[heap] Improve IncrementalMarking::UpdateMarkingDequeAfterScavenge.

src/heap/incremental-marking.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/incremental-marking.h"
 
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/conversions.h"
 #include "src/heap/gc-idle-time-handler.h"
@@ skipping 777 matching lines @@
   int limit = marking_deque->top();
   HeapObject** array = marking_deque->array();
   int new_top = current;
 
   Map* filler_map = heap_->one_pointer_filler_map();
 
   while (current != limit) {
     HeapObject* obj = array[current];
     DCHECK(obj->IsHeapObject());
     current = ((current + 1) & mask);
-    if (heap_->InNewSpace(obj)) {
+    // Only pointers to from space have to be updated.
+    if (heap_->InFromSpace(obj)) {
       MapWord map_word = obj->map_word();
+      // There may be objects on the marking deque that do not exist anymore,
+      // e.g. left trimmed objects or objects from the root set (frames).
+      // If these object are dead at scavenging time, their marking deque
+      // entries will not point to forwarding addresses. Hence, we can discard
+      // them.
       if (map_word.IsForwardingAddress()) {
         HeapObject* dest = map_word.ToForwardingAddress();
         array[new_top] = dest;
         new_top = ((new_top + 1) & mask);
         DCHECK(new_top != marking_deque->bottom());
 #ifdef DEBUG
         MarkBit mark_bit = Marking::MarkBitFrom(obj);
         DCHECK(Marking::IsGrey(mark_bit) ||
                (obj->IsFiller() && Marking::IsWhite(mark_bit)));
 #endif
@@ skipping 424 matching lines @@
 void IncrementalMarking::IncrementIdleMarkingDelayCounter() {
   idle_marking_delay_counter_++;
 }
 
 
 void IncrementalMarking::ClearIdleMarkingDelayCounter() {
   idle_marking_delay_counter_ = 0;
 }
 }  // namespace internal
 }  // namespace v8