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Unified Bytecode Expansion Contract

Date: 2026-06-01 Scope: Shared contract for parallel unified-bytecode lane work.

Source-Of-Truth Surfaces

  • Unified opcode surface: src/Asynkron.JsEngine/Execution/UnifiedBytecode/UnifiedBytecodeProgram.cs (UnifiedBytecodeOpCode)
  • Unified compiler ownership: src/Asynkron.JsEngine/Execution/UnifiedBytecode/UnifiedBytecodeCompiler.cs
  • Unified VM ownership: src/Asynkron.JsEngine/Execution/UnifiedBytecode/UnifiedBytecodeVirtualMachine.cs
  • Production eligibility and decline taxonomy: src/Asynkron.JsEngine/Execution/UnifiedBytecode/UnifiedBytecodeProductionEligibility.cs
  • Statement diagnostics support surface: src/Asynkron.JsEngine/Execution/StatementInstructionDiagnosticsCodec.cs (IsSupportedKind)
  • Expression bytecode capability map: docs/expression-bytecode-coverage.md
  • Current production routing evidence: docs/performance/unified-bytecode-branch-production-routing.md
  • Primary sync route coverage: docs/performance/unified-bytecode-primary-sync-route-coverage.md

No-Mixed-Execution Rule

Production-eligible unified programs are all-or-nothing VM execution. Accepted programs must execute fully in UnifiedBytecodeVirtualMachine and must not fallback into ExpressionProgram, ExecutionPlanRunner, or AST evaluators. The bounded ApplyBindingTarget bridge is the current exception: the VM owns dispatch and operand state, then applies an already-lowered BindingTargetProgram for assignment destructuring parity instead of falling back to expression or statement interpretation.

Ordinary Sync Routing Boundary

  • Ordinary sync invocation keeps the dedicated simple-return binary and binary-chain fast paths ahead of the broader production unified-bytecode route, matching ADR 0258's route-priority contract.

  • For ordinary sync functions that pass both CanUseProductionUnifiedBytecodeFastPath and UnifiedBytecodeProductionEligibility.Evaluate, the production VM is the primary default attempt before SyncIrCallTrampoline and generic ExecutionPlanRunner interpretation.

  • Current route coverage estimate: 100% of accepted ordinary sync production programs attempt UnifiedBytecodeVirtualMachine before generic IR fallback. This is a selector coverage estimate, not a full ECMAScript function-surface claim; unsupported buckets below remain pre-VM declines.

  • Coverage evidence: docs/performance/unified-bytecode-primary-sync-route-coverage.md

Final Production Decline Status

  • The accepted production scope has no unclassified decline rows: every current UnifiedBytecodeProductionDeclineCode, sync pre-gate, and prototype opcode guard is represented in the checked ledger below with an owning fallback route and proof command.
  • Remaining declines are scoped to non-production-only or not-yet-admitted semantic lanes. They must stay as pre-VM declines until a future slice owns selector, compiler, VM/runtime semantics, public route proof, no-route neighbor proof, and no-mixed-execution source gates together.
  • Benchmark and profile rows are routing evidence only when they also report the unified-bytecode-production-fast-path route-hit signal. Use rtk ./tools/profile <profile> --route-hits for route evidence over manifest workloads without requiring the external profiler.

Current Completion Audit

  • Completing the decline-burndown plan did not make unified bytecode the only execution model. The result is a wider production route plus a checked decline ledger. Any shape that misses the sync pre-gates, production eligibility, or compiler lowering still falls back to the existing IR/expression-bytecode execution paths.
  • Non-undefined new.target is no longer a standalone ordinary-sync production pre-gate. Constructor route eligibility is now owned by the constructor/class activation rows and by concrete opcode/compiler support such as LoadNewTarget and ConstructInvocationBoundary.
  • The current gap is not missing VM switch coverage. The UnifiedBytecodeOpCode inventory and UnifiedBytecodeVirtualMachine switch are expected to stay in lockstep; the current opcode surface has VM handling for every listed opcode. Remaining work is primarily semantic admission and lowering, not adding switch cases for already-declared opcodes.
  • UnifiedBytecodeCompiler now has generated audit coverage for every declared IR instruction record. Function-scoped FunctionDeclarationInstruction entries compile as no-ops after fast activation hoisting installs the callable value, and descriptor-backed runtime declarations lower to DeclareFunction. Block-scoped function declarations behind unsupported lexical environment shapes can still decline through the surrounding PushEnvironment or call boundary, but they are no longer a stale compiler-decline row. ClassDeclarationInstruction is VM-owned through DeclareClass, with environment-backed lexical declaration installation and class-value creation handled inside the unified VM. Static synchronous BindingVariableDeclarationInstruction shapes are now VM-owned through ApplyDeclarationBindingTarget, while binding defaults, computed binding names, assignment targets, awaited declarations, and using declarations still decline before VM execution.
  • Expression lowering is still the largest surface. ExpressionOpKind contains more shapes than the general unified lowering loop accepts. Several operations are admitted only through narrow shape helpers, while remaining optional-call chain shapes and some call/reference helpers remain outside general unified bytecode lowering. Private delete syntax is rejected before bytecode eligibility.
  • The remaining direct general-expression lowering gaps are drift-checked under ### General Expression Lowering Gaps (current). Property set/update operations plus the SwapTopTwo, DuplicateTopTwo, and RotateTopThreeRight stack operations are no longer helper-only compiler shapes; they lower through the general expression loop, subject to the same private-name and name-inference semantic guards as the owned property-write helpers.
  • Optional-chain short-circuit provenance is now VM-owned for the general expression loop. JumpIfShortCircuited lowers directly to unified bytecode, and the VM tracks a packed side flag per operand-stack slot so short-circuited undefined remains distinct from ordinary undefined.
  • There is no retained discard-specific production decline. EvaluateAndDiscard compiles the supported expression program first and appends Pop, so discarded statements route when their underlying operation family is already selector/compiler/VM-owned. Remaining discarded-expression declines are declines of the underlying family, such as dynamic lookup, unsupported private neighbors, optional-chain neighbors, or other semantic lanes not yet admitted.
  • The current ExpressionOpKind names with no unified compiler reference list is empty. Computed super-property operations also route their required EnsureSuperReference op through the general unified expression loop.
  • Slot update lowering is now VM-owned for activation-resolved ordinary-sync shapes via UpdateSlot. The opcode is used by IncrementSlotInstruction and activation-resolved UpdateIdentifier, while dynamic/with-backed updates continue to use UpdateDynamicIdentifier.
  • Public class methods with active or captured private-name scopes are no longer blanket pre-gated when their body otherwise satisfies production unified-bytecode routing. The invocation bridge threads those private-name scopes into the VM, so #name in receiver can execute through PrivateFieldIn and direct private named reads/writes/updates can execute through GetNamedProperty / SetNamedProperty / UpdateNamedProperty. Direct private named compound and logical writes execute through GetNamedPropertyForCompoundSet plus SetNamedProperty when the receiver chain is otherwise the admitted activation-resolved named-write shape. Direct private named method calls can execute through PrepareNamedCallTarget. Private member deletes are parser early errors, so they no longer enter production eligibility.
  • Simple-return arrows can route with captured lexical this / new.target values. The production invocation bridge resolves those captured values before entering the VM and avoids sloppy-call this coercion for arrows. Arrows that need a lexical-this environment or super binding still decline.
  • Simple base class constructors with public or private instance fields and simple derived constructors with public or private instance fields can route through production unified bytecode. The production constructor bridge performs the existing base pre-body field initialization/private branding before VM entry and consumes the derived pending field initializer after super(...).
  • Resumable async/generator unified bytecode is narrower than ordinary sync production bytecode. The resumable eligibility opcode allow-list is now audited against the ExecuteResumable switch, so opcodes already implemented by the resumable VM cannot remain stale declines. Broad async/generator control flow, calls, dynamic lookup, arguments, awaited iterator sources, and async-generator delegated yield shapes still decline.
  • Generator-lowered synthetic resume and driver targets (__yield_lower_resume*, internal yield* state slots) are now added to the unified slot layout when the original activation analysis did not include them. Parenthesized yield (left && right), return yield, and yield* state/result shapes therefore compile and route without borrowing parameter slots.
  • ApplyBindingTarget is the one explicit bridge inside accepted VM execution: the VM owns dispatch and stack/slot state, then applies an already-lowered BindingTargetProgram for assignment destructuring parity. This is not a fallback into ExecutionPlanRunner, but it is also not fully native unified bytecode.
  • The contract drift test now verifies exact membership for the current production decline inventory and checked ledger rows. A stale extra decline row should fail before it can mask completed widening work.

Current Support Matrix

Unified Opcode Inventory (current)

  • LoadSlot
  • LoadDynamicIdentifier
  • LoadThis
  • LoadNewTarget
  • LoadImportMeta
  • LoadTemplateObject
  • LoadLiteral
  • LoadRegexLiteral
  • StoreSlot
  • UpdateSlot
  • InitializeSlot
  • DeclareDynamicVar
  • StoreDynamicIdentifier
  • ResolveDynamicIdentifierReference
  • LoadDynamicIdentifierReference
  • StoreDynamicIdentifierReference
  • PopDynamicIdentifierReference
  • Binary
  • RequireObjectCoercible
  • ResolvePropertyKey
  • GetNamedProperty
  • GetNamedPropertyOptional
  • GetComputedProperty
  • GetNamedPropertyForCompoundSet
  • GetComputedPropertyForCompoundSet
  • SetNamedProperty
  • SetComputedProperty
  • EnsureSuperReference
  • GetNamedSuperProperty
  • GetComputedSuperProperty
  • SetNamedSuperProperty
  • SetComputedSuperProperty
  • UpdateNamedSuperProperty
  • UpdateComputedSuperProperty
  • UpdateNamedProperty
  • UpdateComputedProperty
  • UpdateDynamicIdentifier
  • TypeOf
  • TypeOfIdentifier
  • TypeOfDynamicIdentifier
  • DeleteDynamicIdentifier
  • DeleteNamedProperty
  • DeleteComputedProperty
  • UnaryPlus
  • UnaryMinus
  • UnaryLogicalNot
  • UnaryBitwiseNot
  • UnaryVoid
  • PrivateFieldIn
  • ToString
  • Pop
  • DuplicateTop
  • DuplicateTopTwo
  • SwapTopTwo
  • RotateTopThreeRight
  • CreateArray
  • ArrayPush
  • ArrayPushHole
  • ArraySpread
  • CreateObject
  • DefineObjectProperty
  • DefineComputedObjectProperty
  • DefineObjectMethod
  • DefineComputedObjectMethod
  • DefineObjectAccessor
  • DefineComputedObjectAccessor
  • ObjectSpread
  • Jump
  • JumpWithDriverCleanup
  • JumpIfFalse
  • JumpIfShortCircuitFalse
  • JumpIfShortCircuitTrue
  • JumpIfShortCircuitNotNullish
  • JumpIfShortCircuited
  • JumpIfNullishReplaceUndefined (Note: Jump was already in the inventory; ConditionalExpression (?:) is now admitted via ADR 0297 using the existing Jump, JumpIfShortCircuitFalse, and Pop opcodes without new additions.)
  • Return
  • ReturnUndefined
  • Throw
  • ThrowReferenceError
  • Break
  • Continue
  • PushEnvironment
  • PopEnvironment
  • EnterTry
  • EnterCatch
  • LeaveTry
  • EndFinally
  • EnterWith
  • LeaveWith
  • TdzHeadInit
  • IteratorInit
  • IteratorMoveNext
  • IteratorClose
  • ForInInit
  • ForInMoveNext
  • ArrayDestructuringInit
  • ArrayDestructuringElement
  • ArrayDestructuringRest
  • ArrayDestructuringClose
  • ObjectDestructuringInit
  • ObjectDestructuringProperty
  • ObjectDestructuringRest
  • ObjectDestructuringClose
  • PrepareIdentifierCallTarget
  • PrepareIdentifierOptionalCallTarget
  • PrepareDynamicIdentifierCallTarget
  • PrepareNamedCallTarget
  • PrepareComputedCallTarget
  • PrepareNamedOptionalCallTarget
  • PrepareComputedOptionalCallTarget
  • PrepareNamedSuperCallTarget
  • PrepareComputedSuperCallTarget
  • CallInvocationBoundary
  • ConstructInvocationBoundary
  • SuperConstructInvocationBoundary
  • DeclareClass
  • DeclareFunction
  • LoadFunctionLiteral
  • Yield
  • StoreResumeValue
  • AwaitAndDiscard
  • AwaitedReturn
  • YieldStar
  • LoadClassLiteral
  • ApplyBindingTarget
  • ApplyDeclarationBindingTarget
  • EnsureHasName

Production Decline Families (current)

  • None
  • AsyncLikeFunction
  • GeneratorFunction
  • CapturedOrDynamicActivation
  • ArgumentsObjectDependency
  • ArrowLexicalThisDependency
  • ClassConstructorActivation
  • CallDependency
  • DynamicLookupDependency
  • PropertyReadBoundaryOutOfScope
  • PropertyWriteDependency
  • PropertyUpdateDependency
  • DeleteDependency
  • SuperPropertyDependency
  • OptionalChainDependency
  • ObjectLiteralOrSpreadDependency
  • PrivateFieldDependency
  • ForInDriverStateDependency
  • DestructuringDependency
  • LabelControlFlow
  • UnsupportedPlanShape
  • CallInvocationBoundary

Checked Production Decline Ledger

This ledger is a drift-guarded baseline for widening work. It distinguishes ordinary sync pre-gates from UnifiedBytecodeProductionEligibility declines: pre-gates fall through to the existing sync route before eligibility runs, eligibility declines keep the plan out of the production VM, and accepted rows must still obey the no-mixed-execution rule.

Eligibility Decline Rows

Decline code Owning source / current example Current fallback route Planned batch / lane Proof command
None UnifiedBytecodeProductionEligibility.Accept for accepted programs, for example function passThrough(x) { var y = x; return y; } Production unified-bytecode VM Baseline accepted route rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_LinearSlotLiteralReturnPlan_Accepts"
AsyncLikeFunction Evaluate activation gate and awaited with-object plan decline Existing async / awaited IR route Resumable async/generator lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_AsyncLikeActivation_DeclinesBeforePlanInspection"
GeneratorFunction Evaluate activation gate for ordinary sync production routing Existing generator IR route Resumable async/generator lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_GeneratorActivation_DeclinesBeforePlanInspection"
CapturedOrDynamicActivation Activation descriptor HasCapturedOrDynamicActivation, including captured function scope or unresolved dynamic activation outside the with-backed lane Existing sync IR / environment route Dynamic activation lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ActivationDependencies_DeclineBeforeCompile"
ArgumentsObjectDependency Activation descriptor and expression scan for real arguments object access or call targets; implicit typeof arguments routes as a literal "object", while parameter/lexical arguments reads, typeof, updates, and call targets route as activation slots Existing sync IR / arguments-object route Arguments object lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ArgumentsAccess_DeclinesWithArgumentsDependency"
ArrowLexicalThisDependency Activation descriptor gate for arrow lexical this / new.target ownership before ordinary sync routing Existing arrow invocation route Arrow route lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_OrdinarySyncActivationDescriptorBlockers_DeclineBeforeCompile"
ClassConstructorActivation Activation descriptor gate for class constructor activation outside the admitted simple base constructor route and explicit derived-constructor super(...) route with post-super this body reads/writes Constructor route Constructor boundary lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionConstructCallTests&FullyQualifiedName~Constructor"
CallDependency Direct eval outside the one-argument non-spread eval-identifier boundary, out-of-boundary call-target preparation, complex call arguments excluding admitted simple/binary template-literal substitutions, simple/binary computed object keys, and zero-argument activation-resolved identifier-call computed object keys, and descriptor-level non-parameter callee calls Existing sync IR call route Wider call invocation lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ActivationDependencies_DeclineBeforeCompile"
DynamicLookupDependency Unresolved identifier loads/stores/update outside the admitted ordinary and with-backed dynamic-name paths Existing sync IR / environment lookup route Dynamic-name lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_WithThenOutsideDynamicIdentifier_DeclinesWithDynamicLookupDependency"
PropertyReadBoundaryOutOfScope Named/computed property reads outside the admitted activation-resolved boundaries Existing sync IR property route Property read widening lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ComputedPropertyReadOutsideFirstBoundary_DeclinesWithBoundaryCode"
PropertyWriteDependency Property writes and compound/logical property writes outside the admitted direct property-write shapes, supported computed expression-key mutation shapes, simple nested named receiver assignment shape, nested named compound-write shape, and nested named logical-write shape Existing sync IR property-write route Property write widening lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_LogicalAndAssignment_UnsupportedShapes_DeclineWithExplicitCodes"
PropertyUpdateDependency Property and identifier update expressions outside the admitted direct update, computed expression-key update, and simple nested named receiver update boundary Existing sync IR update route Property update lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_NestedNamedPropertyUpdate_AcceptsOwnedPropertyOpcodes"
DeleteDependency delete expressions outside the admitted ordinary named/computed property delete lane, ordinary dynamic-key computed delete lane, and with-backed dynamic-name delete lane Existing sync IR delete route Delete semantics lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_NestedComputedPropertyDeleteDynamicKey_AcceptsOrdinaryDynamicNameOpcode"
SuperPropertyDependency Out-of-boundary super call targets; super property reads/writes/updates are admitted by dedicated VM opcodes Existing class / constructor route for remaining call-target shapes Super semantics lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_SuperPropertyAccess_AcceptsOwnedOpcodes"
OptionalChainDependency Optional chains outside the admitted optional property-read, optional-call, and exact optional named/computed delete boundaries Existing sync IR optional-chain route Optional-chain widening lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_OptionalChainNamedPrefixPlainCallExpressionPlan_AcceptsExecutableInvocationBoundary"
ObjectLiteralOrSpreadDependency Object/array spread sources outside simple operands and direct named member-call spread sources, and object methods/accessors only when they appear inside restricted simple literal spans Existing sync IR literal/spread route for remaining spans Literal/spread lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ObjectLiteralUnsupportedFeatures_DeclineWithExplicitCodes"
PrivateFieldDependency Private-name operations outside the admitted routes; #name in obj, direct private named reads/writes/updates, direct private named compound/logical writes, and direct private named method calls are VM-owned when the surrounding class method is otherwise production-eligible. Private member deletes are parser early errors before production eligibility. Existing private-name route for remaining private member access Private-name lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_PrivateFieldIn_AcceptsAndVmChecksPrivateBrand"
ForInDriverStateDependency Unsupported for-in driver state such as awaited object source Existing for-in IR driver route Driver-state lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~IsSupportedForInInit_AwaitedSource_Declines"
DestructuringDependency Awaited binding values, unsupported destructuring driver shapes, and targets outside the admitted driver or descriptor-backed lanes; declaration defaults and computed binding names route through ApplyDeclarationBindingTarget Existing destructuring IR route for remaining shapes Destructuring driver lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_DeclarationDestructuringDescriptorShapes_AcceptDescriptorOpcode"
LabelControlFlow Labeled break/continue that exits an intervening iterator/for-in driver loop not directly targeted by the abrupt jump Existing IR loop-control route Multi-driver labeled cleanup lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_LabeledBreakCrossingDriverLoop_DeclinesWithLabelControlFlow"
UnsupportedPlanShape Missing activation slot metadata, unsupported instruction families, unsupported compiler shapes, unsupported resumable opcodes, and unknown production opcode defaults Existing execution-plan route Statement/control-flow ownership lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~ExpressionProgramCoverageMapTests&FullyQualifiedName~UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"
CallInvocationBoundary Plan-structural call invocation outside the currently executable call boundary, separate from descriptor-level CallDependency Existing sync IR call route Wider call invocation lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~ExpressionProgramCoverageMapTests&FullyQualifiedName~UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"

Sync Production Pre-Gate Rows

These rows are owned by TypedAstEvaluator.SyncFunctionInvoker.CanUseProductionUnifiedBytecodeFastPath. They decline before UnifiedBytecodeProductionEligibility.Evaluate, so they do not always have a UnifiedBytecodeProductionDeclineCode.

Pre-gate key Owning source / current example Current fallback route Planned batch / lane Proof command
pre-gate:IsClassConstructor Class constructor invokers outside the admitted simple base constructor route and explicit derived-constructor super(...) route with post-super this body reads/writes Constructor route Constructor boundary lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:IsArrowFunction Arrow functions outside the admitted simple-return route whose lowered body proves no super, closure-variable, or dynamic-identifier dependency. Captured lexical this / new.target values are VM-owned; lexical-this environments remain a separate pre-gate. Existing arrow invocation route Arrow route lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:IsAsyncLike Async and formerly-async ordinary invokers Async route Resumable async/generator lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_AsyncLikeActivation_DeclinesBeforePlanInspection"
pre-gate:IsGenerator Generator functions before ordinary sync production routing Generator route Resumable async/generator lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_GeneratorActivation_DeclinesBeforePlanInspection"
pre-gate:IsDefaultDerivedConstructor Default derived class constructor setup Constructor route Constructor boundary lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:hasParameterExpressions Default/rest/destructured parameter expressions Existing parameter environment route Parameter semantics lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:hasOnlySimpleIdentifierParameters Non-simple parameter lists Existing parameter environment route Parameter semantics lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:usesArguments Functions that read the real implicit arguments object except the bounded typeof arguments literal route; shadowing parameter/lexical arguments reads, typeof, updates, and calls are activation-slot traffic Existing arguments-object route Arguments object lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ArgumentsAccess_DeclinesWithArgumentsDependency"
pre-gate:needsArgumentsBinding Sloppy mapped arguments binding when an implicit arguments object is needed and not admitted by with-backed dynamic-name routing Existing arguments-object route Arguments object lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:allowIdentifierCache Identifier cache disabled outside the ordinary and with-backed dynamic-name lanes Existing environment lookup route Dynamic-name lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_WithThenOutsideDynamicIdentifier_DeclinesWithDynamicLookupDependency"
pre-gate:lexicalThisEnvironment Lexical this environment captured by arrow / class-derived shapes Existing lexical-this route This-binding lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
pre-gate:superConstructor Derived-constructor super binding dependency outside the bounded production constructor admission path Constructor / super route Super / constructor lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_SuperCall_AcceptsSuperConstructInvocationBoundary"
pre-gate:superPrototype Method home-object super prototype state; admitted for VM-owned super property reads/writes/updates and first-boundary super calls Existing super route for remaining shapes Super semantics lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_SuperPropertyAccess_AcceptsOwnedOpcodes"
pre-gate:activationSlotShape CanUseProductionUnifiedBytecodePlanShape requires activation slots matching root scope, layout, and parameter-slot length unless with-backed dynamic names or environment-backed class declarations own the materialized activation Existing execution-plan route Slot-layout lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~ExpressionProgramCoverageMapTests&FullyQualifiedName~UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"

Prototype Opcode Guard Rows

These rows are owned by UnifiedBytecodeProductionEligibility.TryFindPrototypeOnlyOpcode. The guard is the final post-compile production subset check before VM entry.

Prototype guard key Owning source / current example Current fallback route Planned batch / lane Proof command
prototype-guard:Binary Binary opcodes currently admit the selector-owned production subset: arithmetic (+, -, *, /, %, **), equality/comparison (==, !=, ===, !==, <, <=, >, >=), bitwise/shift (&, ` , ^, <<, >>, >>>), and relational/object tests (in, instanceof); binary operators outside that subset (for example &&, ||, ??) decline as UnsupportedPlanShape` Existing expression route Binary operator widening lane
prototype-guard:Jump Jump and JumpWithDriverCleanup are admitted only for compiler-owned branch, loop, and driver cleanup shapes Existing control-flow route if an unowned jump shape is introduced Control-flow lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_LabeledBreakCrossingDriverLoop_DeclinesWithLabelControlFlow"
prototype-guard:JumpIfFalse JumpIfFalse and short-circuit conditional jump opcodes are admitted only for proven compiler-owned expression and statement control flow Existing control-flow route if an unowned conditional shape is introduced Control-flow lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests&FullyQualifiedName~Evaluate_ConditionalExpression_ThisPropertyConditionAndArms_Accepts"
prototype-guard:DefaultUnsupportedOpcode Any future UnifiedBytecodeOpCode not explicitly listed in the switch declines as UnsupportedPlanShape until the selector, compiler, VM, and proof pack are updated together Existing execution-plan route Opcode ownership lane rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~ExpressionProgramCoverageMapTests&FullyQualifiedName~UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"

Statement Diagnostics Supported Kinds (current)

  • Jump, Break, Continue, SetCompletionValue, BreakableExit
  • EvaluateAndDiscard, AwaitAndDiscard, Throw, Return
  • AssignmentSlot, SimpleVariableDeclaration, BindingVariableDeclaration, StoreResumeValue
  • FunctionDeclaration, ClassDeclaration, PushEnvironment, PopEnvironment

Expression Bytecode Family Groups (current)

  • Loads and identifier references
  • Call target resolution and super setup
  • Array/object construction and property definitions
  • Property get/set/update/delete operations
  • Unary, binary, and conversion operations
  • Control flow, stack mechanics, and invocation
  • Stateful iterator, for-in, and array destructuring driver operations

General Expression Lowering Gaps (current)

  • None.

Production This-Binding Boundary

  • Ordinary sync functions that reference this are admitted to the production unified-bytecode path. The _homeObject is not null blanket rejection was removed from CanUseProductionUnifiedBytecodeFastPath in PR #2633 (ADR 0279).
  • _homeObject is set for all class methods and object-literal methods defined with method-definition syntax. A class method is admitted when its plan body contains no super-property opcodes. The existing property-write boundary still applies: compound read-then-write is admitted for the named-property shape (activation-resolved base — this.prop or box.prop, named key, production binary operator e.g. +=, simple RHS) via TryIsFirstBoundaryNamedCompoundPropertyWriteCandidate, and logical-assignment member writes (&&=, ||=, ??=) via TryIsFirstBoundaryNamedLogicalPropertyWriteCandidate. Direct computed logical writes (box[key] &&= y, box[key] ||= y, box[key] ??= y) are also admitted when they match TryIsFirstBoundaryComputedLogicalPropertyWriteCandidate (activation-resolved base, supported computed-key span, simple RHS, non-optional target). Direct computed assignments, compound writes, logical writes, and updates may use the same supported computed-key expression span as optional computed reads (box[key + suffix] = y, box[key + suffix] += y, box[key + suffix] &&= y, box[key + suffix]++) when the RHS is a simple operand and the operator-specific shape is already admitted. Simple nested named receiver assignments, compound writes, logical writes, and updates (box.child.value = y, box.child.value += y, box.child.value &&= y, box.child.value++) are admitted through TryIsFirstBoundaryNestedNamedPropertyWriteCandidate and prefix-aware TryIsFirstBoundaryNamedCompoundPropertyWriteCandidate and prefix-aware TryIsFirstBoundaryNamedLogicalPropertyWriteCandidate and TryIsFirstBoundaryNestedNamedPropertyUpdateCandidate, which compile the receiver chain as owned GetNamedProperty opcodes before the final SetNamedProperty / UpdateNamedProperty. Optional named and computed read chains, including named-then-computed continuations, may also start after a plain named receiver prefix (box.child?.value, box.child?.[key], box.child?.items[key]), with the prefix emitted as owned GetNamedProperty reads before the optional jump-to-chain-end boundary. Nested named receiver chains ending in a simple computed delete (delete box.child[key], delete box.child[left + right]) are admitted by TryIsFirstBoundaryComputedPropertyDeleteCandidate; optional named deletes (delete box?.value, delete box.child?.value) and optional computed delete chains (delete box?.[key], delete box?.child[key], delete box.child?.[key], and delete box?.child?.[key]) are admitted for activation-resolved receivers, supported computed-key spans, and compiler-owned nullish short-circuit-to-true lowering (ADR 0317 plus the optional delete follow-ups). The compiler emits the named receiver reads and final DeleteNamedProperty / DeleteComputedProperty, while the VM's descriptor-aware delete helper owns strict/sloppy results. Retained declines include unsupported chained optional delete neighbors, private receiver-chain/mutation neighbors outside the admitted direct named shape, dynamic lookup, unsupported computed-key spans, unsupported RHS spans, and optional/super/private neighbors of computed-key mutation shapes. Private member deletes are rejected before this production gate. Note: slot-identifier logical assignment (x &&= y) remains admitted.
  • Super-property reads, writes, and updates are now VM-owned through EnsureSuperReference, GetNamedSuperProperty, GetComputedSuperProperty, SetNamedSuperProperty, SetComputedSuperProperty, UpdateNamedSuperProperty, and UpdateComputedSuperProperty. SuperPropertyDependency remains only as the safety net for super call-target preparation outside the first production invocation boundary.
  • Sloppy-mode this coercion is handled before VM entry: boundThis is computed via CoerceThisValueForNonStrict in TryInvokeProductionUnifiedBytecode, so the VM's LoadThis opcode always receives the correctly coerced value.
  • Simple-return arrow functions whose lowered body has no lexical this, new.target, super, closure-variable, dynamic-identifier, or nested function/class literal dependency may route. Dependency-bearing arrows still decline via the IsArrowFunction, captured-activation, or _lexicalThisEnvironment is not null pre-gates.
  • There is no retained this decline in the production activation descriptor; future shapes that cannot thread this through the VM should introduce a concrete gate with a current failing example instead of carrying a placeholder.

Production Loop-Control Boundary

  • Current production control-flow support is compiler-owned, not source-syntax-owned.
  • Accepted loop-control shapes include branch joins, direct branches, canonical condition-first loop backedges, unlabeled BreakInstruction and ContinueInstruction target jumps, simple for-style update continue targets, and simple do-while consequent backedges proven by PR #2489.
  • Labeled breakable control flow is admitted (ADR 0285): labeled statements, labeled loops, labeled block break, and labeled break/continue route through the same compiler-owned resolved-target path as the unlabeled case. Label resolution is not a source-syntax permission — a labeled construct routes whenever its unlabeled IR topology would route.
  • The one labeled shape still declined with LabelControlFlow is a labeled break/continue that transfers control out of an enclosing iterator/for-in driver loop it is not directly targeting (driver-crossing). The VM's single-level driver cleanup only closes the driver whose break target equals the abrupt jump target, so an intervening inner iterator would be leaked; multi-driver labeled cleanup is the next loop-control widening frontier.
  • Unsupported complex loop/control-flow shapes must decline before VM execution instead of falling back from inside UnifiedBytecodeVirtualMachine.
  • The pre-ADR 0253 break/continue-only decline taxonomy is retired: PR #2489 removed the blanket production pre-scan that rejected every break/continue instruction, and no ordinary sync site should produce a break/continue-specific decline code.

Production Block Lexical Scope Boundary

  • Current production block-scope support is slot-layout-owned, not source-syntax-owned.
  • Accepted block scopes are non-capturing, non-iterating lexical scopes with flat slot mappings. Destructuring-free let / const declarations compile through SimpleVariableDeclaration into active-scope flat slots.
  • UnifiedBytecodeProgram.SlotCount covers root activation slots and admitted block-scope flat slots. Parameter slot metadata preserves formal positions, using -1 for unused or unmapped formals so invocation does not shift later arguments.
  • PushEnvironment initializes the admitted scope's lexical flat slots to JsValue.Uninitialized; PopEnvironment is an owned VM cleanup opcode for the admitted linear shape. Neither opcode creates a JsEnvironment, uses name fallback, or calls back into ExpressionProgram, ExecutionPlanRunner, or AST evaluation.
  • Unsupported binding declaration shapes, unsupported object/array destructuring driver instructions, direct eval / unsupported dynamic lookup, captured activation, per-iteration bindings, and using / await using remain pre-VM declines.

Production With-Backed Dynamic Name Boundary

  • Current production dynamic-name support is with-backed and compiler-gated. The compiler may admit otherwise-supported sync functions that enter a with environment and then use dynamic identifier load, store, update, typeof, delete, assignment-reference, or receiver-aware identifier-call opcodes.
  • EnterWith and LeaveWith create and unwind object environments through the existing JsEnvironment with-binding helpers. The VM-owned opcodes must preserve proxy traps, Symbol.unscopables, strict/sloppy assignment, delete, and receiver binding without falling back into mixed execution.
  • The sync invocation bridge must create an activation environment that already contains function-scoped var bindings before VM execution, so a callee's hoisted var names shadow an outer with object instead of falling through to dynamic lookup.
  • PrepareDynamicIdentifierCallTarget must resolve an active with binding regardless of identifier-cache state. When the binding exists, the receiver is the with binding object and the callee is read through that captured binding.
  • Dynamic identifier support does not make dynamic activation globally admissible. Direct eval is admitted only through the one-argument non-spread eval-identifier call boundary; unresolved non-with dynamic activation, captured dynamic activation, arguments-object dependencies, and async/generator functions still decline before VM execution.

Production Call Invocation Boundary

  • Current executable call support covers activation-resolved identifier calls, direct named member calls whose optional-free named receiver chain is activation-resolved, and direct computed member calls whose receiver chain remains inside the shallow computed-call boundary. Arguments may be simple literal or slot operands, or simple array/object literal spans ([a, b], {x: a, y: b}), including object literal spread entries whose spread source is a simple operand ({...source}), with no computed keys or name inference in that restricted simple-span form (gh2705, ADR 0290). General object method/accessor literal construction is VM-owned outside that restricted span. Computed member keys must also be simple literal or slot operands. For accepted member receiver chains, the call receiver is the final resolved receiver object, not the root object.
  • Synchronous spread calls are admitted (gh2676): f(...args), f(...a, ...b), obj.method(...args), and mixed f(a, ...b, c). Each argument (positional or spread) lowers to one value-producing load; the CallInvocationBoundary operand low 16 bits hold the pushed argument value count and the high bits hold a spread-mask reference (spreadMaskIndex + 1, where 0 means no spread). The VM flattens spread iterables left-to-right at the boundary via the shared TypedAstEvaluator.EnumerateSpread helper, preserving iteration order and side-effects, then invokes through the existing callable helpers with receiver-as-this. Optional identifier spread calls such as fn?.(...args) skip argument lowering when fn is nullish and otherwise use the same spread-mask boundary. Optional-start computed member spread calls such as a?.box[key](...args) use the same spread-mask boundary after the optional receiver has been resolved. Direct eval is admitted only for the one-argument non-spread eval-identifier shape; multi-argument/spread direct eval and super spread stay declined.
  • Synchronous construct calls are admitted (gh2690 plus the construct-boundary widening): new F(...), new F(...args), new box.Ctor(...), and new box[key](...) when the constructor/key/argument subexpressions are already production-safe. Construct results may feed trailing plain named property reads such as new F(...args).x. The constructor value and each logical argument are pushed left-to-right by their own loads (no receiver/this), then a single ConstructInvocationBoundary opcode invokes [[Construct]] with the constructor itself as new.target, mirroring the spec-conformant construct reference helper. Its operand uses the same low-16-bit argument count plus high-bit spread-mask reference encoding as CallInvocationBoundary; the VM flattens spread iterables at the construct boundary before calling ReflectHelper.Construct. A non-constructor target throws TypeError at the boundary.
  • Derived-constructor super(...) calls are admitted through SuperConstructInvocationBoundary, including spread arguments such as super(...args). The VM resolves the dynamic super constructor from the current super binding, flattens spread iterables at the boundary when needed, invokes [[Construct]] with the caller's new.target, initializes this, and preserves the existing double-super-call ReferenceError.
  • Accepted identifier-call programs use PrepareIdentifierCallTarget or PrepareIdentifierOptionalCallTarget followed by CallInvocationBoundary; the VM resolves the callable from unified bytecode-owned slot state and invokes it through existing callable invocation helpers with the active EvaluationContext and caller JsEnvironment when the callee needs environment-aware or debug-aware invocation state. The optional variant packs a nullish short-circuit jump target and jumps past argument lowering and the call boundary when the callee slot is nullish.
  • Accepted named member-call programs use PrepareNamedCallTarget followed by CallInvocationBoundary; the VM keeps the final receiver on the stack, loads the named callee from that receiver, and invokes through the existing receiver-as-this stack contract.
  • Accepted computed member-call programs use PrepareComputedCallTarget followed by CallInvocationBoundary; the VM keeps the final receiver on the stack, preserves nullish-receiver-before-key-coercion ordering, consumes the computed key with normal property-key semantics, loads the callee from that receiver, and invokes through the existing receiver-as-this stack contract.
  • Accepted named super-member call programs use PrepareNamedSuperCallTarget followed by CallInvocationBoundary; the VM resolves the named property against the active super binding and invokes with the derived receiver as this.
  • Accepted computed super-member call programs use PrepareComputedSuperCallTarget followed by CallInvocationBoundary; the VM evaluates the key before resolving the super-member callee and invokes with the derived receiver as this.
  • Ordinary non-optional identifier/member/super call-target opcodes now lower through the general expression loop. Direct eval and optional-call shapes still use the first-boundary call-target helper because they carry boundary-local directness metadata or packed nullish short-circuit jump targets.
  • Accepted optional call programs either use a jump-owned optional-start prefix followed by an ordinary call-target preparation opcode (a?.b.c(args), a.x?.b.c(args), a?.b[k](args)) or use PrepareIdentifierOptionalCallTarget, PrepareNamedOptionalCallTarget, or PrepareComputedOptionalCallTarget followed by CallInvocationBoundary. The optional preparation opcodes pack a call-target constant index (low 16 bits) and a nullish short-circuit jump target (high 16 bits) into a single operand. The VM checks the receiver or callee for nullish before argument evaluation proceeds; if nullish, the stack top becomes Undefined and execution jumps past the call boundary. Admitted callee/receiver-optional patterns include callee-optional identifier calls (fn?.(args)), receiver-optional named calls (box?.read(args)), callee-optional named calls (box.read?.(args)), and callee-optional computed calls (box[key]?.(args)). The IsOptionalReceiverCheck flag in UnifiedBytecodeCallTarget distinguishes the two named variants. (Optional member calls admitted as of gh2689; ADR 0289.)
  • Direct eval programs use PrepareIdentifierCallTarget followed by CallInvocationBoundary with the direct-eval operand flag. The admitted shape is exactly one non-spread eval-identifier argument; the VM invokes the eval host with the caller environment and syncs mutated caller slots back into unified bytecode-owned slots before execution continues.
  • Private-adjacent member targets outside the admitted direct named method-call shape, arguments-object dependencies, unsupported non-with dynamic lookup, deeper computed-member call receiver chains, complex receiver/key shapes, spread-onto-optional calls, and receiver-binding-sensitive adjacent families still decline before VM execution. (Synchronous spread calls are admitted as of gh2676; synchronous non-spread construct calls are admitted as of gh2690.)
  • Accepted programs must still satisfy the no-mixed-execution rule: no callback into ExpressionProgram, ExecutionPlanRunner, or AST evaluation is allowed from UnifiedBytecodeVirtualMachine.

Production Driver-State Boundary

  • Current driver-state support is VM-owned and descriptor-backed. Driver instructions compile to UnifiedBytecodeDriverDescriptor entries rather than unrelated stack-op translations or AST payloads.
  • break compiles as JumpWithDriverCleanup; before transferring control the VM closes active driver states whose descriptor break target matches the jump target. continue remains a plain same-loop Jump.
  • Accepted sync iterator driver shapes include IteratorInit, IteratorMoveNext, and IteratorClose when the iterable source is lowered to synchronous expression bytecode, the iterator kind is Sync, and no awaited source program is required. Slice A (#2678) also admits a TDZ head environment (for (const x of …) / for (let x of …)): the compiler emits a TdzHeadInit instruction that resolves the head bindings to flat slots, and the VM marks them JsValue.Uninitialized before the source is evaluated so a read of the head binding inside the source throws a ReferenceError.
  • Accepted for-in driver shapes include ForInInit and ForInMoveNext when the object source is lowered to synchronous expression bytecode and no awaited source program is required. Slice A (#2678) also admits a TDZ head environment (for (const k in …) / for (let k in …)) via the same TdzHeadInit mechanism. The VM owns driver state and skips deleted properties during enumeration.
  • Accepted array destructuring driver shapes include ArrayDestructuringInit, ArrayDestructuringElement, ArrayDestructuringRest, and ArrayDestructuringClose when the source is lowered to expression bytecode and element/rest targets resolve to unified flat slots. Normal and abrupt cleanup close the destructuring iterator from VM-owned driver state.
  • Accepted object destructuring driver shapes include ObjectDestructuringInit, ObjectDestructuringProperty, ObjectDestructuringRest, and ObjectDestructuringClose when the source is lowered to expression bytecode, all property keys are static identifiers (no computed keys), there are no defaults or nested patterns, and every property and rest target resolves to a unified flat slot. The VM coerces the source with ToObject, reads properties in source order (observable getter side effects preserved), and a trailing rest target collects the remaining own enumerable keys minus the consumed ones. Abrupt completion (a non-coercible source or a throwing getter) closes the VM-owned driver state.
  • Accepted expression-level assignment destructuring shapes that lower through ApplyBindingTarget are admitted through a descriptor-backed BindingTargetProgram constant. The production VM duplicates the assignment RHS per expression bytecode stack semantics, applies the binding target in BindingMode.Assign, and syncs unified slots with the activation environment before and after the bridge so existing computed-key, default, rest, nested, TDZ, and iterator-closing semantics stay centralized.
  • Object destructuring with computed/dynamic keys, defaults, nested patterns, or rest targets that cannot resolve to unified slots outside the descriptor-backed assignment lane, async iterator drivers, awaited driver sources, dynamic-name shapes, and targets that cannot resolve to unified slots still decline before VM execution. Sync-driver TDZ head environments are now admitted (Slice A, #2678); async-kind and awaited-source TDZ heads remain declined.

Production Resumable Boundary

  • Current production resumable support is a separate async/generator route with VM-owned suspension state. Accepted programs preserve program counter, operand stack, slots, pending await promise, pending resume payload, and completion state inside the unified runtime.
  • Accepted resumable generator shapes include simple yield / resume-value storage bodies that compile through Yield and StoreResumeValue, plus sync yield* bodies whose delegated .next(), .return(value), and .throw(value) resume behavior is owned by the VM YieldStar opcode.
  • Accepted resumable async shapes include simple awaited discard and awaited return bodies that compile through AwaitAndDiscard and AwaitedReturn.
  • this-dependent async and generator programs are admitted (resumable-route counterpart to the ordinary sync this support; see Production This-Binding Boundary above and ADR 0283). The strict/sloppy-coerced boundThis is computed in the async and sync-generator invokers via CoerceThisValueForNonStrict and stored on UnifiedBytecodeResumeState at construction so it survives suspension/resume across yield/await. The resumable LoadThis opcode pushes state.ThisValue. Property reads such as this.x remain outside the resumable opcode set and decline independently of the this-binding gate.
  • Captured/dynamic activation, arguments objects, new.target, calls, dynamic lookup, labels, iterator/destructuring drivers, unsupported expression payloads, and unmodeled statement families still decline before VM execution.
  • Async-generator yield*, awaited delegated sources, and unsupported delegated expression payloads remain outside production resumable routing until their promise/async-iterator settlement semantics are modeled by the VM. Focused issue #2955 coverage pins delegated async-generator .return(value) and .throw(value) on the existing IR async-generator path with no unified-bytecode-resumable-* route log.

Reserved Ownership Lanes (planned, not implemented)

  • Compiler-owned control-flow widening lanes
  • VM-owned property and assignment semantics lanes
  • Expression-op translation/normalization lanes
  • Statement-storage/diagnostics compaction lanes
  • Production-eligibility boundary and decline-taxonomy lanes

Reserved lanes define ownership for parallel work and do not imply runtime support today.

Iterator/Destructuring Model Boundary

  • IteratorInitInstruction, IteratorMoveNextInstruction, and IteratorCloseInstruction are eligible only for the synchronous lowered iterator-driver model described above.
  • ForInInitInstruction and ForInMoveNextInstruction are eligible only for the lowered for-in driver model described above. Unsupported for-in driver shapes still decline with ForInDriverStateDependency.
  • ArrayDestructuringInitInstruction, ArrayDestructuringElementInstruction, ArrayDestructuringRestInstruction, and ArrayDestructuringCloseInstruction are eligible only for the lowered direct-slot array destructuring model described above. Unsupported destructuring shapes still decline with DestructuringDependency.
  • ObjectDestructuringInitInstruction, ObjectDestructuringPropertyInstruction, ObjectDestructuringRestInstruction, and ObjectDestructuringCloseInstruction are eligible only for the lowered direct-slot object destructuring model described above (static keys, identifier targets, no defaults, no nested patterns, optional identifier rest). Static nested binding declarations with identifier/rest targets are admitted through ApplyDeclarationBindingTarget. Computed/dynamic-name keys, defaults, assignment targets, awaited binding values, and using declarations still decline with DestructuringDependency or UnsupportedPlanShape. ADR 0284 records the model-first decision and admit/decline boundary.
  • ExpressionOpKind.ApplyBindingTarget is eligible for ordinary sync production assignment destructuring when the expression compiler can lift the lowered BindingTargetProgram into the unified program descriptor table.
  • ApplyDeclarationBindingTarget is eligible for ordinary sync var / let / const binding declarations whose lowered BindingTargetProgram contains only static identifier/array/object/rest targets with no default or computed subprograms. This lane preserves existing binding-target semantics as a bridge; it does not make unsupported destructuring driver shapes production-eligible.
  • Decision for this lane: model-first. Any future widening must preserve explicit driver-state descriptors and pre-VM declines for shapes that would require mixed IR/AST execution.

Ranked Next Unsupported Buckets (current boundary)

  1. Activation model gaps are still larger than any single expression-family neighbor. Ordinary sync production routing still pre-gates async-like functions, generators, arrows needing lexical-this environments or super bindings, closure or dynamic identifier dependencies, or non-simple bodies, real arguments object and sloppy mapped-arguments dependencies, non-simple parameter lists, default/rest/destructured parameter expressions, broader class constructor routes beyond simple base constructors and the bounded explicit derived super(...) path, default derived constructors, and captured activation outside the explicit with-backed dynamic-name lane. Resumable unified bytecode exists, but EvaluateResumable admits only a small instruction/opcode subset compared with ordinary sync production bytecode.
  2. Wider call invocation remains a high-impact unsupported bucket. Synchronous spread calls are now admitted (gh2676). Optional calls are now admitted (gh2689, ADR 0289): box?.read(args), box.read?.(args), and box[key]?.(args). Synchronous construct calls (new F(...), spread arguments, member/computed constructor targets, and trailing plain named result reads such as new F(...args).x) are now admitted (gh2690, ADR 0286 plus construct-boundary widening). The first bounded super invocation shapes are now admitted too (PR #2862, ADR 0307): derived-constructor super(...), including spread arguments, plus named/computed super-member calls. Simple array and object literal arguments (fn([a, b]), fn({x: a})) are now admitted (gh2705, ADR 0290). Optional-start computed member spread calls (a?.box[key](...args)) are also admitted through the shared spread-mask invocation boundary, and named-prefix optional plain calls such as a.x?.box.read(value) route through the same optional-call machinery after the prefix is resolved by owned named-property reads. Simple object literal spread entries are now admitted through the ObjectSpread opcode when their spread source is a simple operand. Direct eval outside the one-argument non-spread eval-identifier boundary, private-adjacent member targets outside the admitted direct named method-call shape, complex receiver/key shapes, non-simple literal argument spans, and receiver-binding-sensitive adjacent families beyond the direct activation-resolved and with-backed dynamic-identifier boundaries must still decline before VM execution.
  3. Property and assignment widening is no longer a blanket property-read/write gap, but several member-expression neighbors remain outside production: private receiver-chain/mutation neighbors outside the admitted direct named shapes, optional-chain neighbors outside the admitted read/call/delete shapes, richer computed-key spans, unsupported RHS spans, optional/super/private mutation neighbors, and dynamic lookup outside the explicit with-backed lane. Private member deletes are parser early errors, not production declines.
  4. Driver-state widening is next. Sync-driver TDZ head environments (for (const x of …) / for (let k in …)) are now admitted via the TdzHeadInit instruction (Slice A, #2678; see ADR 0288). Async iterator drivers and awaited iterator/for-in sources remain outside the admitted boundary and must decline before VM execution.
  5. Destructuring widening is still model-first. Simple array and object destructuring driver shapes are admitted (static keys, identifier targets, no defaults/nested patterns, optional identifier rest), and expression-level assignment destructuring that lowers through ApplyBindingTarget is admitted through the descriptor-backed binding-target bridge. Generic binding declarations, unsupported driver shapes, and targets outside the direct-slot or descriptor-backed assignment lanes remain outside the admitted boundary (DestructuringDependency).
  6. Dynamic lookup families remain outside the admitted boundary (DynamicLookupDependency) except for the ordinary dynamic-name environment path and the explicit with-backed dynamic name slice above. Direct eval outside the admitted one-argument non-spread eval-identifier boundary, unresolved dynamic activation, and unsupported unresolved lookup shapes still decline before VM execution.
  7. Label-dependent control flow is now admitted (ADR 0285): labeled statements, labeled loops, labeled block break, and labeled break/continue route through the compiler-owned resolved-target path. The remaining LabelControlFlow decline is narrow — a labeled break/continue that crosses (exits) an enclosing iterator/for-in driver loop it is not directly targeting. Multi-driver labeled cleanup is the next loop-control widening frontier.

Proof Commands

rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~ExpressionProgramCoverageMapTests"
rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionEligibilityTests"
rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodePrototypeTests"
rtk dotnet test tests/Asynkron.JsEngine.Tests --filter "FullyQualifiedName~UnifiedBytecodeProductionInvocationTests"
rtk rg "EvaluateExpression\\(|ProfileEvaluateExpression\\(" src/Asynkron.JsEngine/Ast/TypedAstEvaluator.ExecutionPlanRunner*
rtk ./tools/profile forloop --route-hits
rtk ./tools/profile propertyaccess --route-hits
rtk ./tools/profile functioncalls-lite --route-hits
rtk ./tools/profile activation-noargs-lite --route-hits
rtk ./tools/profile forofiteration --route-hits
rtk ./tools/profile forloop --memory