Epic: Design: IR-only execution

[rogeralsing]

Context

The engine currently has an IR execution plan (ExecutionPlan) and interpreter (ExecutionPlanRunner), but many IR instructions still embed AST nodes and evaluate them via EvaluateForJsValue/EvaluateExpression at runtime. The goal is to eliminate all runtime AST evaluation and optionally add a .NET IL compilation backend.

Phases 0–2 are complete: statement-level AST delegation is removed, expression bytecode is introduced, and AST evaluators are quarantined. Current progress is ~78%, with the remaining work focused on eliminating the last AST seams (captured with chains, edge-case dynamic scopes) and the future IL backend phases.

Success Criteria

Phase 1–3 (AST Elimination) — ~78% complete

• No StatementInstruction escape hatches in emitted plans
• IR lowering for for..in, with, block lexical environments, destructuring declarations
• Expression bytecode replaces ExpressionNode.EvaluateExpression operands
• AST evaluators quarantined to analysis/debug only
• Zero AST evaluator calls during any runtime execution path (remaining: captured with chain scripts/eval, ambient with entry)
• AssertNoAstEvaluation passes for all sync execution paths

Phase 4 — IL Backend for Sync Bytecode

• BytecodePlan compiles to DynamicMethod delegate
• Tiered execution: interpret first N calls, compile on hot threshold
• Cache compiled delegates keyed by plan identity + layout id
• Kill switch + "dynamic code supported" guard

Phase 5 — IL Backend for Generator/Async

• Compile step function: delegate StepResult Step(ref GeneratorState state, ResumeMode mode, JsValue arg, ExecutionFrame frame)
• Yield suspension points preserved
• Try/finally semantics across suspension
• Pending await plumbing

Rules

• AST walkers must only be used for parsing + analysis, never at runtime
• Runtime helpers must operate on JsValue/env/context, never on AST nodes
• Interpreter fallback must always be available (IL backend is optional)
• with-closure quarantine must remain intact for captured with environments
• All changes must pass existing Test262 and internal test suites without regression

NFRs

• Memory footprint must not regress (baseline: forloop --memory = 7.05 MB)
• IL backend must have tiering to avoid compile-time overhead for cold functions
• Debug mapping (PC → source span) must be preserved for diagnostics

Sub-Issues

• #398 — Phase 0: Inventory + Invariants (closed or N/A)
• Task 1: Remove statement-level AST delegation #399 — Phase 1: Remove statement-level AST delegation (CLOSED)
• Task 2: Introduce expression bytecode #400 — Phase 2: Introduce expression bytecode (CLOSED)
• Task 3: Remove / quarantine AST evaluators #401 — Phase 3: Remove/quarantine AST evaluators (CLOSED)
• #402 — Phase 4: IL backend for sync bytecode (not yet created)
• #403 — Phase 5: IL backend for generator/async (not yet created)
• Continue IR work #723 — Continue IR work (OPEN, stage:build)

Quality Gates

• All internal unit tests pass (dotnet test tests/Asynkron.JsEngine.Tests)
• AST-eval seam scan returns 0: rg 'EvaluateExpression\(|ProfileEvaluateExpression\(' src/Asynkron.JsEngine/Ast/TypedAstEvaluator.ExecutionPlanRunner* | wc -l
• AssertNoAstEvaluation debug tests pass for all execution paths
• Memory baseline maintained (./tools/profile forloop --memory ≤ 7.05 MB)
• Test262 Language and BuiltIns runsettings show no regressions
• No roslynator warnings in changed files

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