[vm] Remove transition_sentinel and detection of cyclic initialization of legacy static fields

Since null safety, all static fields with initializers are implicitly
late. This change cleans up transition_sentinel which was used in
the detection of cyclic initialization of legacy static fields.

TEST=ci

Change-Id: I6a990dc8ba030f5bd40eb0b86706cbfb0f725e33
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373520
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/vm_service/lib/src/vm_service.dart

@@ -2389,7 +2389,7 @@ abstract class SentinelKind {
   /// Indicates that a variable or field has not been initialized.
   static const String kNotInitialized = 'NotInitialized';
 
-  /// Indicates that a variable or field is in the process of being initialized.
+  /// Deprecated, no longer used.
   static const String kBeingInitialized = 'BeingInitialized';
 
   /// Indicates that a variable has been eliminated by the optimizing compiler.
@@ -2513,8 +2513,6 @@ class AllocationProfile extends Response {
 /// If the field is uninitialized, the `value` will be the `NotInitialized`
 /// [Sentinel].
 ///
-/// If the field is being initialized, the `value` will be the
-/// `BeingInitialized` [Sentinel].
 class BoundField {
   static BoundField? parse(Map<String, dynamic>? json) =>
       json == null ? null : BoundField._fromJson(json);
@@ -2567,9 +2565,6 @@ class BoundField {
 /// If the variable is uninitialized, the `value` will be the `NotInitialized`
 /// [Sentinel].
 ///
-/// If the variable is being initialized, the `value` will be the
-/// `BeingInitialized` [Sentinel].
-///
 /// If the variable has been optimized out by the compiler, the `value` will be
 /// the `OptimizedOut` [Sentinel].
 class BoundVariable extends Response {

runtime/observatory/lib/src/elements/sentinel_value.dart

@@ -55,8 +55,6 @@ class SentinelValueElement extends CustomElement implements Renderable {
       case M.SentinelKind.notInitialized:
         return 'This object will be initialized once it is accessed by '
             'the program.';
-      case M.SentinelKind.initializing:
-        return 'This object is currently being initialized.';
       case M.SentinelKind.optimizedOut:
         return 'This object is no longer needed and has been removed by the '
             'optimizing compiler.';

runtime/observatory/lib/src/elements/sentinel_view.dart

@@ -90,8 +90,6 @@ class SentinelViewElement extends CustomElement implements Renderable {
       case M.SentinelKind.notInitialized:
         return 'This object will be initialized once it is accessed by '
             'the program.';
-      case M.SentinelKind.initializing:
-        return 'This object is currently being initialized.';
       case M.SentinelKind.optimizedOut:
         return 'This object is no longer needed and has been removed by the '
             'optimizing compiler.';

runtime/observatory/lib/src/models/objects/sentinel.dart

@@ -14,9 +14,6 @@ enum SentinelKind {
   /// Indicates that a variable or field has not been initialized.
   notInitialized,
 
-  /// Indicates that a variable or field is in the process of being initialized.
-  initializing,
-
   /// Indicates that a variable has been eliminated by the optimizing compiler.
   optimizedOut,
 

runtime/observatory/lib/src/service/object.dart

@@ -3319,8 +3319,6 @@ M.SentinelKind stringToSentinelKind(String s) {
       return M.SentinelKind.expired;
     case 'NotInitialized':
       return M.SentinelKind.notInitialized;
-    case 'BeingInitialized':
-      return M.SentinelKind.initializing;
     case 'OptimizedOut':
       return M.SentinelKind.optimizedOut;
     case 'Free':

runtime/vm/app_snapshot.cc

@@ -6815,8 +6815,6 @@ class VMSerializationRoots : public SerializationRoots {
 
     s->AddBaseObject(Object::null(), "Null", "null");
     s->AddBaseObject(Object::sentinel().ptr(), "Null", "sentinel");
-    s->AddBaseObject(Object::transition_sentinel().ptr(), "Null",
-                     "transition_sentinel");
     s->AddBaseObject(Object::optimized_out().ptr(), "Null", "<optimized out>");
     s->AddBaseObject(Object::empty_array().ptr(), "Array", "<empty_array>");
     s->AddBaseObject(Object::empty_instantiations_cache_array().ptr(), "Array",
@@ -6940,7 +6938,6 @@ class VMDeserializationRoots : public DeserializationRoots {
 
     d->AddBaseObject(Object::null());
     d->AddBaseObject(Object::sentinel().ptr());
-    d->AddBaseObject(Object::transition_sentinel().ptr());
     d->AddBaseObject(Object::optimized_out().ptr());
     d->AddBaseObject(Object::empty_array().ptr());
     d->AddBaseObject(Object::empty_instantiations_cache_array().ptr());

runtime/vm/compiler/aot/precompiler.cc

@@ -1217,8 +1217,7 @@ void Precompiler::AddConstObject(const class Instance& instance) {
     return;
   }
 
-  if (instance.ptr() == Object::sentinel().ptr() ||
-      instance.ptr() == Object::transition_sentinel().ptr()) {
+  if (instance.ptr() == Object::sentinel().ptr()) {
     return;
   }
 
@@ -1332,8 +1331,6 @@ void Precompiler::AddField(const Field& field) {
     auto field_table = field.is_shared() ? IG->shared_initial_field_table()
                                          : IG->initial_field_table();
     const Object& value = Object::Handle(Z, field_table->At(field.field_id()));
-    // Should not be in the middle of initialization while precompiling.
-    ASSERT(value.ptr() != Object::transition_sentinel().ptr());
 
     if (value.ptr() != Object::sentinel().ptr() &&
         value.ptr() != Object::null()) {

runtime/vm/compiler/backend/il.cc

@@ -4571,22 +4571,18 @@ void LoadStaticFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
       return;
     }
     ASSERT(field().has_initializer());
+    ASSERT(field().is_late());
     auto object_store = compiler->isolate_group()->object_store();
     const Field& original_field = Field::ZoneHandle(field().Original());
 
-    compiler::Label no_call, call_initializer;
+    compiler::Label no_call;
     __ CompareObject(result, Object::sentinel());
-    if (!field().is_late()) {
-      __ BranchIf(EQUAL, &call_initializer);
-      __ CompareObject(result, Object::transition_sentinel());
-    }
     __ BranchIf(NOT_EQUAL, &no_call);
 
     auto& stub = Code::ZoneHandle(compiler->zone());
-    __ Bind(&call_initializer);
     if (field().needs_load_guard()) {
       stub = object_store->init_static_field_stub();
-    } else if (field().is_late()) {
+    } else {
       // The stubs below call the initializer function directly, so make sure
       // one is created.
       original_field.EnsureInitializerFunction();
@@ -4598,11 +4594,6 @@ void LoadStaticFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
               : (field().is_final()
                      ? object_store->init_late_final_static_field_stub()
                      : object_store->init_late_static_field_stub());
-    } else {
-      // We call to runtime for non-late fields because the stub would need to
-      // catch any exception generated by the initialization function to change
-      // the value of the static field from the transition sentinel to null.
-      stub = object_store->init_static_field_stub();
     }
 
     __ LoadObject(InitStaticFieldABI::kFieldReg, original_field);

runtime/vm/compiler/backend/il_serializer.cc

@@ -1692,8 +1692,6 @@ void FlowGraphSerializer::WriteObjectImpl(const Object& x,
     case kSentinelCid:
       if (x.ptr() == Object::sentinel().ptr()) {
         Write<uint8_t>(0);
-      } else if (x.ptr() == Object::transition_sentinel().ptr()) {
-        Write<uint8_t>(1);
       } else if (x.ptr() == Object::optimized_out().ptr()) {
         Write<uint8_t>(2);
       } else {
@@ -1978,8 +1976,6 @@ const Object& FlowGraphDeserializer::ReadObjectImpl(intptr_t cid,
       switch (Read<uint8_t>()) {
         case 0:
           return Object::sentinel();
-        case 1:
-          return Object::transition_sentinel();
         case 2:
           return Object::optimized_out();
         default:

runtime/vm/exceptions.cc

@@ -1202,10 +1202,6 @@ ObjectPtr Exceptions::Create(ExceptionType type, const Array& arguments) {
       constructor_name = &Symbols::DotCreate();
       break;
 #endif
-    case kCyclicInitializationError:
-      library = Library::CoreLibrary();
-      class_name = &Symbols::_CyclicInitializationError();
-      break;
     case kCompileTimeError:
       library = Library::CoreLibrary();
       class_name = &Symbols::_CompileTimeError();

runtime/vm/exceptions.h

@@ -67,7 +67,6 @@ class Exceptions : AllStatic {
     kAssertion,
     kType,
     kAbstractClassInstantiation,
-    kCyclicInitializationError,
     kCompileTimeError,
     kLateFieldAssignedDuringInitialization,
     kLateFieldNotInitialized,

runtime/vm/hash_table.h

@@ -100,7 +100,7 @@ struct WeakAcqRelStorageTraits : WeakArrayStorageTraits {
 
 class HashTableBase : public ValueObject {
  public:
-  static const Object& UnusedMarker() { return Object::transition_sentinel(); }
+  static const Object& UnusedMarker() { return Object::sentinel(); }
   static const Object& DeletedMarker() { return Object::null_object(); }
 };
 
@@ -156,7 +156,7 @@ class HashTableBase : public ValueObject {
 // Each entry contains a key, followed by zero or more payload components,
 // and has 3 possible states: unused, occupied, or deleted.
 // The header tracks the number of entries in each state.
-// Any object except the backing storage array and Object::transition_sentinel()
+// Any object except the backing storage array and Object::sentinel()
 // may be stored as a key. Any object may be stored in a payload.
 //
 // Parameters

runtime/vm/isolate_reload.cc

@@ -2208,8 +2208,7 @@ class FieldInvalidator {
         // At that point it doesn't have the field table setup yet.
         if (field_table->IsReadyToUse()) {
           value_ = field_table->At(field_id);
-          if ((value_.ptr() != Object::sentinel().ptr()) &&
-              (value_.ptr() != Object::transition_sentinel().ptr())) {
+          if (value_.ptr() != Object::sentinel().ptr()) {
             CheckValueType(value_, field);
           }
         }

runtime/vm/message_snapshot.cc

@@ -27,8 +27,6 @@
 namespace dart {
 
 static Dart_CObject cobj_sentinel = {Dart_CObject_kUnsupported, {false}};
-static Dart_CObject cobj_transition_sentinel = {Dart_CObject_kUnsupported,
-                                                {false}};
 static Dart_CObject cobj_dynamic_type = {Dart_CObject_kUnsupported, {false}};
 static Dart_CObject cobj_void_type = {Dart_CObject_kUnsupported, {false}};
 static Dart_CObject cobj_empty_type_arguments = {Dart_CObject_kUnsupported,
@@ -3216,7 +3214,6 @@ MessageDeserializationCluster* BaseDeserializer::ReadCluster() {
 void MessageSerializer::AddBaseObjects() {
   AddBaseObject(Object::null());
   AddBaseObject(Object::sentinel().ptr());
-  AddBaseObject(Object::transition_sentinel().ptr());
   AddBaseObject(Object::empty_array().ptr());
   AddBaseObject(Object::dynamic_type().ptr());
   AddBaseObject(Object::void_type().ptr());
@@ -3228,7 +3225,6 @@ void MessageSerializer::AddBaseObjects() {
 void MessageDeserializer::AddBaseObjects() {
   AddBaseObject(Object::null());
   AddBaseObject(Object::sentinel().ptr());
-  AddBaseObject(Object::transition_sentinel().ptr());
   AddBaseObject(Object::empty_array().ptr());
   AddBaseObject(Object::dynamic_type().ptr());
   AddBaseObject(Object::void_type().ptr());
@@ -3240,7 +3236,6 @@ void MessageDeserializer::AddBaseObjects() {
 void ApiMessageSerializer::AddBaseObjects() {
   AddBaseObject(PredefinedCObjects::cobj_null());
   AddBaseObject(&cobj_sentinel);
-  AddBaseObject(&cobj_transition_sentinel);
   AddBaseObject(PredefinedCObjects::cobj_empty_array());
   AddBaseObject(&cobj_dynamic_type);
   AddBaseObject(&cobj_void_type);
@@ -3252,7 +3247,6 @@ void ApiMessageSerializer::AddBaseObjects() {
 void ApiMessageDeserializer::AddBaseObjects() {
   AddBaseObject(PredefinedCObjects::cobj_null());
   AddBaseObject(&cobj_sentinel);
-  AddBaseObject(&cobj_transition_sentinel);
   AddBaseObject(PredefinedCObjects::cobj_empty_array());
   AddBaseObject(&cobj_dynamic_type);
   AddBaseObject(&cobj_void_type);

runtime/vm/object.cc

@@ -830,7 +830,6 @@ void Object::Init(IsolateGroup* isolate_group) {
   // Allocate and initialize the sentinel values.
   {
     *sentinel_ ^= Sentinel::New();
-    *transition_sentinel_ ^= Sentinel::New();
   }
 
   // Allocate and initialize optimizing compiler constants.
@@ -1316,8 +1315,6 @@ void Object::Init(IsolateGroup* isolate_group) {
   ASSERT(empty_async_exception_handlers_->IsExceptionHandlers());
   ASSERT(!sentinel_->IsSmi());
   ASSERT(sentinel_->IsSentinel());
-  ASSERT(!transition_sentinel_->IsSmi());
-  ASSERT(transition_sentinel_->IsSentinel());
   ASSERT(!unknown_constant_->IsSmi());
   ASSERT(unknown_constant_->IsSentinel());
   ASSERT(!non_constant_->IsSmi());
@@ -12311,7 +12308,6 @@ bool Field::IsUninitialized() const {
   Thread* thread = Thread::Current();
   const FieldTable* field_table = thread->isolate()->field_table();
   const ObjectPtr raw_value = field_table->At(field_id());
-  ASSERT(raw_value != Object::transition_sentinel().ptr());
   return raw_value == Object::sentinel().ptr();
 }
 
@@ -12399,40 +12395,25 @@ ErrorPtr Field::InitializeStatic() const {
   ASSERT(IsOriginal());
   ASSERT(is_static());
   if (StaticValue() == Object::sentinel().ptr()) {
+    ASSERT(is_late());
     auto& value = Object::Handle();
-    if (is_late()) {
-      if (!has_initializer()) {
-        Exceptions::ThrowLateFieldNotInitialized(String::Handle(name()));
-        UNREACHABLE();
-      }
-      value = EvaluateInitializer();
-      if (value.IsError()) {
-        return Error::Cast(value).ptr();
-      }
-      if (is_final() && (StaticValue() != Object::sentinel().ptr())) {
-        Exceptions::ThrowLateFieldAssignedDuringInitialization(
-            String::Handle(name()));
-        UNREACHABLE();
-      }
-    } else {
-      SetStaticValue(Object::transition_sentinel());
-      value = EvaluateInitializer();
-      if (value.IsError()) {
-        SetStaticValue(Object::null_instance());
-        return Error::Cast(value).ptr();
-      }
+    if (!has_initializer()) {
+      Exceptions::ThrowLateFieldNotInitialized(String::Handle(name()));
+      UNREACHABLE();
+    }
+    value = EvaluateInitializer();
+    if (value.IsError()) {
+      return Error::Cast(value).ptr();
+    }
+    if (is_final() && (StaticValue() != Object::sentinel().ptr())) {
+      Exceptions::ThrowLateFieldAssignedDuringInitialization(
+          String::Handle(name()));
+      UNREACHABLE();
     }
     ASSERT(value.IsNull() || value.IsInstance());
     SetStaticValue(value.IsNull() ? Instance::null_instance()
                                   : Instance::Cast(value));
     return Error::null();
-  } else if (StaticValue() == Object::transition_sentinel().ptr()) {
-    ASSERT(!is_late());
-    const Array& ctor_args = Array::Handle(Array::New(1));
-    const String& field_name = String::Handle(name());
-    ctor_args.SetAt(0, field_name);
-    Exceptions::ThrowByType(Exceptions::kCyclicInitializationError, ctor_args);
-    UNREACHABLE();
   }
   return Error::null();
 }
@@ -12824,7 +12805,6 @@ StaticTypeExactnessState StaticTypeExactnessState::Compute(
     bool print_trace /* = false */) {
   ASSERT(!value.IsNull());  // Should be handled by the caller.
   ASSERT(value.ptr() != Object::sentinel().ptr());
-  ASSERT(value.ptr() != Object::transition_sentinel().ptr());
 
   Thread* thread = Thread::Current();
   Zone* const zone = thread->zone();
@@ -18754,8 +18734,6 @@ SentinelPtr Sentinel::New() {
 const char* Sentinel::ToCString() const {
   if (ptr() == Object::sentinel().ptr()) {
     return "sentinel";
-  } else if (ptr() == Object::transition_sentinel().ptr()) {
-    return "transition_sentinel";
   } else if (ptr() == Object::unknown_constant().ptr()) {
     return "unknown_constant";
   } else if (ptr() == Object::non_constant().ptr()) {

runtime/vm/object.h

@@ -447,9 +447,6 @@ class Object {
   //
   // - sentinel is a value that cannot be produced by Dart code. It can be used
   // to mark special values, for example to distinguish "uninitialized" fields.
-  // - transition_sentinel is a value marking that we are transitioning from
-  // sentinel, e.g., computing a field value. Used to detect circular
-  // initialization.
   // - unknown_constant and non_constant are optimizing compiler's constant
   // propagation constants.
   // - optimized_out results from deopt environment pruning or failure to
@@ -480,7 +477,6 @@ class Object {
   V(Array, synthetic_getter_parameter_types)                                   \
   V(Array, synthetic_getter_parameter_names)                                   \
   V(Sentinel, sentinel)                                                        \
-  V(Sentinel, transition_sentinel)                                             \
   V(Sentinel, unknown_constant)                                                \
   V(Sentinel, non_constant)                                                    \
   V(Sentinel, optimized_out)                                                   \
@@ -7580,9 +7576,6 @@ class ContextScope : public Object {
 // - Object::sentinel() is a value that cannot be produced by Dart code.
 // It can be used to mark special values, for example to distinguish
 // "uninitialized" fields.
-// - Object::transition_sentinel() is a value marking that we are transitioning
-// from sentinel, e.g., computing a field value. Used to detect circular
-// initialization of static fields.
 // - Object::unknown_constant() and Object::non_constant() are optimizing
 // compiler's constant propagation constants.
 // - Object::optimized_out() result from deopt environment pruning or failure

runtime/vm/object_graph.cc

@@ -1137,9 +1137,6 @@ class Pass2Visitor : public ObjectVisitor,
       if (obj == Object::sentinel().ptr()) {
         writer_->WriteUnsigned(kNameData);
         writer_->WriteUtf8("uninitialized");
-      } else if (obj == Object::transition_sentinel().ptr()) {
-        writer_->WriteUnsigned(kNameData);
-        writer_->WriteUtf8("initializing");
       } else {
         writer_->WriteUnsigned(kNoData);
       }

runtime/vm/object_service.cc

@@ -1129,12 +1129,6 @@ void Sentinel::PrintJSONImpl(JSONStream* stream, bool ref) const {
     jsobj.AddProperty("kind", "NotInitialized");
     jsobj.AddProperty("valueAsString", "<not initialized>");
     return;
-  } else if (ptr() == Object::transition_sentinel().ptr()) {
-    JSONObject jsobj(stream);
-    jsobj.AddProperty("type", "Sentinel");
-    jsobj.AddProperty("kind", "BeingInitialized");
-    jsobj.AddProperty("valueAsString", "<being initialized>");
-    return;
   } else if (ptr() == Object::optimized_out().ptr()) {
     JSONObject jsobj(stream);
     jsobj.AddProperty("type", "Sentinel");

runtime/vm/object_test.cc

@@ -6367,16 +6367,6 @@ ISOLATE_UNIT_TEST_CASE(PrintJSONPrimitives) {
         "\"valueAsString\":\"<not initialized>\"}",
         js.ToCString());
   }
-  // Transition sentinel reference
-  {
-    JSONStream js;
-    Object::transition_sentinel().PrintJSON(&js, true);
-    EXPECT_STREQ(
-        "{\"type\":\"Sentinel\","
-        "\"kind\":\"BeingInitialized\","
-        "\"valueAsString\":\"<being initialized>\"}",
-        js.ToCString());
-  }
 }
 
 #endif  // !PRODUCT

runtime/vm/runtime_entry.cc

@@ -3826,8 +3826,7 @@ DEFINE_RUNTIME_ENTRY(InitInstanceField, 2) {
   Object& result = Object::Handle(zone, field.InitializeInstance(instance));
   ThrowIfError(result);
   result = instance.GetField(field);
-  ASSERT((result.ptr() != Object::sentinel().ptr()) &&
-         (result.ptr() != Object::transition_sentinel().ptr()));
+  ASSERT(result.ptr() != Object::sentinel().ptr());
   arguments.SetReturn(result);
 }
 
@@ -3836,8 +3835,7 @@ DEFINE_RUNTIME_ENTRY(InitStaticField, 1) {
   Object& result = Object::Handle(zone, field.InitializeStatic());
   ThrowIfError(result);
   result = field.StaticValue();
-  ASSERT((result.ptr() != Object::sentinel().ptr()) &&
-         (result.ptr() != Object::transition_sentinel().ptr()));
+  ASSERT(result.ptr() != Object::sentinel().ptr());
   arguments.SetReturn(result);
 }
 

runtime/vm/service/service.md

@@ -1891,9 +1891,6 @@ _Instance_.
 If the field is uninitialized, the _value_ will be the
 _NotInitialized_ [Sentinel](#sentinel).
 
-If the field is being initialized, the _value_ will be the
-_BeingInitialized_ [Sentinel](#sentinel).
-
 ### BoundVariable
 
 ```
@@ -1918,9 +1915,6 @@ in a _Frame_.
 If the variable is uninitialized, the _value_ will be the
 _NotInitialized_ [Sentinel](#sentinel).
 
-If the variable is being initialized, the _value_ will be the
-_BeingInitialized_ [Sentinel](#sentinel).
-
 If the variable has been optimized out by the compiler, the _value_
 will be the _OptimizedOut_ [Sentinel](#sentinel).
 
@@ -4188,7 +4182,7 @@ enum SentinelKind {
   // Indicates that a variable or field has not been initialized.
   NotInitialized,
 
-  // Indicates that a variable or field is in the process of being initialized.
+  // Deprecated, no longer used.
   BeingInitialized,
 
   // Indicates that a variable has been eliminated by the optimizing compiler.

runtime/vm/symbols.h

@@ -291,7 +291,6 @@ class ObjectPointerVisitor;
   V(_ConstMap, "_ConstMap")                                                    \
   V(_ConstSet, "_ConstSet")                                                    \
   V(_ControllerSubscription, "_ControllerSubscription")                        \
-  V(_CyclicInitializationError, "_CyclicInitializationError")                  \
   V(_DeletedEnumPrefix, "Deleted enum value from ")                            \
   V(_DeletedEnumSentinel, "_deleted_enum_sentinel")                            \
   V(_Double, "_Double")                                                        \

sdk/lib/_internal/vm/lib/errors_patch.dart

@@ -132,30 +132,6 @@ class StateError {
   }
 }
 
-/// Error thrown when a lazily initialized variable cannot be initialized.
-///
-/// Cyclic dependencies are no longer detected at runtime in null safe code.
-/// Such code will fail in other ways instead,
-/// possibly with a [StackOverflowError].
-///
-/// Will be removed when support for non-null-safe code is discontinued.
-@Deprecated("Remove when no longer supporting non-null-safe code.")
-class _CyclicInitializationError extends Error {
-  final String? variableName;
-  @pragma("vm:entry-point")
-  _CyclicInitializationError([this.variableName]);
-  String toString() {
-    var variableName = this.variableName;
-    return variableName == null
-        ? "Reading static variable during its initialization"
-        : "Reading static variable '$variableName' during its initialization";
-  }
-
-  static _throwNew(String variableName) {
-    throw new _CyclicInitializationError(variableName);
-  }
-}
-
 @patch
 class NoSuchMethodError {
   final Object? _receiver;