Reland "[heap profiler] Refactor: Replace indices with HeapEntry*"

This is a reland of 69a502ce

TBR=ulan@chromium.org

Original change's description:
> [heap profiler] Refactor: Replace indices with HeapEntry*
>
> Change-Id: I0c176f66711d45e2f59d527f3133a1afbf825ec3
> Reviewed-on: https://chromium-review.googlesource.com/1229613
> Commit-Queue: Alexei Filippov <alph@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#56245}

Change-Id: I416da19eb38a1a55f5e2f5897062bc1ca454ac34
Reviewed-on: https://chromium-review.googlesource.com/1246802
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56250}

src/profiler/heap-snapshot-generator-inl.h

@@ -13,51 +13,41 @@
 namespace v8 {
 namespace internal {
 
-
 HeapEntry* HeapGraphEdge::from() const {
   return &snapshot()->entries()[from_index()];
 }
 
-
-Isolate* HeapGraphEdge::isolate() const {
-  return snapshot()->profiler()->isolate();
-}
-
+Isolate* HeapGraphEdge::isolate() const { return to_entry_->isolate(); }
 
 HeapSnapshot* HeapGraphEdge::snapshot() const {
   return to_entry_->snapshot();
 }
 
-
-int HeapEntry::index() const {
-  return static_cast<int>(this - &snapshot_->entries().front());
-}
-
-
 int HeapEntry::set_children_index(int index) {
-  children_index_ = index;
+  // Note: children_count_ and children_end_index_ are parts of a union.
   int next_index = index + children_count_;
-  children_count_ = 0;
+  children_end_index_ = index;
   return next_index;
 }
 
 void HeapEntry::add_child(HeapGraphEdge* edge) {
-  *(children_begin() + children_count_++) = edge;
+  snapshot_->children()[children_end_index_++] = edge;
 }
 
-HeapGraphEdge* HeapEntry::child(int i) { return *(children_begin() + i); }
+HeapGraphEdge* HeapEntry::child(int i) { return children_begin()[i]; }
+
+std::vector<HeapGraphEdge*>::iterator HeapEntry::children_begin() const {
+  return index_ == 0 ? snapshot_->children().begin()
+                     : snapshot_->entries()[index_ - 1].children_end();
+}
 
-std::deque<HeapGraphEdge*>::iterator HeapEntry::children_begin() {
-  DCHECK_GE(children_index_, 0);
-  SLOW_DCHECK(
-      children_index_ < static_cast<int>(snapshot_->children().size()) ||
-      (children_index_ == static_cast<int>(snapshot_->children().size()) &&
-       children_count_ == 0));
-  return snapshot_->children().begin() + children_index_;
+std::vector<HeapGraphEdge*>::iterator HeapEntry::children_end() const {
+  DCHECK_GE(children_end_index_, 0);
+  return snapshot_->children().begin() + children_end_index_;
 }
 
-std::deque<HeapGraphEdge*>::iterator HeapEntry::children_end() {
-  return children_begin() + children_count_;
+int HeapEntry::children_count() const {
+  return static_cast<int>(children_end() - children_begin());
 }
 
 Isolate* HeapEntry::isolate() const { return snapshot_->profiler()->isolate(); }

test/cctest/test-heap-profiler.cc

@@ -157,12 +157,9 @@ static Optional<SourceLocation> GetLocation(const v8::HeapSnapshot* s,
                                             const v8::HeapGraphNode* node) {
   const i::HeapSnapshot* snapshot = reinterpret_cast<const i::HeapSnapshot*>(s);
   const std::vector<SourceLocation>& locations = snapshot->locations();
-  const int index =
-      const_cast<i::HeapEntry*>(reinterpret_cast<const i::HeapEntry*>(node))
-          ->index();
-
+  const i::HeapEntry* entry = reinterpret_cast<const i::HeapEntry*>(node);
   for (const auto& loc : locations) {
-    if (loc.entry_index == index) {
+    if (loc.entry_index == entry->index()) {
       return Optional<SourceLocation>(loc);
     }
   }
@@ -223,7 +220,7 @@ static bool ValidateSnapshot(const v8::HeapSnapshot* snapshot, int depth = 3) {
     entry->value = reinterpret_cast<void*>(ref_count + 1);
   }
   uint32_t unretained_entries_count = 0;
-  std::vector<i::HeapEntry>& entries = heap_snapshot->entries();
+  std::deque<i::HeapEntry>& entries = heap_snapshot->entries();
   for (i::HeapEntry& entry : entries) {
     v8::base::HashMap::Entry* map_entry = visited.Lookup(
         reinterpret_cast<void*>(&entry),
@@ -1003,21 +1000,6 @@ TEST(HeapEntryIdsAndGC) {
   CHECK_EQ(b1->GetId(), b2->GetId());
 }
 
-
-TEST(HeapSnapshotRootPreservedAfterSorting) {
-  LocalContext env;
-  v8::HandleScope scope(env->GetIsolate());
-  v8::HeapProfiler* heap_profiler = env->GetIsolate()->GetHeapProfiler();
-  const v8::HeapSnapshot* snapshot = heap_profiler->TakeHeapSnapshot();
-  CHECK(ValidateSnapshot(snapshot));
-  const v8::HeapGraphNode* root1 = snapshot->GetRoot();
-  const_cast<i::HeapSnapshot*>(reinterpret_cast<const i::HeapSnapshot*>(
-      snapshot))->GetSortedEntriesList();
-  const v8::HeapGraphNode* root2 = snapshot->GetRoot();
-  CHECK_EQ(root1, root2);
-}
-
-
 namespace {
 
 class TestJSONStream : public v8::OutputStream {