[heap] Don't read current bucket twice when interating over slot set.

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 <stack>
 
 #include "src/allocation.h"
@@ skipping 150 matching lines @@
   //
   // Sample usage:
   // Iterate([](Address slot_address) {
   //    if (good(slot_address)) return KEEP_SLOT;
   //    else return REMOVE_SLOT;
   // });
   template <typename Callback>
   int Iterate(Callback callback, EmptyBucketMode mode) {
     int new_count = 0;
     for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
-      if (bucket[bucket_index].Value() != nullptr) {
+      base::AtomicValue<uint32_t>* current_bucket =
+          bucket[bucket_index].Value();
+      if (current_bucket != nullptr) {
         int in_bucket_count = 0;
-        base::AtomicValue<uint32_t>* current_bucket =
-            bucket[bucket_index].Value();
         int cell_offset = bucket_index * kBitsPerBucket;
         for (int i = 0; i < kCellsPerBucket; i++, cell_offset += kBitsPerCell) {
           if (current_bucket[i].Value()) {
             uint32_t cell = current_bucket[i].Value();
             uint32_t old_cell = cell;
             uint32_t new_cell = cell;
             while (cell) {
               int bit_offset = base::bits::CountTrailingZeros32(cell);
               uint32_t bit_mask = 1u << bit_offset;
               uint32_t slot = (cell_offset + bit_offset) << kPointerSizeLog2;
@@ skipping 296 matching lines @@
   Address page_start_;
   base::AtomicValue<Chunk*> chunk_;
   base::Mutex to_be_freed_chunks_mutex_;
   std::stack<Chunk*> to_be_freed_chunks_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SLOT_SET_H