Promotion is the backup strategy when semi-space copy fails and vice versa.

src/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_H_
 #define V8_HEAP_H_
 
 #include <cmath>
 
 #include "src/allocation.h"
@@ skipping 1182 matching lines @@
   void UpdateNewSpaceReferencesInExternalStringTable(
       ExternalStringTableUpdaterCallback updater_func);
 
   void UpdateReferencesInExternalStringTable(
       ExternalStringTableUpdaterCallback updater_func);
 
   void ProcessWeakReferences(WeakObjectRetainer* retainer);
 
   void VisitExternalResources(v8::ExternalResourceVisitor* visitor);
 
-  // Helper function that governs the promotion policy from new space to
-  // old.  If the object's old address lies below the new space's age
-  // mark or if we've already filled the bottom 1/16th of the to space,
-  // we try to promote this object.
+  // An object should be promoted if the object has survived a
+  // scavenge operation.
   inline bool ShouldBePromoted(Address old_address, int object_size);
 
   void ClearJSFunctionResultCaches();
 
   void ClearNormalizedMapCaches();
 
   GCTracer* tracer() { return tracer_; }
 
   // Returns the size of objects residing in non new spaces.
   intptr_t PromotedSpaceSizeOfObjects();
@@ skipping 1562 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_