Reland "[in-place weak refs] Replace Heap::script_list with a WeakArrayList."

This reverts commit cfccdf88.

Reason for revert: Was a faulty test.

Original change's description:
> Revert "[in-place weak refs] Replace Heap::script_list with a WeakArrayList."
> 
> This reverts commit 06f556d8.
> 
> Reason for revert: gc stress failures
> 
> Original change's description:
> > [in-place weak refs] Replace Heap::script_list with a WeakArrayList.
> > 
> > BUG=v8:7308
> > 
> > Change-Id: Idd527fc2a2f5bde635441a511e424a83558ab969
> > Reviewed-on: https://chromium-review.googlesource.com/1143467
> > Commit-Queue: Marja Hölttä <marja@chromium.org>
> > Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> > Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#54635}
> 
> TBR=ulan@chromium.org,marja@chromium.org,jgruber@chromium.org
> 
> Change-Id: I6908fa42fcf4a8916a1dc49b865526094f950c66
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: v8:7308
> Reviewed-on: https://chromium-review.googlesource.com/1148280
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Marja Hölttä <marja@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54636}

TBR=ulan@chromium.org,marja@chromium.org,jgruber@chromium.org

Change-Id: I2451ed0237b69bf17bbe13f4929568ebe2abfc67
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7308
Reviewed-on: https://chromium-review.googlesource.com/1148340Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54639}

src/debug/debug.cc

@@ -1621,12 +1621,12 @@ Handle<FixedArray> Debug::GetLoadedScripts() {
   isolate_->heap()->CollectAllGarbage(Heap::kFinalizeIncrementalMarkingMask,
                                       GarbageCollectionReason::kDebugger);
   Factory* factory = isolate_->factory();
-  if (!factory->script_list()->IsFixedArrayOfWeakCells()) {
+  if (!factory->script_list()->IsWeakArrayList()) {
     return factory->empty_fixed_array();
   }
-  Handle<FixedArrayOfWeakCells> array =
-      Handle<FixedArrayOfWeakCells>::cast(factory->script_list());
-  Handle<FixedArray> results = factory->NewFixedArray(array->Length());
+  Handle<WeakArrayList> array =
+      Handle<WeakArrayList>::cast(factory->script_list());
+  Handle<FixedArray> results = factory->NewFixedArray(array->length());
   int length = 0;
   {
     Script::Iterator iterator(isolate_);

src/heap/factory.cc

@@ -1556,8 +1556,10 @@ Handle<Script> Factory::NewScriptWithId(Handle<String> source, int script_id,
                                     SKIP_WRITE_BARRIER);
   script->set_flags(0);
   script->set_host_defined_options(*empty_fixed_array());
-  heap->set_script_list(
-      *FixedArrayOfWeakCells::Add(isolate(), script_list(), script));
+  Handle<WeakArrayList> scripts = script_list();
+  scripts = WeakArrayList::AddToEnd(isolate(), scripts,
+                                    MaybeObjectHandle::Weak(script));
+  heap->set_script_list(*scripts);
   LOG(isolate(), ScriptEvent(Logger::ScriptEventType::kCreate, script_id));
   return script;
 }
@@ -1582,8 +1584,10 @@ Handle<Script> Factory::CloneScript(Handle<Script> script) {
   new_script->set_eval_from_position(script->eval_from_position());
   new_script->set_flags(script->flags());
   new_script->set_host_defined_options(script->host_defined_options());
-  heap->set_script_list(
-      *FixedArrayOfWeakCells::Add(isolate(), script_list(), new_script));
+  Handle<WeakArrayList> scripts = script_list();
+  scripts = WeakArrayList::AddToEnd(isolate(), scripts,
+                                    MaybeObjectHandle::Weak(new_script));
+  heap->set_script_list(*scripts);
   LOG(isolate(), ScriptEvent(Logger::ScriptEventType::kCreate, script_id));
   return new_script;
 }

src/heap/heap.cc

@@ -5016,6 +5016,34 @@ void CompactFixedArrayOfWeakCells(Isolate* isolate, Object* object) {
     array->Compact<FixedArrayOfWeakCells::NullCallback>(isolate);
   }
 }
+
+Handle<WeakArrayList> CompactWeakArrayList(Heap* heap,
+                                           Handle<WeakArrayList> array) {
+  if (array->length() == 0) {
+    return array;
+  }
+  int new_length = array->CountLiveWeakReferences();
+  if (new_length == array->length()) {
+    return array;
+  }
+
+  Handle<WeakArrayList> new_array = WeakArrayList::EnsureSpace(
+      heap->isolate(),
+      handle(ReadOnlyRoots(heap).empty_weak_array_list(), heap->isolate()),
+      new_length);
+  // Allocation might have caused GC and turned some of the elements into
+  // cleared weak heap objects. Count the number of live references again and
+  // fill in the new array.
+  int copy_to = 0;
+  for (int i = 0; i < array->length(); i++) {
+    MaybeObject* element = array->Get(i);
+    if (element->IsClearedWeakHeapObject()) continue;
+    new_array->Set(copy_to++, element);
+  }
+  new_array->set_length(copy_to);
+  return new_array;
+}
+
 }  // anonymous namespace
 
 void Heap::CompactFixedArraysOfWeakCells() {
@@ -5042,8 +5070,12 @@ void Heap::CompactFixedArraysOfWeakCells() {
 
   // Find known FixedArrayOfWeakCells and compact them.
   CompactFixedArrayOfWeakCells(isolate(), noscript_shared_function_infos());
-  CompactFixedArrayOfWeakCells(isolate(), script_list());
   CompactFixedArrayOfWeakCells(isolate(), weak_stack_trace_list());
+
+  // Find known WeakArrayLists and compact them.
+  Handle<WeakArrayList> scripts(script_list(), isolate());
+  scripts = CompactWeakArrayList(this, scripts);
+  set_script_list(*scripts);
 }
 
 void Heap::AddRetainedMap(Handle<Map> map) {

src/heap/object-stats.cc

@@ -709,8 +709,8 @@ void ObjectStatsCollectorImpl::CollectGlobalStatistics() {
       nullptr,
       FixedArrayOfWeakCells::cast(heap_->noscript_shared_function_infos()),
       ObjectStats::NOSCRIPT_SHARED_FUNCTION_INFOS_TYPE);
-  RecordSimpleVirtualObjectStats(
-      nullptr, FixedArrayOfWeakCells::cast(heap_->script_list()),
+  RecordSimpleVirtualObjectStats(nullptr,
+                                 WeakArrayList::cast(heap_->script_list()),
                                  ObjectStats::SCRIPT_LIST_TYPE);
 
   // HashTable.

src/heap/setup-heap-internal.cc

@@ -766,7 +766,7 @@ void Heap::CreateInitialObjects() {
 
   set_feedback_vectors_for_profiling_tools(roots.undefined_value());
 
-  set_script_list(Smi::kZero);
+  set_script_list(roots.empty_weak_array_list());
 
   Handle<NumberDictionary> slow_element_dictionary = NumberDictionary::New(
       isolate(), 1, TENURED_READ_ONLY, USE_CUSTOM_MINIMUM_CAPACITY);

src/objects.cc

@@ -10587,6 +10587,16 @@ Handle<WeakArrayList> WeakArrayList::EnsureSpace(Isolate* isolate,
   return array;
 }
 
+int WeakArrayList::CountLiveWeakReferences() const {
+  int live_weak_references = 0;
+  for (int i = 0; i < length(); i++) {
+    if (Get(i)->IsWeakHeapObject()) {
+      ++live_weak_references;
+    }
+  }
+  return live_weak_references;
+}
+
 // static
 Handle<WeakArrayList> PrototypeUsers::Add(Isolate* isolate,
                                           Handle<WeakArrayList> array,
@@ -10640,14 +10650,7 @@ WeakArrayList* PrototypeUsers::Compact(Handle<WeakArrayList> array, Heap* heap,
   if (array->length() == 0) {
     return *array;
   }
-  // Count the amount of live references.
-  int new_length = kFirstIndex;
-  for (int i = kFirstIndex; i < array->length(); i++) {
-    MaybeObject* element = array->Get(i);
-    if (element->IsSmi()) continue;
-    if (element->IsClearedWeakHeapObject()) continue;
-    ++new_length;
-  }
+  int new_length = array->CountLiveWeakReferences();
   if (new_length == array->length()) {
     return *array;
   }
@@ -13818,8 +13821,13 @@ MaybeHandle<SharedFunctionInfo> Script::FindSharedFunctionInfo(
 Script::Iterator::Iterator(Isolate* isolate)
     : iterator_(isolate->heap()->script_list()) {}
 
-
-Script* Script::Iterator::Next() { return iterator_.Next<Script>(); }
+Script* Script::Iterator::Next() {
+  Object* o = iterator_.Next();
+  if (o != nullptr) {
+    return Script::cast(o);
+  }
+  return nullptr;
+}
 
 SharedFunctionInfo::ScriptIterator::ScriptIterator(Isolate* isolate,
                                                    Script* script)

src/objects/fixed-array-inl.h

@@ -8,6 +8,7 @@
 #include "src/objects/fixed-array.h"
 
 #include "src/objects/bigint.h"
+#include "src/objects/maybe-object-inl.h"
 
 // Has to be the last include (doesn't have include guards):
 #include "src/objects/object-macros.h"
@@ -286,6 +287,18 @@ MaybeObject** WeakArrayList::data_start() {
   return HeapObject::RawMaybeWeakField(this, kHeaderSize);
 }
 
+HeapObject* WeakArrayList::Iterator::Next() {
+  if (array_ != nullptr) {
+    while (index_ < array_->length()) {
+      MaybeObject* item = array_->Get(index_++);
+      DCHECK(item->IsWeakHeapObject() || item->IsClearedWeakHeapObject());
+      if (!item->IsClearedWeakHeapObject()) return item->ToWeakHeapObject();
+    }
+    array_ = nullptr;
+  }
+  return nullptr;
+}
+
 Object* FixedArrayOfWeakCells::Get(int index) const {
   Object* raw = FixedArray::cast(this)->get(index + kFirstIndex);
   if (raw->IsSmi()) return raw;

src/objects/fixed-array.h

@@ -363,11 +363,28 @@ class WeakArrayList : public HeapObject {
   static const int kMaxCapacity =
       (FixedArray::kMaxSize - kHeaderSize) / kPointerSize;
 
- protected:
   static Handle<WeakArrayList> EnsureSpace(Isolate* isolate,
                                            Handle<WeakArrayList> array,
                                            int length);
 
+  // Returns the number of non-cleaned weak references in the array.
+  int CountLiveWeakReferences() const;
+
+  class Iterator {
+   public:
+    explicit Iterator(WeakArrayList* array) : index_(0), array_(array) {}
+
+    inline HeapObject* Next();
+
+   private:
+    int index_;
+    WeakArrayList* array_;
+#ifdef DEBUG
+    DisallowHeapAllocation no_gc_;
+#endif  // DEBUG
+    DISALLOW_COPY_AND_ASSIGN(Iterator);
+  };
+
  private:
   static int OffsetOfElementAt(int index) {
     return kHeaderSize + index * kPointerSize;

src/objects/script.h

@@ -182,7 +182,7 @@ class Script : public Struct, public NeverReadOnlySpaceObject {
     Script* Next();
 
    private:
-    FixedArrayOfWeakCells::Iterator iterator_;
+    WeakArrayList::Iterator iterator_;
     DISALLOW_COPY_AND_ASSIGN(Iterator);
   };
 

src/roots.h

@@ -232,7 +232,7 @@ namespace internal {
   V(NameDictionary, public_symbol_table, PublicSymbolTable)                  \
   V(NameDictionary, api_symbol_table, ApiSymbolTable)                        \
   V(NameDictionary, api_private_symbol_table, ApiPrivateSymbolTable)         \
-  V(Object, script_list, ScriptList)                                         \
+  V(WeakArrayList, script_list, ScriptList)                                  \
   V(SimpleNumberDictionary, code_stubs, CodeStubs)                           \
   V(FixedArray, materialized_objects, MaterializedObjects)                   \
   V(FixedArray, microtask_queue, MicrotaskQueue)                             \

src/snapshot/object-deserializer.cc

@@ -85,8 +85,9 @@ void ObjectDeserializer::CommitPostProcessedObjects() {
         ScriptEvent(Logger::ScriptEventType::kDeserialize, script->id()));
     LOG(isolate(), ScriptDetails(*script));
     // Add script to list.
-    Handle<Object> list =
-        FixedArrayOfWeakCells::Add(isolate(), factory->script_list(), script);
+    Handle<WeakArrayList> list = factory->script_list();
+    list = WeakArrayList::AddToEnd(isolate(), list,
+                                   MaybeObjectHandle::Weak(script));
     heap->SetRootScriptList(*list);
   }
 }