Implement shrinking of paged spaces during sweeping.

src/spaces.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1217 matching lines @@
 
   // Grow the space by adding available bytes.  They are initially marked as
   // being in use (part of the size), but will normally be immediately freed,
   // putting them on the free list and removing them from size_.
   void ExpandSpace(int size_in_bytes) {
     capacity_ += size_in_bytes;
     size_ += size_in_bytes;
     ASSERT(size_ >= 0);
   }
 
+  // Shrink the space by removing available bytes.  Since shrinking is done
+  // during sweeping, bytes have been marked as being in use (part of the size)
+  // and are hereby freed.
+  void ShrinkSpace(int size_in_bytes) {
+    capacity_ -= size_in_bytes;
+    size_ -= size_in_bytes;
+    ASSERT(size_ >= 0);
+  }
+
   // Allocate from available bytes (available -> size).
   void AllocateBytes(intptr_t size_in_bytes) {
     size_ += size_in_bytes;
     ASSERT(size_ >= 0);
   }
 
   // Free allocated bytes, making them available (size -> available).
   void DeallocateBytes(intptr_t size_in_bytes) {
     size_ -= size_in_bytes;
     ASSERT(size_ >= 0);
@@ skipping 229 matching lines @@
   // Give a block of memory to the space's free list.  It might be added to
   // the free list or accounted as waste.
   // If add_to_freelist is false then just accounting stats are updated and
   // no attempt to add area to free list is made.
   int Free(Address start, int size_in_bytes) {
     int wasted = free_list_.Free(start, size_in_bytes);
     accounting_stats_.DeallocateBytes(size_in_bytes - wasted);
     return size_in_bytes - wasted;
   }
 
-  int FreeOrUnmapPage(Page* page, Address start, int size_in_bytes);
-
   // Set space allocation info.
   void SetTop(Address top, Address limit) {
     ASSERT(top == limit ||
            Page::FromAddress(top) == Page::FromAddress(limit - 1));
     allocation_info_.top = top;
     allocation_info_.limit = limit;
   }
 
   void Allocate(int bytes) {
     accounting_stats_.AllocateBytes(bytes);
   }
 
   void IncreaseCapacity(int size) {
     accounting_stats_.ExpandSpace(size);
   }
 
-  // Releases half of unused pages.
-  void Shrink();
+  // Releases an unused page and shrinks the space.
+  void ReleasePage(Page* page);
+
+  // Releases all of the unused pages.
+  void ReleaseAllUnusedPages();
 
   // The dummy page that anchors the linked list of pages.
   Page* anchor() { return &anchor_; }
 
 #ifdef DEBUG
   // Print meta info and objects in this space.
   virtual void Print();
 
   // Verify integrity of this space.
   virtual void Verify(ObjectVisitor* visitor);
@@ skipping 1092 matching lines @@
   }
   // Must be small, since an iteration is used for lookup.
   static const int kMaxComments = 64;
 };
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_