[heap] Only insert new slot set entries.

src/heap/slot-set.h

 // Copyright 2016 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.
 
 #ifndef V8_SLOT_SET_H
 #define V8_SLOT_SET_H
 
 #include "src/allocation.h"
 #include "src/base/atomic-utils.h"
 #include "src/base/bits.h"
@@ skipping 30 matching lines @@
   // This method should only be called on the main thread because concurrent
   // allocation of the bucket is not thread-safe.
   void Insert(int slot_offset) {
     int bucket_index, cell_index, bit_index;
     SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
     base::AtomicValue<uint32_t>* current_bucket = bucket[bucket_index].Value();
     if (current_bucket == nullptr) {
       current_bucket = AllocateBucket();
       bucket[bucket_index].SetValue(current_bucket);
     }
-    current_bucket[cell_index].SetBit(bit_index);
+    if (!(current_bucket[cell_index].Value() & (1u << bit_index))) {
+      current_bucket[cell_index].SetBit(bit_index);
+    }
   }
 
   // The slot offset specifies a slot at address page_start_ + slot_offset.
   void Remove(int slot_offset) {
     int bucket_index, cell_index, bit_index;
     SlotToIndices(slot_offset, &bucket_index, &cell_index, &bit_index);
     base::AtomicValue<uint32_t>* current_bucket = bucket[bucket_index].Value();
     if (current_bucket != nullptr) {
       uint32_t cell = current_bucket[cell_index].Value();
       if (cell) {
@@ skipping 348 matching lines @@
   };
 
   Address page_start_;
   base::AtomicValue<Chunk*> chunk_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SLOT_SET_H