Simplify synthetic roots creation in heap snapshot

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index cb9edaf2b458a048612be9ec014a70005e8278dd..4a4c914e4ee0ddde726c857dd117f17dfee21505 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -188,7 +188,6 @@ HeapSnapshot::HeapSnapshot(HeapProfiler* profiler,
       uid_(uid),
       root_index_(HeapEntry::kNoEntry),
       gc_roots_index_(HeapEntry::kNoEntry),
-      natives_root_index_(HeapEntry::kNoEntry),
       max_snapshot_js_object_id_(0) {
   STATIC_ASSERT(
       sizeof(HeapGraphEdge) ==
@@ -217,6 +216,18 @@ void HeapSnapshot::RememberLastJSObjectId() {
 }
 
 
+void HeapSnapshot::AddSyntheticRootEntries() {
+  AddRootEntry();
+  AddGcRootsEntry();
+  SnapshotObjectId id = HeapObjectsMap::kGcRootsFirstSubrootId;
+  for (int tag = 0; tag < VisitorSynchronization::kNumberOfSyncTags; tag++) {
+    AddGcSubrootEntry(tag, id);
+    id += HeapObjectsMap::kObjectIdStep;
+  }
+  DCHECK(HeapObjectsMap::kFirstAvailableObjectId == id);
+}
+
+
 HeapEntry* HeapSnapshot::AddRootEntry() {
   DCHECK(root_index_ == HeapEntry::kNoEntry);
   DCHECK(entries_.is_empty());  // Root entry must be the first one.
@@ -243,15 +254,11 @@ HeapEntry* HeapSnapshot::AddGcRootsEntry() {
 }
 
 
-HeapEntry* HeapSnapshot::AddGcSubrootEntry(int tag) {
+HeapEntry* HeapSnapshot::AddGcSubrootEntry(int tag, SnapshotObjectId id) {
   DCHECK(gc_subroot_indexes_[tag] == HeapEntry::kNoEntry);
   DCHECK(0 <= tag && tag < VisitorSynchronization::kNumberOfSyncTags);
-  HeapEntry* entry = AddEntry(
-      HeapEntry::kSynthetic,
-      VisitorSynchronization::kTagNames[tag],
-      HeapObjectsMap::GetNthGcSubrootId(tag),
-      0,
-      0);
+  HeapEntry* entry = AddEntry(HeapEntry::kSynthetic,
+                              VisitorSynchronization::kTagNames[tag], id, 0, 0);
   gc_subroot_indexes_[tag] = entry->index();
   return entry;
 }
@@ -771,20 +778,6 @@ void HeapObjectsSet::SetTag(Object* obj, const char* tag) {
 }
 
 
-HeapObject* const V8HeapExplorer::kInternalRootObject =
-    reinterpret_cast<HeapObject*>(
-        static_cast<intptr_t>(HeapObjectsMap::kInternalRootObjectId));
-HeapObject* const V8HeapExplorer::kGcRootsObject =
-    reinterpret_cast<HeapObject*>(
-        static_cast<intptr_t>(HeapObjectsMap::kGcRootsObjectId));
-HeapObject* const V8HeapExplorer::kFirstGcSubrootObject =
-    reinterpret_cast<HeapObject*>(
-        static_cast<intptr_t>(HeapObjectsMap::kGcRootsFirstSubrootId));
-HeapObject* const V8HeapExplorer::kLastGcSubrootObject =
-    reinterpret_cast<HeapObject*>(
-        static_cast<intptr_t>(HeapObjectsMap::kFirstAvailableObjectId));
-
-
 V8HeapExplorer::V8HeapExplorer(
     HeapSnapshot* snapshot,
     SnapshottingProgressReportingInterface* progress,
@@ -809,16 +802,7 @@ HeapEntry* V8HeapExplorer::AllocateEntry(HeapThing ptr) {
 
 
 HeapEntry* V8HeapExplorer::AddEntry(HeapObject* object) {
-  if (object == kInternalRootObject) {
-    snapshot_->AddRootEntry();
-    return snapshot_->root();
-  } else if (object == kGcRootsObject) {
-    HeapEntry* entry = snapshot_->AddGcRootsEntry();
-    return entry;
-  } else if (object >= kFirstGcSubrootObject && object < kLastGcSubrootObject) {
-    HeapEntry* entry = snapshot_->AddGcSubrootEntry(GetGcSubrootOrder(object));
-    return entry;
-  } else if (object->IsJSFunction()) {
+  if (object->IsJSFunction()) {
     JSFunction* func = JSFunction::cast(object);
     SharedFunctionInfo* shared = func->shared();
     const char* name = shared->bound() ? "native_bind" :
@@ -965,41 +949,6 @@ class SnapshotFiller {
 };
 
 
-class GcSubrootsEnumerator : public ObjectVisitor {
- public:
-  GcSubrootsEnumerator(
-      SnapshotFiller* filler, V8HeapExplorer* explorer)
-      : filler_(filler),
-        explorer_(explorer),
-        previous_object_count_(0),
-        object_count_(0) {
-  }
-  void VisitPointers(Object** start, Object** end) {
-    object_count_ += end - start;
-  }
-  void Synchronize(VisitorSynchronization::SyncTag tag) {
-    // Skip empty subroots.
-    if (previous_object_count_ != object_count_) {
-      previous_object_count_ = object_count_;
-      filler_->AddEntry(V8HeapExplorer::GetNthGcSubrootObject(tag), explorer_);
-    }
-  }
- private:
-  SnapshotFiller* filler_;
-  V8HeapExplorer* explorer_;
-  intptr_t previous_object_count_;
-  intptr_t object_count_;
-};
-
-
-void V8HeapExplorer::AddRootEntries(SnapshotFiller* filler) {
-  filler->AddEntry(kInternalRootObject, this);
-  filler->AddEntry(kGcRootsObject, this);
-  GcSubrootsEnumerator enumerator(filler, this);
-  heap_->IterateRoots(&enumerator, VISIT_ALL);
-}
-
-
 const char* V8HeapExplorer::GetSystemEntryName(HeapObject* object) {
   switch (object->map()->instance_type()) {
     case MAP_TYPE:
@@ -1855,9 +1804,6 @@ class RootsReferencesExtractor : public ObjectVisitor {
   void FillReferences(V8HeapExplorer* explorer) {
     DCHECK(strong_references_.length() <= all_references_.length());
     Builtins* builtins = heap_->isolate()->builtins();
-    for (int i = 0; i < reference_tags_.length(); ++i) {
-      explorer->SetGcRootsReference(reference_tags_[i].tag);
-    }
     int strong_index = 0, all_index = 0, tags_index = 0, builtin_index = 0;
     while (all_index < all_references_.length()) {
       bool is_strong = strong_index < strong_references_.length()
@@ -1900,10 +1846,15 @@ bool V8HeapExplorer::IterateAndExtractReferences(
     SnapshotFiller* filler) {
   filler_ = filler;
 
+  // Create references to the synthetic roots.
+  SetRootGcRootsReference();
+  for (int tag = 0; tag < VisitorSynchronization::kNumberOfSyncTags; tag++) {
+    SetGcRootsReference(static_cast<VisitorSynchronization::SyncTag>(tag));
+  }
+
   // Make sure builtin code objects get their builtin tags
   // first. Otherwise a particular JSFunction object could set
   // its custom name to a generic builtin.
-  SetRootGcRootsReference();
   RootsReferencesExtractor extractor(heap_);
   heap_->IterateRoots(&extractor, VISIT_ONLY_STRONG);
   extractor.SetCollectingAllReferences();
@@ -2619,6 +2570,8 @@ bool HeapSnapshotGenerator::GenerateSnapshot() {
   debug_heap->Verify();
 #endif
 
+  snapshot_->AddSyntheticRootEntries();
+
   if (!FillReferences()) return false;
 
   snapshot_->FillChildren();
@@ -2659,7 +2612,6 @@ void HeapSnapshotGenerator::SetProgressTotal(int iterations_count) {
 
 bool HeapSnapshotGenerator::FillReferences() {
   SnapshotFiller filler(snapshot_, &entries_);
-  v8_heap_explorer_.AddRootEntries(&filler);
   return v8_heap_explorer_.IterateAndExtractReferences(&filler)
       && dom_explorer_.IterateAndExtractReferences(&filler);
 }