Re-land "Concurrently unmap free pages."

src/heap/heap.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.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 1405 matching lines @@
 
   inline Isolate* isolate();
 
   void CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags);
   void CallGCEpilogueCallbacks(GCType gc_type, GCCallbackFlags flags);
 
   inline bool OldGenerationAllocationLimitReached();
 
   void QueueMemoryChunkForFree(MemoryChunk* chunk);
   void FilterStoreBufferEntriesOnAboutToBeFreedPages();
+  void FreeQueuedChunks(MemoryChunk* list_head);
   void FreeQueuedChunks();
+  void WaitUntilUnmappingOfFreeChunksCompleted();
 
   int gc_count() const { return gc_count_; }
 
   bool RecentIdleNotificationHappened();
 
   // Completely clear the Instanceof cache (to stop it keeping objects alive
   // around a GC).
   inline void CompletelyClearInstanceofCache();
 
   // The roots that have an index less than this are always in old space.
@@ skipping 156 matching lines @@
   // An ArrayBuffer moved from new space to old space.
   void PromoteArrayBuffer(Object* buffer);
 
   bool HasLowAllocationRate();
   bool HasHighFragmentation();
   bool HasHighFragmentation(intptr_t used, intptr_t committed);
 
   bool ShouldOptimizeForMemoryUsage() { return optimize_for_memory_usage_; }
 
  private:
+  class UnmapFreeMemoryTask;
+
   static const int kInitialStringTableSize = 2048;
   static const int kInitialEvalCacheSize = 64;
   static const int kInitialNumberStringCacheSize = 256;
 
   Heap();
 
   int current_gc_flags() { return current_gc_flags_; }
   void set_current_gc_flags(int flags) {
     current_gc_flags_ = flags;
     DCHECK(!ShouldFinalizeIncrementalMarking() ||
@@ skipping 657 matching lines @@
 
   // Currently set GC flags that are respected by all GC components.
   int current_gc_flags_;
 
   ExternalStringTable external_string_table_;
 
   VisitorDispatchTable<ScavengingCallback> scavenging_visitors_table_;
 
   MemoryChunk* chunks_queued_for_free_;
 
+  base::Semaphore pending_unmap_job_semaphore_;
+
   base::Mutex relocation_mutex_;
 
   int gc_callbacks_depth_;
 
   bool deserialization_complete_;
 
   bool concurrent_sweeping_enabled_;
 
   // |live_array_buffers_| maps externally allocated memory used as backing
   // store for ArrayBuffers to the length of the respective memory blocks.
@@ skipping 397 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_