After performing scavenge, hint OS that data in from spaces are no more neede...

src/spaces.h

Index: src/spaces.h
===================================================================
--- src/spaces.h	(revision 3342)
+++ src/spaces.h	(working copy)
@@ -440,6 +440,11 @@
   // and false otherwise.
   static bool UncommitBlock(Address start, size_t size);
 
+  // Give OS a hint that data of this contiguous block of memory are no longer
+  // needed and could be discarded.  This does NOT uncommit the block though.
+  static inline bool ResetBlock(Address start, size_t size,
+                                Executability executable);
+
   // Attempts to allocate the requested (non-zero) number of pages from the
   // OS.  Fewer pages might be allocated than requested. If it fails to
   // allocate memory for the OS or cannot allocate a single page, this
@@ -1130,6 +1135,7 @@
   bool is_committed() { return committed_; }
   bool Commit();
   bool Uncommit();
+  inline bool Reset();
 
 #ifdef DEBUG
   virtual void Print();
@@ -1389,6 +1395,10 @@
     return from_space_.Uncommit();
   }
 
+  bool ResetFromSpace() {
+    return from_space_.Reset();
+  }
+
  private:
   // The semispaces.
   SemiSpace to_space_;