[heap] Concurrently free empty slot set buckets.

test/unittests/heap/slot-set-unittest.cc

 // 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.
 
 #include <limits>
 
 #include "src/globals.h"
 #include "src/heap/slot-set.h"
 #include "src/heap/spaces.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 34 matching lines @@
 
 TEST(SlotSet, Iterate) {
   SlotSet set;
   set.SetPageStart(0);
   for (int i = 0; i < Page::kPageSize; i += kPointerSize) {
     if (i % 7 == 0) {
       set.Insert(i);
     }
   }
 
-  set.Iterate([](Address slot_address) {
-    uintptr_t intaddr = reinterpret_cast<uintptr_t>(slot_address);
-    if (intaddr % 3 == 0) {
-      return KEEP_SLOT;
-    } else {
-      return REMOVE_SLOT;
-    }
-  });
+  set.Iterate(
+      [](Address slot_address) {
+        uintptr_t intaddr = reinterpret_cast<uintptr_t>(slot_address);
+        if (intaddr % 3 == 0) {
+          return KEEP_SLOT;
+        } else {
+          return REMOVE_SLOT;
+        }
+      },
+      SlotSet::KEEP_EMPTY_BUCKETS);
 
   for (int i = 0; i < Page::kPageSize; i += kPointerSize) {
     if (i % 21 == 0) {
       EXPECT_TRUE(set.Lookup(i));
     } else {
       EXPECT_FALSE(set.Lookup(i));
     }
   }
 }
 
@@ skipping 100 matching lines @@
         EXPECT_EQ(0, i % 2);
         ++iterated;
         return KEEP_SLOT;
       },
       TypedSlotSet::KEEP_EMPTY_CHUNKS);
   EXPECT_EQ(added / 2, iterated);
 }
 
 }  // namespace internal
 }  // namespace v8