Commit an patch from Gabor Greif in Mar 2005. This eliminates the tail

pointer from ilist, storing it in the prev pointer of the first node in the
list instead.

This shrinks ilist from 8 to 4 bytes, BasicBlock from 40->36 bytes, Function
from 76->68 bytes, Module from 52->44 bytes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36210 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/ilist

@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 //
 // This file defines classes to implement an intrusive doubly linked list class
-// (ie each node of the list must contain a next and previous field for the
+// (i.e. each node of the list must contain a next and previous field for the
 // list.
 //
 // The ilist_traits trait class is used to gain access to the next and previous
@@ -22,7 +22,7 @@
 // really want to know if it's empty.
 //
 // The ilist class is implemented by allocating a 'tail' node when the list is
-// created (using ilist_traits<>::createEndMarker()).  This tail node is
+// created (using ilist_traits<>::createSentinel()).  This tail node is
 // absolutely required because the user must be able to compute end()-1. Because
 // of this, users of the direct next/prev links will see an extra link on the
 // end of the list, which should be ignored.
@@ -134,7 +134,7 @@ public:
   // Increment and decrement operators...
   ilist_iterator &operator--() {      // predecrement - Back up
     NodePtr = Traits::getPrev(NodePtr);
-    assert(NodePtr && "--'d off the beginning of an ilist!");
+    assert(Traits::getNext(NodePtr) && "--'d off the beginning of an ilist!");
     return *this;
   }
   ilist_iterator &operator++() {      // preincrement - Advance
@@ -213,12 +213,20 @@ template<typename NodeTy> struct simplify_type<const ilist_iterator<NodeTy> > {
 //===----------------------------------------------------------------------===//
 //
 // iplist - The subset of list functionality that can safely be used on nodes of
-// polymorphic types, ie a heterogeneus list with a common base class that holds
-// the next/prev pointers...
+// polymorphic types, i.e. a heterogenous list with a common base class that
+// holds the next/prev pointers...
 //
 template<typename NodeTy, typename Traits=ilist_traits<NodeTy> >
 class iplist : public Traits {
-  NodeTy *Head, *Tail;
+  NodeTy *Head;
+
+  // Use the prev node pointer of 'head' as the tail pointer.  This is really a
+  // circularly linked list where we snip the 'next' link from the sentinel node
+  // back to the first node in the list (to preserve assertions about going off
+  // the end of the list).
+  NodeTy *getTail() { return getPrev(Head); }
+  const NodeTy *getTail() const { return getPrev(Head); }
+  void setTail(NodeTy *N) { setPrev(Head, N); }
 
   static bool op_less(NodeTy &L, NodeTy &R) { return L < R; }
   static bool op_equal(NodeTy &L, NodeTy &R) { return L == R; }
@@ -235,28 +243,28 @@ public:
   typedef std::reverse_iterator<const_iterator>  const_reverse_iterator;
   typedef std::reverse_iterator<iterator>  reverse_iterator;
 
-  iplist() : Head(Traits::createSentinel()), Tail(Head) {
+  iplist() : Head(Traits::createSentinel()) {
     setNext(Head, 0);
-    setPrev(Head, 0);
+    setTail(Head);
   }
-  ~iplist() { clear(); Traits::destroySentinel(Tail); }
+  ~iplist() { clear(); Traits::destroySentinel(getTail()); }
 
   // Iterator creation methods.
   iterator begin()             { return iterator(Head); }
   const_iterator begin() const { return const_iterator(Head); }
-  iterator end()               { return iterator(Tail); }
-  const_iterator end() const   { return const_iterator(Tail); }
+  iterator end()               { return iterator(getTail()); }
+  const_iterator end() const   { return const_iterator(getTail()); }
 
   // reverse iterator creation methods.
   reverse_iterator rbegin()            { return reverse_iterator(end()); }
   const_reverse_iterator rbegin() const{ return const_reverse_iterator(end()); }
   reverse_iterator rend()              { return reverse_iterator(begin()); }
-  const_reverse_iterator rend() const  {return const_reverse_iterator(begin());}
+  const_reverse_iterator rend() const  { return const_reverse_iterator(begin()); }
 
 
   // Miscellaneous inspection routines.
   size_type max_size() const { return size_type(-1); }
-  bool empty() const { return Head == Tail; }
+  bool empty() const { return Head == getTail(); }
 
   // Front and back accessor functions...
   reference front() {
@@ -269,17 +277,16 @@ public:
   }
   reference back() {
     assert(!empty() && "Called back() on empty list!");
-    return *getPrev(Tail);
+    return *getPrev(getTail());
   }
   const_reference back() const {
     assert(!empty() && "Called back() on empty list!");
-    return *getPrev(Tail);
+    return *getPrev(getTail());
   }
 
   void swap(iplist &RHS) {
     abort();     // Swap does not use list traits callback correctly yet!
     std::swap(Head, RHS.Head);
-    std::swap(Tail, RHS.Tail);
   }
 
   iterator insert(iterator where, NodeTy *New) {
@@ -287,7 +294,7 @@ public:
     setNext(New, CurNode);
     setPrev(New, PrevNode);
 
-    if (PrevNode)
+    if (CurNode != Head)  // Is PrevNode off the beginning of the list?
       setNext(PrevNode, New);
     else
       Head = New;
@@ -303,7 +310,7 @@ public:
     NodeTy *NextNode = getNext(Node);
     NodeTy *PrevNode = getPrev(Node);
 
-    if (PrevNode)
+    if (Node != Head)  // Is PrevNode off the beginning of the list?
       setNext(PrevNode, NextNode);
     else
       Head = NextNode;
@@ -331,7 +338,15 @@ private:
   //
   void transfer(iterator position, iplist &L2, iterator first, iterator last) {
     assert(first != last && "Should be checked by callers");
+
     if (position != last) {
+      // Note: we have to be careful about the case when we move the first node
+      // in the list.  This node is the list sentinel node and we can't move it.
+      NodeTy *ThisSentinel = getTail();
+      setTail(0);
+      NodeTy *L2Sentinel = L2.getTail();
+      L2.setTail(0);
+
       // Remove [first, last) from its old position.
       NodeTy *First = &*first, *Prev = getPrev(First);
       NodeTy *Next = last.getNodePtrUnchecked(), *Last = getPrev(Next);
@@ -357,6 +372,10 @@ private:
       setPrev(PosNext, Last);
 
       transferNodesFromList(L2, First, PosNext);
+
+      // Now that everything is set, restore the pointers to the list sentinals.
+      L2.setTail(L2Sentinel);
+      setTail(ThisSentinel);
     }
   }