[vm] Fix quadratic cost of SNAPSHOT_BACKTRACE.

Observed this taking ~16% of debug gen_snapshot time.
Observed this taking ~52% of debug AppJIT training time.

TEST=ci
Change-Id: I945e4f3a8794e607e1402150a10f0d328fa1e677
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506260
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/app_snapshot.cc

@@ -636,7 +636,6 @@ class Serializer : public ThreadStackResource {
 
 #if defined(SNAPSHOT_BACKTRACE)
   ObjectPtr current_parent_;
-  GrowableArray<Object*> parent_pairs_;
 #endif
 
 #if defined(DART_PRECOMPILER)
@@ -7754,8 +7753,7 @@ Serializer::Serializer(Thread* thread,
       profile_writer_(profile_writer)
 #if defined(SNAPSHOT_BACKTRACE)
       ,
-      current_parent_(Object::null()),
-      parent_pairs_()
+      current_parent_(Object::null())
 #endif
 #if defined(DART_PRECOMPILER)
       ,
@@ -8759,8 +8757,7 @@ void Serializer::Push(ObjectPtr object, intptr_t cid_override) {
       num_written_objects_++;
     }
 #if defined(SNAPSHOT_BACKTRACE)
-    parent_pairs_.Add(&Object::Handle(zone_, object));
-    parent_pairs_.Add(&Object::Handle(zone_, current_parent_));
+    heap()->SetSnapshotParent(object, &Object::Handle(zone_, current_parent_));
 #endif
   }
 }
@@ -8851,19 +8848,17 @@ void Serializer::UnexpectedObject(ObjectPtr raw_object, const char* message) {
 
 #if defined(SNAPSHOT_BACKTRACE)
 ObjectPtr Serializer::ParentOf(ObjectPtr object) const {
-  for (intptr_t i = 0; i < parent_pairs_.length(); i += 2) {
-    if (parent_pairs_[i]->ptr() == object) {
-      return parent_pairs_[i + 1]->ptr();
-    }
+  Object* parent = heap()->GetSnapshotParent(object);
+  if (parent != nullptr) {
+    return parent->ptr();
   }
   return Object::null();
 }
 
 ObjectPtr Serializer::ParentOf(const Object& object) const {
-  for (intptr_t i = 0; i < parent_pairs_.length(); i += 2) {
-    if (parent_pairs_[i]->ptr() == object.ptr()) {
-      return parent_pairs_[i + 1]->ptr();
-    }
+  Object* parent = heap()->GetSnapshotParent(object.ptr());
+  if (parent != nullptr) {
+    return parent->ptr();
   }
   return Object::null();
 }
@@ -9104,6 +9099,9 @@ ZoneGrowableArray<Object*>* Serializer::Serialize(SerializationRoots* roots) {
   PrintSnapshotSizes();
 
   heap()->ResetObjectIdTable();
+#if defined(SNAPSHOT_BACKTRACE)
+  heap()->ResetSnapshotParentTable();
+#endif
 
   return objects_;
 }

runtime/vm/globals.h

@@ -149,6 +149,10 @@ const intptr_t kDefaultNewGenSemiMaxSize = (kWordSize <= 4) ? 8 : 16;
 #define TARGET_HAS_FAST_WRITE_WRITE_FENCE 1
 #define HOST_HAS_FAST_WRITE_WRITE_FENCE 1
 
+#if defined(DEBUG)
+#define SNAPSHOT_BACKTRACE
+#endif
+
 // The expression OFFSET_OF(type, field) computes the byte-offset of
 // the specified field relative to the containing type.
 //

runtime/vm/heap/heap.cc

@@ -879,6 +879,13 @@ void Heap::ResetObjectIdTable() {
   old_weak_tables_[kObjectIds]->Reset();
 }
 
+#if defined(SNAPSHOT_BACKTRACE)
+void Heap::ResetSnapshotParentTable() {
+  new_weak_tables_[kSnapshotParents]->Reset();
+  old_weak_tables_[kSnapshotParents]->Reset();
+}
+#endif
+
 intptr_t Heap::GetWeakEntry(ObjectPtr raw_obj, WeakSelector sel) const {
   if (raw_obj->IsImmediateOrOldObject()) {
     return old_weak_tables_[sel]->GetValue(raw_obj);

runtime/vm/heap/heap.h

@@ -47,6 +47,9 @@ class Heap {
 #endif
     kCanonicalHashes,
     kObjectIds,
+#if defined(SNAPSHOT_BACKTRACE)
+    kSnapshotParents,
+#endif
     kLoadingUnits,
 #if !defined(PRODUCT) || defined(FORCE_INCLUDE_SAMPLING_HEAP_PROFILER)
     kHeapSamplingData,
@@ -202,6 +205,16 @@ class Heap {
   }
   void ResetObjectIdTable();
 
+#if defined(SNAPSHOT_BACKTRACE)
+  void SetSnapshotParent(ObjectPtr obj, Object* parent) {
+    SetWeakEntry(obj, kSnapshotParents, reinterpret_cast<intptr_t>(parent));
+  }
+  Object* GetSnapshotParent(ObjectPtr obj) const {
+    return reinterpret_cast<Object*>(GetWeakEntry(obj, kSnapshotParents));
+  }
+  void ResetSnapshotParentTable();
+#endif
+
   void SetLoadingUnit(ObjectPtr raw_obj, intptr_t unit_id) {
     ASSERT(Thread::Current()->IsDartMutatorThread());
     SetWeakEntry(raw_obj, kLoadingUnits, unit_id);

runtime/vm/image_snapshot.h

@@ -22,10 +22,6 @@
 #include "vm/type_testing_stubs.h"
 #include "vm/v8_snapshot_writer.h"
 
-#if defined(DEBUG)
-#define SNAPSHOT_BACKTRACE
-#endif
-
 namespace dart {
 
 // Forward declarations.