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/wavereadlaneat-constant-zero-to-readfirst.md contains the canonical detection logic + GPU reasoning.
TL;DR
WaveReadLaneAt(x, lane) is the general lane-index broadcast: every active lane in the wave receives the value of x from the lane numbered lane. The hardware contract is general-purpose — lane may be any value in [0, WaveGetLaneCount()), may differ across waves, and is required to be uniform across the active lanes for the result to be defined. Because lane is a runtime input from the compiler's perspective, the lowering must emit the lane-index broadcast in its general form: on AMD RDNA 2/3 that is v_readlane_b32 (a SALU instruction) plus the bookkeeping required to broadcast back to VGPRs; on NVIDIA Turing and Ada Lovelace, the SHFL.IDX shuffle with a per-lane index argument; on Intel Xe-HPG, the SLM-backed lane-index path. None of these are expensive in absolute terms, but they all encode "the lane index is variable" and the compiler cannot strip the index plumbing.
What the rule fires on
Calls of the form WaveReadLaneAt(x, 0) where the second argument is the integer literal 0 (or a static const integer that folds to zero at parse time). The match is on the literal lane index, not on the value of x. The rule does not fire on WaveReadLaneAt(x, K) for any non-zero compile-time constant K (that case has its own portability concern when wave size is not pinned and is queued as a Phase 3 reflection-aware rule, wavereadlaneat-constant-non-zero-portability), and does not fire on WaveReadLaneAt(x, dynamic) calls where the lane index is a runtime expression.
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 wavereadlaneat-constant-zero-to-readfirst.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- math overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.