groupshared-overwrite-before-barrier
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/groupshared-overwrite-before-barrier.md contains the canonical detection logic + GPU reasoning.
TL;DR
GPU memory ordering for groupshared / LDS is barrier-relative: a write becomes visible to other lanes in the workgroup only after a GroupMemoryBarrier* is executed by both the writer and the reader. With no intervening barrier, a write that is shadowed by a later write from the same thread is unobservable to every other thread — the second write completely supersedes the first in the LDS bank, and no consumer wave ever sees the original value. On AMD RDNA 2/3 the LDS write coalescer may fold back-to-back same-address stores into a single transaction, but the wasted instruction issue still occupies the LDS write port. On NVIDIA Ada the shared-memory atomic / store unit serialises the two writes; on Intel Xe-HPG the SLM behaves similarly. In every case the first write costs a full LDS write cycle and produces zero observable effect.
What the rule fires on
A write to a groupshared cell gs[i] followed by a second write to the same cell from the same thread on every CFG path, with no GroupMemoryBarrier, GroupMemoryBarrierWithGroupSync, AllMemoryBarrier, or equivalent intervening synchronisation that could have made the first write observable to other threads. The classic case is gs[gi] = init_value; gs[gi] = computed_value; in a straight-line block. The rule also fires when the second store is conditional but dominates the join (every path from the first write reaches the second).
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 groupshared-overwrite-before-barrier.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- workgroup overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.