Reland^2 "[string] Support shared strings in Value{Serializer,Deserializer}"

This is a reland of 5320fe8d58

Changes since revert:
- Remove stale DCHECK in deserializer

Original change's description:
> Reland "[string] Support shared strings in Value{Serializer,Deserializer}"
>
> This is a reland of 3cb4039cd1
>
> Changes since revert:
> - Fix FLAG_stress_scavenge interaction with shared Isolate
> - Use the shared Isolate's global handles to keep shared values
>   alive in transit during a postMessage
>
> Original change's description:
> > [string] Support shared strings in Value{Serializer,Deserializer}
> >
> > When FLAG_shared_string_table is true, postMessaging strings will share
> > instead of copy.
> >
> > Note that not all operations on shared strings are supported, and shared
> > strings may be slower than non-shared strings for some operations.

Bug: v8:12007
Change-Id: I70782978ed05558615eca03bafc4c12eba3644ca
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3417189
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78791}

include/v8-value-serializer-version.h

@@ -17,7 +17,7 @@
 
 namespace v8 {
 
-constexpr uint32_t CurrentValueSerializerFormatVersion() { return 14; }
+constexpr uint32_t CurrentValueSerializerFormatVersion() { return 15; }
 
 }  // namespace v8
 

include/v8-value-serializer.h

@@ -67,6 +67,23 @@ class V8_EXPORT ValueSerializer {
 
     virtual Maybe<uint32_t> GetWasmModuleTransferId(
         Isolate* isolate, Local<WasmModuleObject> module);
+
+    /**
+     * Returns whether shared values are supported. GetSharedValueId is only
+     * called if SupportsSharedValues() returns true.
+     */
+    virtual bool SupportsSharedValues() const;
+
+    /**
+     * Called when the ValueSerializer serializes a value that is shared across
+     * Isolates. The embedder must return an ID for the object. This function
+     * must be idempotent for the same object. When deserializing, the ID will
+     * be passed to ValueDeserializer::Delegate::GetSharedValueFromId as
+     * |shared_value_id|.
+     */
+    virtual Maybe<uint32_t> GetSharedValueId(Isolate* isolate,
+                                             Local<Value> shared_value);
+
     /**
      * Allocates memory for the buffer of at least the size provided. The actual
      * size (which may be greater or equal) is written to |actual_size|. If no
@@ -166,17 +183,30 @@ class V8_EXPORT ValueDeserializer {
 
     /**
      * Get a WasmModuleObject given a transfer_id previously provided
-     * by ValueSerializer::GetWasmModuleTransferId
+     * by ValueSerializer::Delegate::GetWasmModuleTransferId
      */
     virtual MaybeLocal<WasmModuleObject> GetWasmModuleFromId(
         Isolate* isolate, uint32_t transfer_id);
 
     /**
      * Get a SharedArrayBuffer given a clone_id previously provided
-     * by ValueSerializer::GetSharedArrayBufferId
+     * by ValueSerializer::Delegate::GetSharedArrayBufferId
      */
     virtual MaybeLocal<SharedArrayBuffer> GetSharedArrayBufferFromId(
         Isolate* isolate, uint32_t clone_id);
+
+    /**
+     * Returns whether shared values are supported. GetSharedValueFromId is only
+     * called if SupportsSharedValues() returns true.
+     */
+    virtual bool SupportsSharedValues() const;
+
+    /**
+     * Get a value shared across Isolates given a shared_value_id provided by
+     * ValueSerializer::Delegate::GetSharedValueId.
+     */
+    virtual MaybeLocal<Value> GetSharedValueFromId(Isolate* isolate,
+                                                   uint32_t shared_value_id);
   };
 
   ValueDeserializer(Isolate* isolate, const uint8_t* data, size_t size);

src/api/api.cc

@@ -3306,6 +3306,17 @@ Maybe<uint32_t> ValueSerializer::Delegate::GetWasmModuleTransferId(
   return Nothing<uint32_t>();
 }
 
+bool ValueSerializer::Delegate::SupportsSharedValues() const { return false; }
+
+Maybe<uint32_t> ValueSerializer::Delegate::GetSharedValueId(
+    Isolate* v8_isolate, Local<Value> shared_value) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
+  isolate->ScheduleThrow(*isolate->factory()->NewError(
+      isolate->error_function(), i::MessageTemplate::kDataCloneError,
+      Utils::OpenHandle(*shared_value)));
+  return Nothing<uint32_t>();
+}
+
 void* ValueSerializer::Delegate::ReallocateBufferMemory(void* old_buffer,
                                                         size_t size,
                                                         size_t* actual_size) {
@@ -3395,6 +3406,17 @@ MaybeLocal<WasmModuleObject> ValueDeserializer::Delegate::GetWasmModuleFromId(
   return MaybeLocal<WasmModuleObject>();
 }
 
+bool ValueDeserializer::Delegate::SupportsSharedValues() const { return false; }
+
+MaybeLocal<Value> ValueDeserializer::Delegate::GetSharedValueFromId(
+    Isolate* v8_isolate, uint32_t shared_value_id) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
+  isolate->ScheduleThrow(*isolate->factory()->NewError(
+      isolate->error_function(),
+      i::MessageTemplate::kDataCloneDeserializationError));
+  return MaybeLocal<Value>();
+}
+
 MaybeLocal<SharedArrayBuffer>
 ValueDeserializer::Delegate::GetSharedArrayBufferFromId(Isolate* v8_isolate,
                                                         uint32_t id) {

src/d8/d8.cc

@@ -28,6 +28,7 @@
 #include "include/v8-initialization.h"
 #include "include/v8-inspector.h"
 #include "include/v8-json.h"
+#include "include/v8-locker.h"
 #include "include/v8-profiler.h"
 #include "include/v8-wasm.h"
 #include "src/api/api-inl.h"
@@ -45,6 +46,7 @@
 #include "src/debug/debug-interface.h"
 #include "src/deoptimizer/deoptimizer.h"
 #include "src/diagnostics/basic-block-profiler.h"
+#include "src/execution/v8threads.h"
 #include "src/execution/vm-state-inl.h"
 #include "src/flags/flags.h"
 #include "src/handles/maybe-handles.h"
@@ -2594,11 +2596,18 @@ void Shell::QuitOnce(v8::FunctionCallbackInfo<v8::Value>* args) {
                       ->Int32Value(args->GetIsolate()->GetCurrentContext())
                       .FromMaybe(0);
   WaitForRunningWorkers();
-  args->GetIsolate()->Exit();
+  Isolate* isolate = args->GetIsolate();
+  isolate->Exit();
+
   // As we exit the process anyway, we do not dispose the platform and other
-  // global data. Other isolates might still be running, so disposing here can
-  // cause them to crash.
-  OnExit(args->GetIsolate(), false);
+  // global data and manually unlock to quell DCHECKs. Other isolates might
+  // still be running, so disposing here can cause them to crash.
+  i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
+  if (i_isolate->thread_manager()->IsLockedByCurrentThread()) {
+    i_isolate->thread_manager()->Unlock();
+  }
+
+  OnExit(isolate, false);
   base::OS::ExitProcess(exit_code);
 }
 
@@ -4862,6 +4871,30 @@ class Serializer : public ValueSerializer::Delegate {
 
   void FreeBufferMemory(void* buffer) override { base::Free(buffer); }
 
+  bool SupportsSharedValues() const override { return true; }
+
+  Maybe<uint32_t> GetSharedValueId(Isolate* isolate,
+                                   Local<Value> shared_value) override {
+    DCHECK_NOT_NULL(data_);
+    for (size_t index = 0; index < data_->shared_values_.size(); ++index) {
+      if (data_->shared_values_[index] == shared_value) {
+        return Just<uint32_t>(static_cast<uint32_t>(index));
+      }
+    }
+
+    size_t index = data_->shared_values_.size();
+    // Shared values in transit are kept alive by global handles in the shared
+    // isolate. No code ever runs in the shared Isolate, so locking it does not
+    // contend with long-running tasks.
+    {
+      DCHECK_EQ(reinterpret_cast<i::Isolate*>(isolate)->shared_isolate(),
+                reinterpret_cast<i::Isolate*>(Shell::shared_isolate));
+      v8::Locker locker(Shell::shared_isolate);
+      data_->shared_values_.emplace_back(Shell::shared_isolate, shared_value);
+    }
+    return Just<uint32_t>(static_cast<uint32_t>(index));
+  }
+
  private:
   Maybe<bool> PrepareTransfer(Local<Context> context, Local<Value> transfer) {
     if (transfer->IsArray()) {
@@ -4928,6 +4961,12 @@ class Serializer : public ValueSerializer::Delegate {
   size_t current_memory_usage_;
 };
 
+void SerializationData::ClearSharedValuesUnderLockIfNeeded() {
+  if (shared_values_.empty()) return;
+  v8::Locker locker(Shell::shared_isolate);
+  shared_values_.clear();
+}
+
 class Deserializer : public ValueDeserializer::Delegate {
  public:
   Deserializer(Isolate* isolate, std::unique_ptr<SerializationData> data)
@@ -4937,6 +4976,12 @@ class Deserializer : public ValueDeserializer::Delegate {
     deserializer_.SetSupportsLegacyWireFormat(true);
   }
 
+  ~Deserializer() {
+    DCHECK_EQ(reinterpret_cast<i::Isolate*>(isolate_)->shared_isolate(),
+              reinterpret_cast<i::Isolate*>(Shell::shared_isolate));
+    data_->ClearSharedValuesUnderLockIfNeeded();
+  }
+
   Deserializer(const Deserializer&) = delete;
   Deserializer& operator=(const Deserializer&) = delete;
 
@@ -4974,6 +5019,17 @@ class Deserializer : public ValueDeserializer::Delegate {
         isolate_, data_->compiled_wasm_modules().at(transfer_id));
   }
 
+  bool SupportsSharedValues() const override { return true; }
+
+  MaybeLocal<Value> GetSharedValueFromId(Isolate* isolate,
+                                         uint32_t id) override {
+    DCHECK_NOT_NULL(data_);
+    if (id < data_->shared_values().size()) {
+      return data_->shared_values().at(id).Get(isolate);
+    }
+    return MaybeLocal<Value>();
+  }
+
  private:
   Isolate* isolate_;
   ValueDeserializer deserializer_;

src/d8/d8.h

@@ -151,6 +151,11 @@ class SerializationData {
   const std::vector<CompiledWasmModule>& compiled_wasm_modules() {
     return compiled_wasm_modules_;
   }
+  const std::vector<v8::Global<v8::Value>>& shared_values() {
+    return shared_values_;
+  }
+
+  void ClearSharedValuesUnderLockIfNeeded();
 
  private:
   struct DataDeleter {
@@ -162,6 +167,7 @@ class SerializationData {
   std::vector<std::shared_ptr<v8::BackingStore>> backing_stores_;
   std::vector<std::shared_ptr<v8::BackingStore>> sab_backing_stores_;
   std::vector<CompiledWasmModule> compiled_wasm_modules_;
+  std::vector<v8::Global<v8::Value>> shared_values_;
 
  private:
   friend class Serializer;

src/execution/v8threads.h

@@ -60,7 +60,7 @@ class ThreadVisitor {
 class ThreadManager {
  public:
   void Lock();
-  void Unlock();
+  V8_EXPORT_PRIVATE void Unlock();
 
   void InitThread(const ExecutionAccess&);
   void ArchiveThread();

src/heap/heap.cc

@@ -1388,7 +1388,7 @@ class V8_NODISCARD GCCallbacksScope {
 };
 
 void Heap::HandleGCRequest() {
-  if (FLAG_stress_scavenge > 0 && stress_scavenge_observer_->HasRequestedGC()) {
+  if (IsStressingScavenge() && stress_scavenge_observer_->HasRequestedGC()) {
     CollectAllGarbage(NEW_SPACE, GarbageCollectionReason::kTesting);
     stress_scavenge_observer_->RequestedGCDone();
   } else if (HighMemoryPressure()) {
@@ -5772,7 +5772,7 @@ void Heap::SetUpSpaces(LinearAllocationArea* new_allocation_info,
     AddAllocationObserversToAllSpaces(stress_marking_observer_,
                                       stress_marking_observer_);
   }
-  if (FLAG_stress_scavenge > 0 && new_space()) {
+  if (IsStressingScavenge()) {
     stress_scavenge_observer_ = new StressScavengeObserver(this);
     new_space()->AddAllocationObserver(stress_scavenge_observer_);
   }
@@ -6006,7 +6006,7 @@ void Heap::TearDown() {
     if (FLAG_stress_marking > 0) {
       PrintMaxMarkingLimitReached();
     }
-    if (FLAG_stress_scavenge > 0) {
+    if (IsStressingScavenge()) {
       PrintMaxNewSpaceSizeReached();
     }
   }
@@ -6031,7 +6031,7 @@ void Heap::TearDown() {
     delete stress_marking_observer_;
     stress_marking_observer_ = nullptr;
   }
-  if (FLAG_stress_scavenge > 0 && new_space()) {
+  if (IsStressingScavenge()) {
     new_space()->RemoveAllocationObserver(stress_scavenge_observer_);
     delete stress_scavenge_observer_;
     stress_scavenge_observer_ = nullptr;
@@ -7372,6 +7372,10 @@ void Heap::IncrementObjectCounters() {
 }
 #endif  // DEBUG
 
+bool Heap::IsStressingScavenge() {
+  return FLAG_stress_scavenge > 0 && new_space();
+}
+
 // StrongRootBlocks are allocated as a block of addresses, prefixed with a
 // StrongRootsEntry pointer:
 //

src/heap/heap.h

@@ -2189,6 +2189,8 @@ class Heap {
     return allocation_type_for_in_place_internalizable_strings_;
   }
 
+  bool IsStressingScavenge();
+
   ExternalMemoryAccounting external_memory_;
 
   // This can be calculated directly from a pointer to the heap; however, it is

src/objects/value-serializer.cc

@@ -50,6 +50,7 @@ namespace internal {
 // Version 13: host objects have an explicit tag (rather than handling all
 //             unknown tags)
 // Version 14: flags for JSArrayBufferViews
+// Version 15: support for shared objects with an explicit tag
 //
 // WARNING: Increasing this value is a change which cannot safely be rolled
 // back without breaking compatibility with data stored on disk. It is
@@ -58,7 +59,7 @@ namespace internal {
 //
 // Recent changes are routinely reverted in preparation for branch, and this
 // has been the cause of at least one bug in the past.
-static const uint32_t kLatestVersion = 14;
+static const uint32_t kLatestVersion = 15;
 static_assert(kLatestVersion == v8::CurrentValueSerializerFormatVersion(),
               "Exported format version must match latest version.");
 
@@ -154,6 +155,8 @@ enum class SerializationTag : uint8_t {
   kArrayBufferView = 'V',
   // Shared array buffer. transferID:uint32_t
   kSharedArrayBuffer = 'u',
+  // A HeapObject shared across Isolates. sharedValueID:uint32_t
+  kSharedObject = 'p',
   // A wasm module object transfer. next value is its index.
   kWasmModuleTransfer = 'w',
   // The delegate is responsible for processing all following data.
@@ -245,6 +248,7 @@ ValueSerializer::ValueSerializer(Isolate* isolate,
                                  v8::ValueSerializer::Delegate* delegate)
     : isolate_(isolate),
       delegate_(delegate),
+      supports_shared_values_(delegate && delegate->SupportsSharedValues()),
       zone_(isolate->allocator(), ZONE_NAME),
       id_map_(isolate->heap(), ZoneAllocationPolicy(&zone_)),
       array_buffer_transfer_map_(isolate->heap(),
@@ -444,7 +448,11 @@ Maybe<bool> ValueSerializer::WriteObject(Handle<Object> object) {
     }
     default:
       if (InstanceTypeChecker::IsString(instance_type)) {
-        WriteString(Handle<String>::cast(object));
+        auto string = Handle<String>::cast(object);
+        if (FLAG_shared_string_table && supports_shared_values_) {
+          return WriteSharedObject(String::Share(isolate_, string));
+        }
+        WriteString(string);
         return ThrowIfOutOfMemory();
       } else if (InstanceTypeChecker::IsJSReceiver(instance_type)) {
         return WriteJSReceiver(Handle<JSReceiver>::cast(object));
@@ -1050,6 +1058,23 @@ Maybe<bool> ValueSerializer::WriteWasmMemory(Handle<WasmMemoryObject> object) {
 }
 #endif  // V8_ENABLE_WEBASSEMBLY
 
+Maybe<bool> ValueSerializer::WriteSharedObject(Handle<HeapObject> object) {
+  // Currently only strings are shareable.
+  DCHECK(String::cast(*object).IsShared());
+  DCHECK(supports_shared_values_);
+  DCHECK_NOT_NULL(delegate_);
+  DCHECK(delegate_->SupportsSharedValues());
+
+  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
+  Maybe<uint32_t> index =
+      delegate_->GetSharedValueId(v8_isolate, Utils::ToLocal(object));
+  RETURN_VALUE_IF_SCHEDULED_EXCEPTION(isolate_, Nothing<bool>());
+
+  WriteTag(SerializationTag::kSharedObject);
+  WriteVarint(index.FromJust());
+  return ThrowIfOutOfMemory();
+}
+
 Maybe<bool> ValueSerializer::WriteHostObject(Handle<JSObject> object) {
   WriteTag(SerializationTag::kHostObject);
   if (!delegate_) {
@@ -1127,6 +1152,7 @@ ValueDeserializer::ValueDeserializer(Isolate* isolate,
       delegate_(delegate),
       position_(data.begin()),
       end_(data.end()),
+      supports_shared_values_(delegate && delegate->SupportsSharedValues()),
       id_map_(isolate->global_handles()->Create(
           ReadOnlyRoots(isolate_).empty_fixed_array())) {}
 
@@ -1136,6 +1162,7 @@ ValueDeserializer::ValueDeserializer(Isolate* isolate, const uint8_t* data,
       delegate_(nullptr),
       position_(data),
       end_(data + size),
+      supports_shared_values_(false),
       id_map_(isolate->global_handles()->Create(
           ReadOnlyRoots(isolate_).empty_fixed_array())) {}
 
@@ -1394,6 +1421,13 @@ MaybeHandle<Object> ValueDeserializer::ReadObjectInternal() {
 #endif  // V8_ENABLE_WEBASSEMBLY
     case SerializationTag::kHostObject:
       return ReadHostObject();
+    case SerializationTag::kSharedObject:
+      if (version_ >= 15 && supports_shared_values_) {
+        return ReadSharedObject();
+      }
+      // If the delegate doesn't support shared values (e.g. older version, or
+      // is for deserializing from storage), treat the tag as unknown.
+      V8_FALLTHROUGH;
     default:
       // Before there was an explicit tag for host objects, all unknown tags
       // were delegated to the host.
@@ -2039,6 +2073,28 @@ MaybeHandle<WasmMemoryObject> ValueDeserializer::ReadWasmMemory() {
 }
 #endif  // V8_ENABLE_WEBASSEMBLY
 
+MaybeHandle<HeapObject> ValueDeserializer::ReadSharedObject() {
+  STACK_CHECK(isolate_, MaybeHandle<HeapObject>());
+  DCHECK_GE(version_, 15);
+  DCHECK(supports_shared_values_);
+  DCHECK_NOT_NULL(delegate_);
+  DCHECK(delegate_->SupportsSharedValues());
+  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
+  uint32_t shared_value_id;
+  Local<Value> shared_value;
+  if (!ReadVarint<uint32_t>().To(&shared_value_id) ||
+      !delegate_->GetSharedValueFromId(v8_isolate, shared_value_id)
+           .ToLocal(&shared_value)) {
+    RETURN_EXCEPTION_IF_SCHEDULED_EXCEPTION(isolate_, HeapObject);
+    return MaybeHandle<HeapObject>();
+  }
+  Handle<HeapObject> shared_object =
+      Handle<HeapObject>::cast(Utils::OpenHandle(*shared_value));
+  // Currently only strings are shareable.
+  DCHECK(String::cast(*shared_object).IsShared());
+  return shared_object;
+}
+
 MaybeHandle<JSObject> ValueDeserializer::ReadHostObject() {
   if (!delegate_) return MaybeHandle<JSObject>();
   STACK_CHECK(isolate_, MaybeHandle<JSObject>());

src/objects/value-serializer.h

@@ -137,6 +137,8 @@ class ValueSerializer {
   Maybe<bool> WriteWasmMemory(Handle<WasmMemoryObject> object)
       V8_WARN_UNUSED_RESULT;
 #endif  // V8_ENABLE_WEBASSEMBLY
+  Maybe<bool> WriteSharedObject(Handle<HeapObject> object)
+      V8_WARN_UNUSED_RESULT;
   Maybe<bool> WriteHostObject(Handle<JSObject> object) V8_WARN_UNUSED_RESULT;
 
   /*
@@ -162,6 +164,7 @@ class ValueSerializer {
   uint8_t* buffer_ = nullptr;
   size_t buffer_size_ = 0;
   size_t buffer_capacity_ = 0;
+  const bool supports_shared_values_;
   bool treat_array_buffer_views_as_host_objects_ = false;
   bool out_of_memory_ = false;
   Zone zone_;
@@ -286,6 +289,7 @@ class ValueDeserializer {
   MaybeHandle<JSObject> ReadWasmModuleTransfer() V8_WARN_UNUSED_RESULT;
   MaybeHandle<WasmMemoryObject> ReadWasmMemory() V8_WARN_UNUSED_RESULT;
 #endif  // V8_ENABLE_WEBASSEMBLY
+  MaybeHandle<HeapObject> ReadSharedObject() V8_WARN_UNUSED_RESULT;
   MaybeHandle<JSObject> ReadHostObject() V8_WARN_UNUSED_RESULT;
 
   /*
@@ -305,6 +309,7 @@ class ValueDeserializer {
   v8::ValueDeserializer::Delegate* const delegate_;
   const uint8_t* position_;
   const uint8_t* const end_;
+  const bool supports_shared_values_;
   uint32_t version_ = 0;
   uint32_t next_id_ = 0;
 

src/runtime/runtime-test.cc

@@ -1460,5 +1460,21 @@ RUNTIME_FUNCTION(Runtime_BigIntMaxLengthBits) {
   return *isolate->factory()->NewNumber(BigInt::kMaxLengthBits);
 }
 
+RUNTIME_FUNCTION(Runtime_IsSameHeapObject) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(2, args.length());
+  CONVERT_ARG_HANDLE_CHECKED(HeapObject, obj1, 0);
+  CONVERT_ARG_HANDLE_CHECKED(HeapObject, obj2, 1);
+  return isolate->heap()->ToBoolean(obj1->address() == obj2->address());
+}
+
+RUNTIME_FUNCTION(Runtime_IsSharedString) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_ARG_HANDLE_CHECKED(HeapObject, obj, 0);
+  return isolate->heap()->ToBoolean(obj->IsString() &&
+                                    Handle<String>::cast(obj)->IsShared());
+}
+
 }  // namespace internal
 }  // namespace v8

src/runtime/runtime.h

@@ -521,6 +521,8 @@ namespace internal {
   F(IsConcatSpreadableProtector, 0, 1)        \
   F(IsConcurrentRecompilationSupported, 0, 1) \
   F(IsDictPropertyConstTrackingEnabled, 0, 1) \
+  F(IsSameHeapObject, 2, 1)                   \
+  F(IsSharedString, 1, 1)                     \
   F(MapIteratorProtector, 0, 1)               \
   F(NeverOptimizeFunction, 1, 1)              \
   F(NewRegExpWithBacktrackLimit, 3, 1)        \

src/snapshot/deserializer.cc

@@ -431,7 +431,6 @@ void Deserializer<IsolateT>::PostProcessNewObject(Handle<Map> map,
           isolate()->string_table()->LookupKey(isolate(), &key);
 
       if (*result != *string) {
-        DCHECK(!string->IsShared());
         string->MakeThin(isolate(), *result);
         // Mutate the given object handle so that the backreference entry is
         // also updated.

test/mjsunit/shared-string.js

@@ -0,0 +1,37 @@
+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Flags: --shared-string-table --allow-natives-syntax
+
+if (this.Worker) {
+
+(function TestSharedStringPostMessage() {
+  let workerScript =
+      `postMessage("started");
+       onmessage = function(str) {
+         if (!%IsSharedString(str)) {
+           throw new Error("str isn't shared");
+         }
+         postMessage(str);
+       };`;
+
+  let worker = new Worker(workerScript, { type: 'string' });
+  let started = worker.getMessage();
+  assertTrue(%IsSharedString(started));
+  assertEquals("started", started);
+
+  // The string literal appears in source and is internalized, so should
+  // already be shared.
+  let str_to_send = 'foo';
+  assertTrue(%IsSharedString(str_to_send));
+  worker.postMessage(str_to_send);
+  let str_received = worker.getMessage();
+  assertTrue(%IsSharedString(str_received));
+  // Object.is and === won't check pointer equality of Strings.
+  assertTrue(%IsSameHeapObject(str_to_send, str_received));
+
+  worker.terminate();
+})();
+
+}