Environment bookkeping has linear time complexity now, not a quadratic one.

src/hydrogen.h

 // 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 432 matching lines @@
 
   explicit HEnvironment(Zone* zone);
 
   HEnvironment* arguments_environment() {
     return outer()->frame_type() == ARGUMENTS_ADAPTOR ? outer() : this;
   }
 
   // Simple accessors.
   Handle<JSFunction> closure() const { return closure_; }
   const ZoneList<HValue*>* values() const { return &values_; }
-  const ZoneList<int>* assigned_variables() const {
+  const GrowableBitVector* assigned_variables() const {
     return &assigned_variables_;
   }
   FrameType frame_type() const { return frame_type_; }
   int parameter_count() const { return parameter_count_; }
   int specials_count() const { return specials_count_; }
   int local_count() const { return local_count_; }
   HEnvironment* outer() const { return outer_; }
   int pop_count() const { return pop_count_; }
   int push_count() const { return push_count_; }
 
@@ skipping 86 matching lines @@
     while (outer->frame_type() != JS_FUNCTION) outer = outer->outer_;
     if (drop_extra) outer->Drop(1);
     return outer;
   }
 
   void AddIncomingEdge(HBasicBlock* block, HEnvironment* other);
 
   void ClearHistory() {
     pop_count_ = 0;
     push_count_ = 0;
-    assigned_variables_.Rewind(0);
+    assigned_variables_.Clear();
   }
 
   void SetValueAt(int index, HValue* value) {
     ASSERT(index < length());
     values_[index] = value;
   }
 
   void PrintTo(StringStream* stream);
   void PrintToStd();
 
@@ skipping 28 matching lines @@
     ASSERT(variable->IsStackAllocated());
     int shift = variable->IsParameter()
         ? 1
         : parameter_count_ + specials_count_;
     return variable->index() + shift;
   }
 
   Handle<JSFunction> closure_;
   // Value array [parameters] [specials] [locals] [temporaries].
   ZoneList<HValue*> values_;
-  ZoneList<int> assigned_variables_;
+  GrowableBitVector assigned_variables_;
   FrameType frame_type_;
   int parameter_count_;
   int specials_count_;
   int local_count_;
   HEnvironment* outer_;
   HEnterInlined* entry_;
   int pop_count_;
   int push_count_;
   BailoutId ast_id_;
   Zone* zone_;
@@ skipping 935 matching lines @@
   const char* filename_;
   HeapStringAllocator string_allocator_;
   StringStream trace_;
   int indent_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_H_