Introduce --always-incremental-marking flag.

src/heap.cc

 // Copyright 2012 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 842 matching lines @@
                                    : isolate_->counters()->gc_compactor());
       next_gc_likely_to_collect_more =
           PerformGarbageCollection(collector, &tracer, gc_callback_flags);
     }
 
     GarbageCollectionEpilogue();
   }
 
   // Start incremental marking for the next cycle. The heap snapshot
   // generator needs incremental marking to stay off after it aborted.
-  if (!mark_compact_collector()->abort_incremental_marking() &&
-      incremental_marking()->IsStopped() &&
-      incremental_marking()->WorthActivating() &&
-      NextGCIsLikelyToBeFull()) {
+  if (incremental_marking()->IsStopped() &&
+      (FLAG_always_incremental_marking ||
+       (!mark_compact_collector()->abort_incremental_marking() &&
+        incremental_marking()->WorthActivating() &&
+        NextGCIsLikelyToBeFull()))) {
     incremental_marking()->Start();
   }
 
   return next_gc_likely_to_collect_more;
 }
 
 
 int Heap::NotifyContextDisposed() {
   if (isolate()->concurrent_recompilation_enabled()) {
     // Flush the queued recompilation tasks.
@@ skipping 6873 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   OS::MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   OS::MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal