Brummel
176821c2e7
The 3020-line docs/DESIGN.md is replaced by the design/ ledger:
design/INDEX.md (sole addressable spine, typed Contracts+Models tables,
polymorphic links — prose file OR authoritative source //!), 14
design/contracts/*.md test-linked invariants + 3 source-link-only
contracts (mangling/env-construction/qualified-xref, no prose file —
code is SoT), 5 design/models/*.md whitepapers, and
docs/journals/2026-05-19-design-decision-records.md (the
relitigation-guard archive — every why/rejected/does-not-do/rollback/
empirical ### moved out at ###-granularity). Clean cut: git rm
docs/DESIGN.md, no stub.
RED-first crates/ailang-core/tests/design_index_pin.rs — the 4-clause
anti-regrowth spine (DESIGN.md-gone / every-INDEX-link-resolves /
every-contract-names-a-resolvable-ratifier /
contracts-carry-no-decision-record-prose) — demonstrably RED before,
GREEN after. Build-atomic by task ordering: design_schema_drift.rs's
include_str! (the only compile-time consumer) retargeted to
design/contracts/data-model.md BEFORE the deletion; its
## Data model/## Pipeline slicer dropped (a simplification the split
enables). 2 NoInstance diagnostics + 2 lockstep E2Es retargeted to
design/contracts/{float-semantics,typeclasses}.md. ~12 agent reading
lists + 5 SKILL bodies + CLAUDE.md + skills/README.md + ~25
code/C/.ail/spec comment xrefs retargeted; OQ7 dangling 'Iter 13b'
cite deleted (no forward target — a pointer would be fiction).
honesty-rule.md rewritten so the rule names the new home
(rationale->journals), resolving the recon-found internal
contradiction; the two docs_honesty_pin.rs:70,72 pinned phrases kept
verbatim+contiguous.
Boss-verified independently: cargo test --workspace 646 passed /
0 failed; design_index_pin 4/4; acceptance grep CLEAN of live
DESIGN.md refs (residuals = only the spec-mandated clause-4
deletion-enforcer). 2 DONE_WITH_CONCERNS routed to the mandatory
milestone-close audit: (a) str-abi.md:23 '(iter str-concat,
2026-05-13)' provenance stamp trips advisory architect_sweeps Sweep-1
— Boss-confirmed byte-identical to DESIGN.md@deeffb1:2062-2065, a
faithfully-migrated PRE-EXISTING anchor (regexes verbatim, only path
retargeted), NOT split-introduced — RATIFY-or-tidy at audit; (b) a
now stale-direction intra-prose 'see Str ABI below' cross-ref in
float-semantics.md — audit-adjudication candidate. Plan defect noted:
Task 9 Step 4's verbatim acceptance grep used a ^./ anchor not
matching the system's grep -rIn output; substance re-verified CLEAN.
Spec grounding-check PASS x2. Journals INDEX + decision-records
pointer appended (Boss-only).
106 lines
5.1 KiB
Markdown
106 lines
5.1 KiB
Markdown
# Float semantics
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## Float semantics
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`Float` is IEEE-754 binary64 (LLVM `double`). One float type ships;
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no `f32` variant. The runtime / codegen contract:
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**Guaranteed:**
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- Every individual builtin (`+`/`-`/`*`/`/`/`neg`/`<`/`==`/...) lowers
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to a single LLVM IR instruction on the Float arm:
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`fadd/fsub/fmul/fdiv double`, `fneg double`, `fcmp olt/ole/ogt/oge
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double`, `fcmp oeq double`, `fcmp une double` (for `!=`). On a
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fixed `(target triple, LLVM version)` pair, the bit pattern of
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the result of any single op is reproducible.
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- NaN and ±Inf propagate per IEEE 754 — no silent collapse to zero,
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no trap. Arithmetic on a NaN operand produces NaN; division by
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zero produces ±Inf; `0.0 / 0.0` produces NaN.
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- `-0.0` and `+0.0` are distinct bit patterns at the canonical-JSON
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hash level (`{"bits":"0000000000000000",...}` vs
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`{"bits":"8000000000000000",...}` — distinct `def_hash`s) but
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compare equal via `==` per IEEE (`fcmp oeq double` returns true
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for `+0 == -0`). This asymmetry is the correct IEEE behaviour;
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it does mean `def_hash`-equality is finer than `==`-equality on
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Float.
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- `==` returns `false` whenever either operand is NaN
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(`fcmp oeq` is the ordered-equal predicate; ordered = both
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operands non-NaN).
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- `!=` returns `true` whenever either operand is NaN (`fcmp une`
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is the unordered-or-not-equal predicate). This matches Rust
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`f64::ne` and IEEE-`!=` exactly.
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- `is_nan` (`fcmp uno double %x, %x`) returns `true` iff `x` is
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NaN. Bit-pattern-based NaN detection without dependence on the
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payload bits.
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- `int_to_float` (`sitofp`) is exact for `|n| < 2^53`,
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round-to-nearest-even otherwise.
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- `float_to_int_truncate` (`@llvm.fptosi.sat.i64.f64`) is total:
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NaN → 0, +Inf → i64::MAX, -Inf → i64::MIN, finite-out-of-range
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saturates, finite-in-range truncates toward zero. Matches Rust
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`as i64` semantics (since 1.45).
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**Unspecified:**
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- FMA contraction. LLVM may fold `fadd (fmul a b) c` into
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`fma a b c`. Bit results may differ between an op-emitted-in-
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isolation pattern and an op-folded-into-FMA pattern.
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- Reassociation. The compiler may reorder a chain like
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`(a + b) + c` into `a + (b + c)`, producing a bit-different
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result on numerically sensitive inputs.
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- Subnormal flushing modes. If the target enables FTZ (flush-to-
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zero) or DAZ (denormals-are-zero), subnormal results round to
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zero; AILang does not enable these flags but does not forbid the
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target from doing so.
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- The exact NaN bit pattern produced by an op. Any quiet NaN bit
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pattern is conformant; `0.0 / 0.0` may produce
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`0x7ff8000000000000` on one target and a different qNaN on
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another.
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- The textual rendering of NaN through `float_to_str` (the runtime
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C helper that backs `instance Show Float` and, post-iter-rpe.1,
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every Float-typed `print` call). The libc `printf("%g", nan)`
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glue used by `float_to_str` is permitted to emit `nan` / `-nan`
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/ `NaN` etc. depending on libc version and the NaN's sign bit;
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AILang does not normalise this, since the prose / surface-print
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paths render NaN as the explicit `"NaN"` spelling and Float
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rendering is for human-readable output, not round-trip.
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The same libc-`%g` rendering applies to `show 1.5` / `show nan` /
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`show inf` via `instance Show Float` (which calls `float_to_str`
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internally — see §"Prelude (built-in) classes" for the Show ship).
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The NaN-spelling caveat above is observable via `do print x` for
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Float-typed `x`; the rendering is libc-version-dependent and
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target-libc-specific. AILang does NOT canonicalise Float textual
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representation; the LLM-author who needs deterministic Float
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rendering for cross-platform test fixtures should bypass `show` /
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`print` and emit a custom formatter.
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These are the Rust / Swift / standard-LLVM defaults — not
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research-grade reproducibility guarantees. The stronger guarantee
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(e.g. Pythonic `float.fromhex`-level bit reproducibility across
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ops) would require `-ffp-contract=off` plus per-op intrinsic
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selection — out of scope for the milestone; revisit only if a real
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use case appears.
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**Form-A serialisation:** Float literals carry the IEEE-754
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bit pattern as a 16-character lowercase hex string in the canonical
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JSON: `{"kind":"float","bits":"<16-hex>"}`. Routing through the
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JSON *string* path (not `serde_json::Number`) preserves bit
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stability across `serde_json` versions and lets NaN / ±Inf
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round-trip through Form-A — JSON numbers cannot represent them.
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**Pattern matching:** `Pattern::Lit` on `Literal::Float` is hard-
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rejected at typecheck (`CheckError::FloatPatternNotAllowed`). IEEE
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semantics make Float patterns semantically dubious — NaN never
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matches via IEEE-`==`, and bit-exact equality is rarely what an
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LLM-author wants. Use ordering operators (`<`, `>`, ...) and
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`is_nan` to discriminate Floats.
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`float_to_str` (Float → Str) and `int_to_str` (Int → Str) are
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fully wired through checker, codegen, and runtime. Both allocate
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a fresh heap-Str slab at the call site (see "Str ABI" below for
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the dual realisation) and carry `ret_mode: Own` so the let-binder
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for the call result is RC-tracked and the slab is freed at scope
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close.
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Ratified by: `crates/ail/tests/eq_float_noinstance.rs`.
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