Precisely sweep scan on scavenge pages and use heap iterator to iterate over them.

src/mark-compact.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/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/cpu-profiler.h"
@@ skipping 4127 matching lines @@
       case PARALLEL_CONSERVATIVE: {
         if (!parallel_sweeping_active) {
           if (FLAG_gc_verbose) {
             PrintF("Sweeping 0x%" V8PRIxPTR " conservatively.\n",
                    reinterpret_cast<intptr_t>(p));
           }
           SweepConservatively<SWEEP_SEQUENTIALLY>(space, NULL, p);
           pages_swept++;
           parallel_sweeping_active = true;
         } else {
-          if (FLAG_gc_verbose) {
-            PrintF("Sweeping 0x%" V8PRIxPTR " conservatively in parallel.\n",
-                   reinterpret_cast<intptr_t>(p));
+          if (p->scan_on_scavenge()) {
+            SweepPrecisely<SWEEP_ONLY, IGNORE_SKIP_LIST, IGNORE_FREE_SPACE>(
+                space, p, NULL);
+            pages_swept++;
+            if (FLAG_gc_verbose) {
+              PrintF("Sweeping 0x%" V8PRIxPTR
+                  " scan on scavenge page precisely.\n",
+                  reinterpret_cast<intptr_t>(p));
+            }
+          } else {
+            if (FLAG_gc_verbose) {
+              PrintF("Sweeping 0x%" V8PRIxPTR " conservatively in parallel.\n",
+                  reinterpret_cast<intptr_t>(p));
+            }
+            p->set_parallel_sweeping(MemoryChunk::PARALLEL_SWEEPING_PENDING);
+            space->IncreaseUnsweptFreeBytes(p);
           }
-          p->set_parallel_sweeping(MemoryChunk::PARALLEL_SWEEPING_PENDING);
-          space->IncreaseUnsweptFreeBytes(p);
         }
         space->set_end_of_unswept_pages(p);
         break;
       }
       case PRECISE: {
         if (FLAG_gc_verbose) {
           PrintF("Sweeping 0x%" V8PRIxPTR " precisely.\n",
                  reinterpret_cast<intptr_t>(p));
         }
         if (space->identity() == CODE_SPACE && FLAG_zap_code_space) {
@@ skipping 325 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal