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

src/heap-inl.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_INL_H_
 #define V8_HEAP_INL_H_
 
 #include <cmath>
 
 #include "src/cpu-profiler.h"
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 }
 
 
 bool Heap::OldGenerationAllocationLimitReached() {
   if (!incremental_marking()->IsStopped()) return false;
   return OldGenerationSpaceAvailable() < 0;
 }
 
 
 bool Heap::ShouldBePromoted(Address old_address, int object_size) {
-  // An object should be promoted if the object has survived a
-  // scavenge operation.
   NewSpacePage* page = NewSpacePage::FromAddress(old_address);
   Address age_mark = new_space_.age_mark();
   return page->IsFlagSet(MemoryChunk::NEW_SPACE_BELOW_AGE_MARK) &&
       (!page->ContainsLimit(age_mark) || old_address < age_mark);
 }
 
 
 void Heap::RecordWrite(Address address, int offset) {
   if (!InNewSpace(address)) store_buffer_.Mark(address + offset);
 }
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 double GCTracer::SizeOfHeapObjects() {
   return (static_cast<double>(heap_->SizeOfObjects())) / MB;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_INL_H_