groupshared-write-then-no-barrier-read
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-write-then-no-barrier-read.md contains the canonical detection logic + GPU reasoning.
TL;DR
The D3D12 compute model guarantees that within a thread group, all threads share LDS / groupshared memory, but it does NOT guarantee any ordering between threads' memory operations unless an explicit barrier is issued. On AMD RDNA 2/3, threads in a wave run in lock-step, but threads in different waves of the same group are scheduled independently and can be hundreds of cycles apart at any given instruction; without a barrier, a read from another wave's slot may return the previous frame's value, the LDS-uninitialised pattern (zero on AMD, undefined on NVIDIA), or a torn write halfway through a non-atomic store. On NVIDIA Turing / Ada, the same applies across warps in a thread block: the SM scheduler issues warps in arbitrary order and the L1/SHM coherence boundary is the explicit barrier instruction. On Intel Xe-HPG, the EU thread scheduler likewise serialises across barrier points only.
What the rule fires on
Compute shader code paths where a thread writes a groupshared location and a different thread (or the same thread on a subsequent iteration of a loop with cross-iteration dependence) reads from the same array region without a GroupMemoryBarrierWithGroupSync or AllMemoryBarrierWithGroupSync between the write and the read. The rule analyses index expressions to determine when reads can target a slot another thread has written: writes indexed by SV_GroupIndex followed by reads indexed by anything other than the same SV_GroupIndex (a neighbour offset, a constant, a transposed coordinate) are the canonical hits.
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-write-then-no-barrier-read.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.