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ADR 0039: Numeric rounding selector ownership

Historical-evidence note: test paths named below record the evidence used when this ADR was accepted; they are not active architecture or release owners. Current ownership is the four-surface ASL tree, with per-ID AVS coverage projected into spec/evidence/release-traceability-readiness.json.

Decision scope​

Semantic-selection and domain-rule portions are superseded by ADR 0047. The mappings recorded below describe the pre-ADR-0047 inventory state. They are not current semantics: scalar values 4–7 now resolve to RNE, and FCVTA now selects RNA (nearest with ties away), as specified by ADR 0047.

Context​

ADR 0047 requires one exact account of rounding taxonomy, selector encodings, operation-local overrides, tie behavior, and rounding-versus-saturation order. The existing PTO surface contains several different selector namespaces:

  • scalar active rounding in CORE_STATE[39:37];
  • fixed rounding selected by FCVTA, FCVTM, FCVTN, FCVTP, and FCVTZ;
  • the three-bit RMode field retained by B.DATR;
  • public conversion modes, target matrix precision controls, quantization stochastic controls, and backend-only cast/part controls.

Shared widths or similar names do not prove that these namespaces have the same encoding or result rule. PTO therefore defines each selector namespace and every explicit mapping between namespaces in its own ASL.

Decision​

The generated spec/evidence/numeric-rounding-selector-contract.json is the fail-closed selector and owner inventory for ADR 0047.

  1. Scalar FRM, scalar fixed conversion overrides, bundle RMode, public API modes, matrix controls, stochastic controls, and backend cast/part controls are distinct namespaces unless an accepted PTO rule maps them.
  2. The portable mathematical core names RNE as nearest with ties to even, RTM as toward negative infinity, RTP as toward positive infinity, and RTZ as toward zero. RTA names away from zero for the fixed FCVTA override.
  3. The five fixed scalar conversion mnemonics have closed selector ownership: FCVTN selects RNE, FCVTM selects RTM, FCVTP selects RTP, FCVTZ selects RTZ, and FCVTA selects RTA. Their operation/type results, range handling, and flags remain profile rules.
  4. The current PTO-v0 scalar active path passes codes 0 through 4 to its profile hook and normalizes codes 5 through 7 to RNE. Whether code 4 is a globally selectable mode and whether codes 5 through 7 reject instead of falling back remain explicit ADR 0047 decisions; this checkpoint does not silently choose either rule.
  5. B.DATR.RMode has a closed three-bit encoding and state binding for all eight values. No numeric meaning is inferred from that binding. Each value requires a named profile rule before a numeric consumer may use it.
  6. Every one of the 18 affected numeric domains, 102 operations, and 25 hooks has an explicit rounding-rule owner row. All domain rounding and saturation-order fields remain null until accepted rules exist.
  7. Unknown or unmapped selectors and missing delegated rules reject before effects under ADR 0037. Backend fallback behavior cannot define a PTO result.

Consequences​

Selector discovery, namespace separation, fixed-override ownership, bundle state binding, and affected-domain ownership are closed. Positive and negative halfway tests cover the five mathematical rounding functions in PTO-v0.

At the time of this decision, ADR 0047 remained open. Closure required an accepted disposition for active FRM codes 4 through 7, exact mappings for public and target selector classes, an accepted rule for all 18 domains, rounding-before-saturation vectors, and any stochastic state or bounded-result contract. This decision does not increment the S5-T2-A2 accepted-decision count or promote maturity beyond M4.

ADR 0047 subsequently accepts those rounding-selection and rounding-point rules. The historical inventory and namespace-separation rationale in this decision remain valid.

Evidence​

  • spec/evidence/numeric-rounding-selector-contract.json
  • scripts/generate-numeric-rounding-selector-contract
  • spec/evidence/numeric-profile-decision-inputs.json
  • spec/evidence/scalar-fsu-totality.json
  • spec/catalog/command-forms.json
  • asl/scalar/floating.asl
  • asl/scalar/dispatch.asl
  • asl/bundle/state.asl
  • asl/bundle/dispatch.asl
  • tests/asl/arch/data-types/rounding/arch-static-rounding-contract-001.asl
  • docs/status/decisions/0028-scalar-fsu-totality-and-profile-boundary.md
  • docs/status/decisions/0037-numeric-profile-identity-and-variation-framework.md
Last updated on by Kevin Zhou