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

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index cb438b164aa20707f0f350faafce9d200bb394f5..fa711f233b59b1f8ab753894d61a2f43e2a150d3 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1200,10 +1200,8 @@ class Heap {
 
   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();