[heap] Minor MC: Add evacuation phase

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index 5103cbf11ef5c5d27836200c0feac34bc7ecbe48..badc048029330fa15c6e882bb8984b9325e6163b 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -282,6 +282,12 @@ class MemoryChunk {
     // triggering on the same page.
     COMPACTION_WAS_ABORTED_FOR_TESTING = 1u << 16,
 
+    // |CANNOT_BE_VERIFIED|: Used for moved pages to indicate that these pages
+    // cannot be verified due to stale pointers in the body. Note that the
+    // objects can still be iterated over, as the size can be correctly
+    // obtained.
+    CANNOT_BE_VERIFIED = 1u << 17,
+
     // |ANCHOR|: Flag is set if page is an anchor.
     ANCHOR = 1u << 17,
   };
@@ -1892,6 +1898,9 @@ class PagedSpace : public Space {
  public:
   typedef PageIterator iterator;
 
+  // Reuse a page for allocation only if it has at least {kPageReuseThreshold}
+  // memory available in its freelist.
+  static const size_t kPageReuseThreshold = 4 * KB;
   static const intptr_t kCompactionMemoryWanted = 500 * KB;
 
   // Creates a space with an id.
@@ -2104,6 +2113,10 @@ class PagedSpace : public Space {
 
   std::unique_ptr<ObjectIterator> GetObjectIterator() override;
 
+  // Removes a page from the space.
+  Page* RemovePageSafe();
+  void AddPage(Page* page);
+
  protected:
   // PagedSpaces that should be included in snapshots have different, i.e.,
   // smaller, initial pages.