Reland of "[heap] Parallel newspace evacuation, semispace copy, and compaction \o/"

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index d7f07b85eb0369689532f9d20271fe916c750ff9..f6bf24652e0e82afed19c440026652121b191a71 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -518,17 +518,19 @@ void Heap::MergeAllocationSitePretenuringFeedback(
     if (map_word.IsForwardingAddress()) {
       site = AllocationSite::cast(map_word.ToForwardingAddress());
     }
-    DCHECK(site->IsAllocationSite());
+
+    // We have not validated the allocation site yet, since we have not
+    // dereferenced the site during collecting information.
+    // This is an inlined check of AllocationMemento::IsValid.
+    if (!site->IsAllocationSite() || site->IsZombie()) continue;
+
     int value =
         static_cast<int>(reinterpret_cast<intptr_t>(local_entry->value));
     DCHECK_GT(value, 0);
 
-    {
-      // TODO(mlippautz): For parallel processing we need synchronization here.
-      if (site->IncrementMementoFoundCount(value)) {
-        global_pretenuring_feedback_->LookupOrInsert(
-            site, static_cast<uint32_t>(bit_cast<uintptr_t>(site)));
-      }
+    if (site->IncrementMementoFoundCount(value)) {
+      global_pretenuring_feedback_->LookupOrInsert(site,
+                                                   ObjectHash(site->address()));
     }
   }
 }
@@ -566,22 +568,24 @@ void Heap::ProcessPretenuringFeedback() {
     bool maximum_size_scavenge = MaximumSizeScavenge();
     for (HashMap::Entry* e = global_pretenuring_feedback_->Start();
          e != nullptr; e = global_pretenuring_feedback_->Next(e)) {
+      allocation_sites++;
       site = reinterpret_cast<AllocationSite*>(e->key);
       int found_count = site->memento_found_count();
-      // The fact that we have an entry in the storage means that we've found
-      // the site at least once.
-      DCHECK_GT(found_count, 0);
-      DCHECK(site->IsAllocationSite());
-      allocation_sites++;
-      active_allocation_sites++;
-      allocation_mementos_found += found_count;
-      if (site->DigestPretenuringFeedback(maximum_size_scavenge)) {
-        trigger_deoptimization = true;
-      }
-      if (site->GetPretenureMode() == TENURED) {
-        tenure_decisions++;
-      } else {
-        dont_tenure_decisions++;
+      // An entry in the storage does not imply that the count is > 0 because
+      // allocation sites might have been reset due to too many objects dying
+      // in old space.
+      if (found_count > 0) {
+        DCHECK(site->IsAllocationSite());
+        active_allocation_sites++;
+        allocation_mementos_found += found_count;
+        if (site->DigestPretenuringFeedback(maximum_size_scavenge)) {
+          trigger_deoptimization = true;
+        }
+        if (site->GetPretenureMode() == TENURED) {
+          tenure_decisions++;
+        } else {
+          dont_tenure_decisions++;
+        }
       }
     }