Speed up debug mode GC.

src/incremental-marking.cc

 // Copyright 2011 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 455 matching lines @@
   // Initialize marking stack.
   Address addr = static_cast<Address>(marking_deque_memory_->address());
   size_t size = marking_deque_memory_->size();
   if (FLAG_force_marking_deque_overflows) size = 64 * kPointerSize;
   marking_deque_.Initialize(addr, addr + size);
 
   ActivateIncrementalWriteBarrier();
 
 #ifdef DEBUG
   // Marking bits are cleared by the sweeper.
-  heap_->mark_compact_collector()->VerifyMarkbitsAreClean();
+  if (FLAG_enable_slow_asserts) {
+    heap_->mark_compact_collector()->VerifyMarkbitsAreClean();
+  }
 #endif
 
   heap_->CompletelyClearInstanceofCache();
   heap_->isolate()->compilation_cache()->MarkCompactPrologue();
 
   if (FLAG_cleanup_code_caches_at_gc) {
     // We will mark cache black with a separate pass
     // when we finish marking.
     MarkObjectGreyDoNotEnqueue(heap_->polymorphic_code_cache());
   }
@@ skipping 336 matching lines @@
   bytes_rescanned_ = 0;
   allocation_marking_factor_ = kInitialAllocationMarkingFactor;
 }
 
 
 int64_t IncrementalMarking::SpaceLeftInOldSpace() {
   return heap_->MaxOldGenerationSize() - heap_->PromotedSpaceSize();
 }
 
 } }  // namespace v8::internal