[heap] Reland concurrent slot filtering in sweeper threads.

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 161 matching lines @@
     for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
       base::AtomicValue<uint32_t>* current_bucket =
           bucket[bucket_index].Value();
       if (current_bucket != nullptr) {
         int in_bucket_count = 0;
         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;
+            uint32_t mask = 0;
             while (cell) {
               int bit_offset = base::bits::CountTrailingZeros32(cell);
               uint32_t bit_mask = 1u << bit_offset;
               uint32_t slot = (cell_offset + bit_offset) << kPointerSizeLog2;
               if (callback(page_start_ + slot) == KEEP_SLOT) {
                 ++in_bucket_count;
               } else {
-                new_cell ^= bit_mask;
+                mask |= bit_mask;
               }
               cell ^= bit_mask;
             }
+            uint32_t new_cell = old_cell & ~mask;
             if (old_cell != new_cell) {
               while (!current_bucket[i].TrySetValue(old_cell, new_cell)) {
                 // If TrySetValue fails, the cell must have changed. We just
                 // have to read the current value of the cell, & it with the
                 // computed value, and retry. We can do this, because this
                 // method will only be called on the main thread and filtering
                 // threads will only remove slots.
                 old_cell = current_bucket[i].Value();
-                new_cell &= old_cell;
+                new_cell = old_cell & ~mask;
               }
             }
           }
         }
         if (mode == PREFREE_EMPTY_BUCKETS && in_bucket_count == 0) {
           PreFreeEmptyBucket(bucket_index);
         }
         new_count += in_bucket_count;
       }
     }
@@ skipping 270 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