diff --git a/include/Support/EquivalenceClasses.h b/include/Support/EquivalenceClasses.h
index 46e626b69b5..127183614be 100644
--- a/include/Support/EquivalenceClasses.h
+++ b/include/Support/EquivalenceClasses.h
@@ -18,6 +18,7 @@
 #define SUPPORT_EQUIVALENCECLASSES_H
 
 #include <map>
+#include <set>
 #include <vector>
 
 namespace llvm {
@@ -26,69 +27,88 @@ template <class ElemTy>
 class EquivalenceClasses {
   // Maps each element to the element that is the leader of its 
   // equivalence class.
-  std::map<ElemTy, ElemTy> Elem2ECLeaderMap;
+  std::map<ElemTy, ElemTy> Elem2LeaderMap;
   
+  // Maintains the set of leaders
+  std::set<ElemTy> LeaderSet;
+
+  // Caches the equivalence class for each leader
+  std::map<ElemTy, std::set<ElemTy> > LeaderToEqClassMap;
+
   // Make Element2 the leader of the union of classes Element1 and Element2
   // Element1 and Element2 are presumed to be leaders of their respective
   // equivalence classes.
   void attach(ElemTy Element1, ElemTy Element2) {
     for (typename std::map<ElemTy, ElemTy>::iterator ElemI = 
-	   Elem2ECLeaderMap.begin(), ElemE = Elem2ECLeaderMap.end(); 
+	   Elem2LeaderMap.begin(), ElemE = Elem2LeaderMap.end(); 
 	 ElemI != ElemE; ++ElemI) {
       if (ElemI->second == Element1)
-	Elem2ECLeaderMap[ElemI->first] = Element2;
+	Elem2LeaderMap[ElemI->first] = Element2;
     }
   }
 
 public:
-  
-  void addElement (ElemTy NewElement) {
-    if (Elem2ECLeaderMap.find(NewElement) == Elem2ECLeaderMap.end())
-      Elem2ECLeaderMap[NewElement] = NewElement;
+  // If an element has not yet in any class, make it a separate new class.
+  // Return the leader of the class containing the element.
+  ElemTy addElement (ElemTy NewElement) {
+    typename std::map<ElemTy, ElemTy>::iterator ElemI = 
+      Elem2LeaderMap.find(NewElement);
+    if (ElemI == Elem2LeaderMap.end()) {
+      Elem2LeaderMap[NewElement] = NewElement;
+      LeaderSet.insert(NewElement);
+      return NewElement;
+    }
+    else
+      return ElemI->second;
   }
   
-  ElemTy findClass(ElemTy Element) {
-    if (Elem2ECLeaderMap.find(Element) == Elem2ECLeaderMap.end())
-      return 0;
-    else 
-      return Elem2ECLeaderMap[Element];
+  ElemTy findClass(ElemTy Element) const {
+    typename std::map<ElemTy, ElemTy>::const_iterator I =
+      Elem2LeaderMap.find(Element);
+    return (I == Elem2LeaderMap.end())? (ElemTy) 0 : I->second;
   }
 
   /// Attach the set with Element1 to the set with Element2 adding Element1 and
   /// Element2 to the set of equivalence classes if they are not there already.
   /// Implication: Make Element1 the element in the smaller set.
+  /// Take Leader[Element1] out of the set of leaders.
   void unionSetsWith(ElemTy Element1, ElemTy Element2) {
     // If either Element1 or Element2 does not already exist, include it
-    if (Elem2ECLeaderMap.find(Element1) == Elem2ECLeaderMap.end())
-      Elem2ECLeaderMap[Element1] = Element1;
-    if (Elem2ECLeaderMap.find(Element2) == Elem2ECLeaderMap.end())
-      Elem2ECLeaderMap[Element2] = Element2;
-
-    attach(Elem2ECLeaderMap[Element1], Elem2ECLeaderMap[Element2]);
+    const ElemTy& leader1 = addElement(Element1);
+    const ElemTy& leader2 = addElement(Element2);
+    assert(leader1 != (ElemTy) 0 && leader2 != (ElemTy) 0);
+    if (leader1 != leader2) {
+      attach(leader1, leader2);
+      LeaderSet.erase(leader1);
+    }
   }
   
-  // Returns a vector containing all the elements in the equivalent class
+  // Returns a vector containing all the elements in the equivalence class
   // including Element1
-  std::vector<ElemTy> getEqClass(ElemTy Element1) {
-    std::vector<ElemTy> EqClass;
+  const std::set<ElemTy> & getEqClass(ElemTy Element1) {
+    assert(Elem2LeaderMap.find(Element1) != Elem2LeaderMap.end());
+    const ElemTy classLeader = Elem2LeaderMap[Element1];
     
-    if (Elem2ECLeaderMap.find(EqClass) == Elem2ECLeaderMap.end())
-      return EqClass;
+    std::set<ElemTy> & EqClass = LeaderToEqClassMap[classLeader];
     
-    ElemTy classLeader = Elem2ECLeaderMap[Element1];
-    for (typename std::map<ElemTy, ElemTy>::iterator ElemI = 
-	   Elem2ECLeaderMap.begin(), ElemE = Elem2ECLeaderMap.end(); 
-	 ElemI != ElemE; ++ElemI) {
-      if (ElemI->second == classLeader)
-	EqClass.push_back(ElemI->first);
+    // If the EqClass vector is empty, it has not been computed yet: do it now
+    if (EqClass.empty()) {
+      for (typename std::map<ElemTy, ElemTy>::iterator
+             ElemI = Elem2LeaderMap.begin(), ElemE = Elem2LeaderMap.end(); 
+           ElemI != ElemE; ++ElemI)
+        if (ElemI->second == classLeader)
+          EqClass.insert(ElemI->first);
+      assert(! EqClass.empty());        // must at least include the leader
     }
     
     return EqClass;
   }
 
-  std::map<ElemTy, ElemTy>& getLeaderMap() {
-    return Elem2ECLeaderMap ;
-  }
+        std::set<ElemTy>& getLeaderSet()       { return LeaderSet; }
+  const std::set<ElemTy>& getLeaderSet() const { return LeaderSet; }
+
+        std::map<ElemTy, ElemTy>& getLeaderMap()       { return Elem2LeaderMap;}
+  const std::map<ElemTy, ElemTy>& getLeaderMap() const { return Elem2LeaderMap;}
 };
 
 } // End llvm namespace
diff --git a/include/llvm/ADT/EquivalenceClasses.h b/include/llvm/ADT/EquivalenceClasses.h
index 46e626b69b5..127183614be 100644
--- a/include/llvm/ADT/EquivalenceClasses.h
+++ b/include/llvm/ADT/EquivalenceClasses.h
@@ -18,6 +18,7 @@
 #define SUPPORT_EQUIVALENCECLASSES_H
 
 #include <map>
+#include <set>
 #include <vector>
 
 namespace llvm {
@@ -26,69 +27,88 @@ template <class ElemTy>
 class EquivalenceClasses {
   // Maps each element to the element that is the leader of its 
   // equivalence class.
-  std::map<ElemTy, ElemTy> Elem2ECLeaderMap;
+  std::map<ElemTy, ElemTy> Elem2LeaderMap;
   
+  // Maintains the set of leaders
+  std::set<ElemTy> LeaderSet;
+
+  // Caches the equivalence class for each leader
+  std::map<ElemTy, std::set<ElemTy> > LeaderToEqClassMap;
+
   // Make Element2 the leader of the union of classes Element1 and Element2
   // Element1 and Element2 are presumed to be leaders of their respective
   // equivalence classes.
   void attach(ElemTy Element1, ElemTy Element2) {
     for (typename std::map<ElemTy, ElemTy>::iterator ElemI = 
-	   Elem2ECLeaderMap.begin(), ElemE = Elem2ECLeaderMap.end(); 
+	   Elem2LeaderMap.begin(), ElemE = Elem2LeaderMap.end(); 
 	 ElemI != ElemE; ++ElemI) {
       if (ElemI->second == Element1)
-	Elem2ECLeaderMap[ElemI->first] = Element2;
+	Elem2LeaderMap[ElemI->first] = Element2;
     }
   }
 
 public:
-  
-  void addElement (ElemTy NewElement) {
-    if (Elem2ECLeaderMap.find(NewElement) == Elem2ECLeaderMap.end())
-      Elem2ECLeaderMap[NewElement] = NewElement;
+  // If an element has not yet in any class, make it a separate new class.
+  // Return the leader of the class containing the element.
+  ElemTy addElement (ElemTy NewElement) {
+    typename std::map<ElemTy, ElemTy>::iterator ElemI = 
+      Elem2LeaderMap.find(NewElement);
+    if (ElemI == Elem2LeaderMap.end()) {
+      Elem2LeaderMap[NewElement] = NewElement;
+      LeaderSet.insert(NewElement);
+      return NewElement;
+    }
+    else
+      return ElemI->second;
   }
   
-  ElemTy findClass(ElemTy Element) {
-    if (Elem2ECLeaderMap.find(Element) == Elem2ECLeaderMap.end())
-      return 0;
-    else 
-      return Elem2ECLeaderMap[Element];
+  ElemTy findClass(ElemTy Element) const {
+    typename std::map<ElemTy, ElemTy>::const_iterator I =
+      Elem2LeaderMap.find(Element);
+    return (I == Elem2LeaderMap.end())? (ElemTy) 0 : I->second;
   }
 
   /// Attach the set with Element1 to the set with Element2 adding Element1 and
   /// Element2 to the set of equivalence classes if they are not there already.
   /// Implication: Make Element1 the element in the smaller set.
+  /// Take Leader[Element1] out of the set of leaders.
   void unionSetsWith(ElemTy Element1, ElemTy Element2) {
     // If either Element1 or Element2 does not already exist, include it
-    if (Elem2ECLeaderMap.find(Element1) == Elem2ECLeaderMap.end())
-      Elem2ECLeaderMap[Element1] = Element1;
-    if (Elem2ECLeaderMap.find(Element2) == Elem2ECLeaderMap.end())
-      Elem2ECLeaderMap[Element2] = Element2;
-
-    attach(Elem2ECLeaderMap[Element1], Elem2ECLeaderMap[Element2]);
+    const ElemTy& leader1 = addElement(Element1);
+    const ElemTy& leader2 = addElement(Element2);
+    assert(leader1 != (ElemTy) 0 && leader2 != (ElemTy) 0);
+    if (leader1 != leader2) {
+      attach(leader1, leader2);
+      LeaderSet.erase(leader1);
+    }
   }
   
-  // Returns a vector containing all the elements in the equivalent class
+  // Returns a vector containing all the elements in the equivalence class
   // including Element1
-  std::vector<ElemTy> getEqClass(ElemTy Element1) {
-    std::vector<ElemTy> EqClass;
+  const std::set<ElemTy> & getEqClass(ElemTy Element1) {
+    assert(Elem2LeaderMap.find(Element1) != Elem2LeaderMap.end());
+    const ElemTy classLeader = Elem2LeaderMap[Element1];
     
-    if (Elem2ECLeaderMap.find(EqClass) == Elem2ECLeaderMap.end())
-      return EqClass;
+    std::set<ElemTy> & EqClass = LeaderToEqClassMap[classLeader];
     
-    ElemTy classLeader = Elem2ECLeaderMap[Element1];
-    for (typename std::map<ElemTy, ElemTy>::iterator ElemI = 
-	   Elem2ECLeaderMap.begin(), ElemE = Elem2ECLeaderMap.end(); 
-	 ElemI != ElemE; ++ElemI) {
-      if (ElemI->second == classLeader)
-	EqClass.push_back(ElemI->first);
+    // If the EqClass vector is empty, it has not been computed yet: do it now
+    if (EqClass.empty()) {
+      for (typename std::map<ElemTy, ElemTy>::iterator
+             ElemI = Elem2LeaderMap.begin(), ElemE = Elem2LeaderMap.end(); 
+           ElemI != ElemE; ++ElemI)
+        if (ElemI->second == classLeader)
+          EqClass.insert(ElemI->first);
+      assert(! EqClass.empty());        // must at least include the leader
     }
     
     return EqClass;
   }
 
-  std::map<ElemTy, ElemTy>& getLeaderMap() {
-    return Elem2ECLeaderMap ;
-  }
+        std::set<ElemTy>& getLeaderSet()       { return LeaderSet; }
+  const std::set<ElemTy>& getLeaderSet() const { return LeaderSet; }
+
+        std::map<ElemTy, ElemTy>& getLeaderMap()       { return Elem2LeaderMap;}
+  const std::map<ElemTy, ElemTy>& getLeaderMap() const { return Elem2LeaderMap;}
 };
 
 } // End llvm namespace