Source: extracted verbatim from the Engineering PRD, docs/prd/09-simd-architecture.md (REQ-DOC-004). From M0 onward this file is the canonical, living ADR record; status changes happen here and in the index, with the PRD text remaining the historical record.
Status: Accepted.
Context: Charter §7.2 no-over-read default; batch tails (n mod W) are the classic over-read temptation.
Alternatives: (1) read-past-end with page-boundary checks — rejected outright: violates the charter's sanitizer-clean pledge, unvendorable; (2) require padded buffers — rejected: that is the _padded variant family, deliberately not shipped in v1 (REQ-MEM-002); (3) scalar tails everywhere — safe but leaves AVX-512 masking value unused; (4) scalar tails on AVX2/NEON + native masking on AVX-512 (selected) — masking is architecturally access-suppressing, keeping the contract while showcasing the ISA (Survey §4.1: "the ISA matters more than the width").
Consequences: tail code is a per-family shared helper (MOD-KCOMMON); guard-page tests place batch ends flush against protected pages for every kernel × ISA (12 §2, REQ-TEST-006).
Reconsideration: ledger evidence that scalar tails dominate small-batch cost for a hot family may motivate _padded variants — via the REQ-MEM-002 amendment path, never silently.