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Correctness, fallback, and determinism

Parent: compilation. Verification: verification. Source anchors: optimize_and_select_javascript, finalize_javascript_candidates, diagnostic rendering in src/compiler.rs, SSA validation in src/optimizer.rs, and the independent evaluator in src/interpreter.rs.

Fallback hierarchy

  1. Frontend, linking, semantic, or configured lowering/optimizer failures are real errors with source diagnostics.
  2. The configured optimizer/emission is retained as the baseline candidate.
  3. An optional optimizer variant that fails to optimize/emit may be discarded without invalidating a program whose configured path succeeds.
  4. Candidate admission/startup limits may reject an alternative; they do not erase the configured baseline.
  5. A parsed peephole clone must parse and validate as a complete generated artifact.
  6. If all optional alternatives lose, emit the configured baseline.

Search never repairs type errors or weakens host boundaries. [optimization].foo = false remains a hard off. An exact compression allowlist usually prevents omitted tactics, but current search-only overlays and the unconditional elide_length_tonumber flip are documented exceptions.

Selection is deterministic for fixed compiler/source/config/profile/toolchain: priority rank, raw length, top-level declaration preference, startup score, lexical JS, and stable plan identity provide total tie-breaking. Terminal topology-preserving search may first prefer more resolved one-byte bindings. Bundle names use source identity and content hashes; manifests record exact output sizes and dependencies.

Semantic oracles

  • Rust unit/conformance tests pin parser, semantic, IR, optimizer, codegen, config, package, and chunk behavior.
  • scripts/verify-matrix.sh compares JS/C/native execution.
  • lilscript-differential interprets checked AST without SSA, targeting shared transform bugs.
  • Paired web cases compare independently authored JS and LilScript only after both pass the observable oracle.

A size win never overrides a semantic failure. A compiler crash, invalid emitted JS, nondeterministic artifact, or baseline-only mismatch is a red result with preserved artifacts for triage. Search ranking an unresolved binding is the same class of failure (ident-05); it is not a smaller program. Current architecture treats identity as blocking further search widening.

SSA validation includes edge uses. In particular, aggregate scalar replacement may not delete a struct definition merely because all instruction uses are field reads; a phi incoming edge is also a live use. Until field-wise phi decomposition exists, the pass retains such aggregates. The structural algorithm corpus owns a loop-carried aggregate regression in addition to the optimizer-level dangling-definition test.