in builds without asserts we do not need to allocate the Next pointer in "ghostly" sentinels

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66415 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 9 deletions

diff --git a/include/llvm/ADT/ilist_node.h b/include/llvm/ADT/ilist_node.h
index dae7475ffa..9c7e6f6a51 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>
-struct ilist_traits;
+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_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