Make use lists into doubly-linked lists.

runtime/vm/intermediate_language.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 55 matching lines @@
 
   static Kind RecognizeKind(const Function& function);
   static const char* KindToCString(Kind kind);
 };
 
 
 class Value : public ZoneAllocated {
  public:
   explicit Value(Definition* definition)
       : definition_(definition),
+        previous_use_(NULL),
         next_use_(NULL),
         instruction_(NULL),
         use_index_(-1),
         reaching_cid_(kIllegalCid) { }
 
   Definition* definition() const { return definition_; }
   void set_definition(Definition* definition) { definition_ = definition; }
 
+  Value* previous_use() const { return previous_use_; }
+  void set_previous_use(Value* previous) { previous_use_ = previous; }
+
   Value* next_use() const { return next_use_; }
   void set_next_use(Value* next) { next_use_ = next; }
 
   Instruction* instruction() const { return instruction_; }
   void set_instruction(Instruction* instruction) { instruction_ = instruction; }
 
   intptr_t use_index() const { return use_index_; }
   void set_use_index(intptr_t index) { use_index_ = index; }
 
   void AddToInputUseList();
@@ skipping 32 matching lines @@
   // the store buffer.
   bool NeedsStoreBuffer() const;
 
   bool Equals(Value* other) const;
 
   void set_reaching_cid(intptr_t cid) { reaching_cid_ = cid; }
   intptr_t reaching_cid() const { return reaching_cid_; }
 
  private:
   Definition* definition_;
+  Value* previous_use_;
   Value* next_use_;
   Instruction* instruction_;
   intptr_t use_index_;
 
   intptr_t reaching_cid_;
 
   DISALLOW_COPY_AND_ASSIGN(Value);
 };
 
 
@@ skipping 4319 matching lines @@
   ForwardInstructionIterator* current_iterator_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FlowGraphVisitor);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_