diff --git a/include/llvm/ADT/ilist_node.h b/include/llvm/ADT/ilist_node.h
index dae7475ffa0..9c7e6f6a513 100644
--- a/include/llvm/ADT/ilist_node.h
+++ b/include/llvm/ADT/ilist_node.h
@@ -18,30 +18,49 @@
 namespace llvm {
 
 template<typename NodeTy>
-struct ilist_nextprev_traits;
+struct ilist_traits;
+
+/// ilist_half_node - Base class that provides prev services for sentinels.
+///
+template<typename NodeTy>
+class ilist_half_node {
+  friend struct ilist_traits<NodeTy>;
+  NodeTy *Prev;
+protected:
+  NodeTy *getPrev() { return Prev; }
+  const NodeTy *getPrev() const { return Prev; }
+  void setPrev(NodeTy *P) { Prev = P; }
+  ilist_half_node() : Prev(0) {}
+};
 
 template<typename NodeTy>
-struct ilist_traits;
+struct ilist_nextprev_traits;
 
 /// ilist_node - Base class that provides next/prev services for nodes
 /// that use ilist_nextprev_traits or ilist_default_traits.
 ///
 template<typename NodeTy>
-class ilist_node {
-private:
+class ilist_node : ilist_half_node<NodeTy> {
   friend struct ilist_nextprev_traits<NodeTy>;
   friend struct ilist_traits<NodeTy>;
-  NodeTy *Prev, *Next;
-  NodeTy *getPrev() { return Prev; }
+  NodeTy *Next;
   NodeTy *getNext() { return Next; }
-  const NodeTy *getPrev() const { return Prev; }
   const NodeTy *getNext() const { return Next; }
-  void setPrev(NodeTy *N) { Prev = N; }
   void setNext(NodeTy *N) { Next = N; }
 protected:
-  ilist_node() : Prev(0), Next(0) {}
+  ilist_node() : Next(0) {}
 };
 
+/// When assertions are off, the Next field of sentinels
+/// will not be accessed. So it is not necessary to allocate
+/// space for it. The following macro selects the most
+/// efficient trais class.
+#ifndef NDEBUG
+#   define ILIST_NODE ilist_node
+#else
+#   define ILIST_NODE ilist_half_node
+#endif
+
 } // End llvm namespace
 
 #endif
diff --git a/include/llvm/BasicBlock.h b/include/llvm/BasicBlock.h
index 072f6152ea7..d0bd25a0db4 100644
--- a/include/llvm/BasicBlock.h
+++ b/include/llvm/BasicBlock.h
@@ -46,7 +46,7 @@ template<> struct ilist_traits<Instruction>
   Instruction *ensureHead(Instruction*) const { return createSentinel(); }
   static void noteHead(Instruction*, Instruction*) {}
 private:
-  mutable ilist_node<Instruction> Sentinel;
+  mutable ILIST_NODE<Instruction> Sentinel;
 };
 
 /// This represents a single basic block in LLVM. A basic block is simply a
diff --git a/include/llvm/CodeGen/MachineBasicBlock.h b/include/llvm/CodeGen/MachineBasicBlock.h
index 56b96ea872a..c7a29c22b71 100644
--- a/include/llvm/CodeGen/MachineBasicBlock.h
+++ b/include/llvm/CodeGen/MachineBasicBlock.h
@@ -26,7 +26,7 @@ class MachineFunction;
 template <>
 struct ilist_traits<MachineInstr> : public ilist_default_traits<MachineInstr> {
 private:
-  mutable ilist_node<MachineInstr> Sentinel;
+  mutable ILIST_NODE<MachineInstr> Sentinel;
 
   // this is only set by the MachineBasicBlock owning the LiveList
   friend class MachineBasicBlock;
diff --git a/include/llvm/CodeGen/MachineFunction.h b/include/llvm/CodeGen/MachineFunction.h
index dc7fa8cb167..e8a847d6336 100644
--- a/include/llvm/CodeGen/MachineFunction.h
+++ b/include/llvm/CodeGen/MachineFunction.h
@@ -37,7 +37,7 @@ class TargetMachine;
 template <>
 struct ilist_traits<MachineBasicBlock>
     : public ilist_default_traits<MachineBasicBlock> {
-  mutable ilist_node<MachineBasicBlock> Sentinel;
+  mutable ILIST_NODE<MachineBasicBlock> Sentinel;
 public:
   MachineBasicBlock *createSentinel() const {
     return static_cast<MachineBasicBlock*>(&Sentinel);
diff --git a/include/llvm/CodeGen/SelectionDAG.h b/include/llvm/CodeGen/SelectionDAG.h
index 7e81e02073c..27da9e6d9be 100644
--- a/include/llvm/CodeGen/SelectionDAG.h
+++ b/include/llvm/CodeGen/SelectionDAG.h
@@ -39,7 +39,7 @@ class FunctionLoweringInfo;
 
 template<> struct ilist_traits<SDNode> : public ilist_default_traits<SDNode> {
 private:
-  mutable ilist_node<SDNode> Sentinel;
+  mutable ILIST_NODE<SDNode> Sentinel;
 public:
   SDNode *createSentinel() const {
     return static_cast<SDNode*>(&Sentinel);
diff --git a/include/llvm/Function.h b/include/llvm/Function.h
index ccc006cfcfb..47273439c1c 100644
--- a/include/llvm/Function.h
+++ b/include/llvm/Function.h
@@ -45,7 +45,7 @@ template<> struct ilist_traits<BasicBlock>
 
   static ValueSymbolTable *getSymTab(Function *ItemParent);
 private:
-  mutable ilist_node<BasicBlock> Sentinel;
+  mutable ILIST_NODE<BasicBlock> Sentinel;
 };
 
 template<> struct ilist_traits<Argument>
@@ -62,7 +62,7 @@ template<> struct ilist_traits<Argument>
 
   static ValueSymbolTable *getSymTab(Function *ItemParent);
 private:
-  mutable ilist_node<Argument> Sentinel;
+  mutable ILIST_NODE<Argument> Sentinel;
 };
 
 class Function : public GlobalValue, public Annotable,