diff --git a/lib/CodeGen/LiveIntervalUnion.h b/lib/CodeGen/LiveIntervalUnion.h
index 90d4aaf88ad..c83578e99c6 100644
--- a/lib/CodeGen/LiveIntervalUnion.h
+++ b/lib/CodeGen/LiveIntervalUnion.h
@@ -95,6 +95,9 @@ public:
   // Remove a live virtual register's segments from this union.
   void extract(LiveInterval &VirtReg);
 
+  // Remove all inserted virtual registers.
+  void clear() { Segments.clear(); ++Tag; }
+
   // Print union, using TRI to translate register names
   void print(raw_ostream &OS, const TargetRegisterInfo *TRI) const;
 
diff --git a/lib/CodeGen/RegAllocBasic.cpp b/lib/CodeGen/RegAllocBasic.cpp
index 708f55688ba..af7da53b89b 100644
--- a/lib/CodeGen/RegAllocBasic.cpp
+++ b/lib/CodeGen/RegAllocBasic.cpp
@@ -235,9 +235,12 @@ void RegAllocBase::init(VirtRegMap &vrm, LiveIntervals &lis) {
   MRI = &vrm.getRegInfo();
   VRM = &vrm;
   LIS = &lis;
-  PhysReg2LiveUnion.init(UnionAllocator, TRI->getNumRegs());
-  // Cache an interferece query for each physical reg
-  Queries.reset(new LiveIntervalUnion::Query[PhysReg2LiveUnion.numRegs()]);
+  const unsigned NumRegs = TRI->getNumRegs();
+  if (NumRegs != PhysReg2LiveUnion.numRegs()) {
+    PhysReg2LiveUnion.init(UnionAllocator, NumRegs);
+    // Cache an interferece query for each physical reg
+    Queries.reset(new LiveIntervalUnion::Query[PhysReg2LiveUnion.numRegs()]);
+  }
 }
 
 void RegAllocBase::LiveUnionArray::clear() {
@@ -251,7 +254,8 @@ void RegAllocBase::LiveUnionArray::clear() {
 }
 
 void RegAllocBase::releaseMemory() {
-  PhysReg2LiveUnion.clear();
+  for (unsigned r = 0, e = PhysReg2LiveUnion.numRegs(); r != e; ++r)
+    PhysReg2LiveUnion[r].clear();
 }
 
 // Visit all the live registers. If they are already assigned to a physical