Source: extracted verbatim from the Engineering PRD, docs/prd/10-benchmark-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 T7 requires every kernel page to lead with the explicit-vs-autovec comparison; a fair autovec baseline must be allowed to use the same ISA as the explicit path — comparing AVX-512 intrinsics against SSE2-constrained scalar codegen would be the strawman-baseline pitfall (Survey §7.5; ADMS 2023 method, Survey §4.5).
Problem: produce compiler-vectorized code at each ISA level without shipping it or polluting the library.
Alternatives: (1) compile whole library at -march=<level> variants — rejected: N library builds, dispatch confusion; (2) benchmark against external engines' kernels — rejected: apples-vs-oranges (Survey §7.5) and license entanglement; (3) bench-tree TUs re-including _scalar_impl.h inside target regions (selected) — exactly the same C++ the scalar backend runs, freely auto-vectorized at each ISA level, existing only in the bench tree.
Consequences: REQ-SIMD-006 purity of _impl.h is load-bearing; the ledger's variant axis carries the comparison; verdicts (including losses) are publication requirements (REQ-LEDGER-011).
Reconsideration: none — this is the methodological core of the product.
Related: REQ-BENCH-010, REQ-SIMD-006, Charter T7, OA §13 (ledger identity).