[heap] Reland uncommit unused large object page memory.

src/base/hashmap.h

 // 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.
 
 // The reason we write our own hash map instead of using unordered_map in STL,
 // is that STL containers use a mutex pool on debug build, which will lead to
 // deadlock when we are using async signal handler.
 
 #ifndef V8_BASE_HASHMAP_H_
 #define V8_BASE_HASHMAP_H_
@@ skipping 43 matching lines @@
   // If an entry with matching key is found, returns that entry.
   // Otherwise, NULL is returned.
   Entry* Lookup(void* key, uint32_t hash) const;
 
   // If an entry with matching key is found, returns that entry.
   // If no matching entry is found, a new entry is inserted with
   // corresponding key, key hash, and NULL value.
   Entry* LookupOrInsert(void* key, uint32_t hash,
                         AllocationPolicy allocator = AllocationPolicy());
 
+  Entry* InsertNew(void* key, uint32_t hash,
+                   AllocationPolicy allocator = AllocationPolicy());
+
   // Removes the entry with matching key.
   // It returns the value of the deleted entry
   // or null if there is no value for such key.
   void* Remove(void* key, uint32_t hash);
 
   // Empties the hash map (occupancy() == 0).
   void Clear();
 
   // The number of (non-empty) entries in the table.
   uint32_t occupancy() const { return occupancy_; }
@@ skipping 53 matching lines @@
 template <class AllocationPolicy>
 typename TemplateHashMapImpl<AllocationPolicy>::Entry*
 TemplateHashMapImpl<AllocationPolicy>::LookupOrInsert(
     void* key, uint32_t hash, AllocationPolicy allocator) {
   // Find a matching entry.
   Entry* p = Probe(key, hash);
   if (p->key != NULL) {
     return p;
   }
 
+  return InsertNew(key, hash, allocator);
+}
+
+template <class AllocationPolicy>
+typename TemplateHashMapImpl<AllocationPolicy>::Entry*
+TemplateHashMapImpl<AllocationPolicy>::InsertNew(void* key, uint32_t hash,
+                                                 AllocationPolicy allocator) {
+  // Find a matching entry.
+  Entry* p = Probe(key, hash);
+  DCHECK(p->key == NULL);
+
   // No entry found; insert one.
   p->key = key;
   p->value = NULL;
   p->hash = hash;
   p->order = occupancy_;
   occupancy_++;
 
   // Grow the map if we reached >= 80% occupancy.
   if (occupancy_ + occupancy_ / 4 >= capacity_) {
     Resize(allocator);
@@ skipping 196 matching lines @@
       return Iterator(this, this->LookupOrInsert(key, key->Hash(), allocator));
     }
     return Iterator(this, this->Lookup(key, key->Hash()));
   }
 };
 
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_HASHMAP_H_