diff --git a/lib/CodeGen/LiveIntervalUnion.cpp b/lib/CodeGen/LiveIntervalUnion.cpp
index dea2283..6e2cd0f 100644
--- a/lib/CodeGen/LiveIntervalUnion.cpp
+++ b/lib/CodeGen/LiveIntervalUnion.cpp
@@ -21,6 +21,9 @@
 using namespace llvm;
 
 // Merge a LiveInterval's segments. Guarantee no overlaps.
+//
+// Consider coalescing adjacent segments to save space, even though it makes
+// extraction more complicated.
 void LiveIntervalUnion::unify(LiveInterval &lvr) {
   // Add this live virtual register to the union
   LiveVirtRegs::iterator pos = std::upper_bound(lvrs_.begin(), lvrs_.end(),
@@ -34,19 +37,21 @@
     LiveSegment segment(lvrI->start, lvrI->end, lvr);
     segPos = segments_.insert(segPos, segment);
     assert(*segPos == segment && "need equal val for equal key");
+#ifndef NDEBUG
+    // check for overlap (inductively)
+    if (segPos != segments_.begin()) {
+      SegmentIter prevPos = segPos;
+      --prevPos;
+      assert(prevPos->end <= segment.start && "overlapping segments" );
+    }
+    SegmentIter nextPos = segPos;
+    ++nextPos;
+    if (nextPos != segments_.end())
+      assert(segment.end <= nextPos->start && "overlapping segments" );
+#endif // NDEBUG
   }
 }
 
-namespace {
-
-// Keep LVRs sorted for fast membership test and extraction.
-struct LessReg
-  : public std::binary_function<LiveInterval*, LiveInterval*, bool> {
-  bool operator()(const LiveInterval *left, const LiveInterval *right) const {
-    return left->reg < right->reg;
-  }
-};
-                    
 // Low-level helper to find the first segment in the range [segI,segEnd) that
 // intersects with a live virtual register segment, or segI.start >= lvr.end
 //
@@ -67,10 +72,8 @@
 // Assuming intervals are disjoint, if an intersection exists, it must be the
 // segment found or immediately behind it. We continue reverse iterating to
 // return the first overlap.
-//
-// FIXME: support extract(), handle tombstones of extracted lvrs.
 typedef LiveIntervalUnion::SegmentIter SegmentIter;
-SegmentIter upperBound(SegmentIter segBegin,
+static SegmentIter upperBound(SegmentIter segBegin,
                        SegmentIter segEnd,
                        const LiveRange &lvrSeg) {
   assert(lvrSeg.end > segBegin->start && "segment iterator precondition");
@@ -84,7 +87,6 @@
   }
   return segI;
 }
-} // end anonymous namespace
 
 // Private interface accessed by Query.
 //
diff --git a/lib/CodeGen/LiveIntervalUnion.h b/lib/CodeGen/LiveIntervalUnion.h
index 3953c59..1eb380f 100644
--- a/lib/CodeGen/LiveIntervalUnion.h
+++ b/lib/CodeGen/LiveIntervalUnion.h
@@ -23,13 +23,16 @@
 
 namespace llvm {
 
-// A LiveSegment is a copy of a LiveRange object used within
-// LiveIntervalUnion. LiveSegment additionally contains a pointer to its
-// original live virtual register (LiveInterval). This allows quick lookup of
-// the live virtual register as we iterate over live segments in a union. Note
-// that LiveRange is misnamed and actually represents only a single contiguous
-// interval within a virtual register's liveness. To limit confusion, in this
-// file we refer it as a live segment.
+/// A LiveSegment is a copy of a LiveRange object used within
+/// LiveIntervalUnion. LiveSegment additionally contains a pointer to its
+/// original live virtual register (LiveInterval). This allows quick lookup of
+/// the live virtual register as we iterate over live segments in a union. Note
+/// that LiveRange is misnamed and actually represents only a single contiguous
+/// interval within a virtual register's liveness. To limit confusion, in this
+/// file we refer it as a live segment.
+///
+/// Note: This currently represents a half-open interval [start,end).
+/// If LiveRange is modified to represent a closed interval, so should this.
 struct LiveSegment {
   SlotIndex start;
   SlotIndex end;
@@ -46,16 +49,10 @@
     return !operator==(ls);
   }
 
-  bool operator<(const LiveSegment &ls) const {
-    return start < ls.start || (start == ls.start && end < ls.end);
-  }
+  // Order segments by starting point only--we expect them to be disjoint.
+  bool operator<(const LiveSegment &ls) const { return start < ls.start; }
 };
 
-/// Compare a live virtual register segment to a LiveIntervalUnion segment.
-inline bool overlap(const LiveRange &lvrSeg, const LiveSegment &liuSeg) {
-  return lvrSeg.start < liuSeg.end && liuSeg.start < lvrSeg.end;
-}
-
 inline bool operator<(SlotIndex V, const LiveSegment &ls) {
   return V < ls.start;
 }
@@ -64,6 +61,11 @@
   return ls.start < V;
 }
 
+/// Compare a live virtual register segment to a LiveIntervalUnion segment.
+inline bool overlap(const LiveRange &lvrSeg, const LiveSegment &liuSeg) {
+  return lvrSeg.start < liuSeg.end && liuSeg.start < lvrSeg.end;
+}
+
 /// Union of live intervals that are strong candidates for coalescing into a
 /// single register (either physical or virtual depending on the context).  We
 /// expect the constituent live intervals to be disjoint, although we may
@@ -100,13 +102,18 @@
 public:
   // default ctor avoids placement new
   LiveIntervalUnion() : repReg_(0) {}
-  
+
+  // Initialize the union by associating it with a representative register
+  // number.
   void init(unsigned repReg) { repReg_ = repReg; }
 
+  // Iterate over all segments in the union of live virtual registers ordered
+  // by their starting position.
   SegmentIter begin() { return segments_.begin(); }
   SegmentIter end() { return segments_.end(); }
 
-  /// FIXME: !!!!!!!!!!! Keeps a non-const ref
+  // Add a live virtual register to this union and merge its segments.
+  // Holds a nonconst reference to the LVR for later maniplution.
   void unify(LiveInterval &lvr);
 
   // FIXME: needed by RegAllocGreedy
diff --git a/lib/CodeGen/RegAllocBase.h b/lib/CodeGen/RegAllocBase.h
index d543635..1534c0d 100644
--- a/lib/CodeGen/RegAllocBase.h
+++ b/lib/CodeGen/RegAllocBase.h
@@ -37,17 +37,29 @@
 #ifndef LLVM_CODEGEN_REGALLOCBASE
 #define LLVM_CODEGEN_REGALLOCBASE
 
-#include "LiveIntervalUnion.h"
-#include "VirtRegMap.h"
-#include "llvm/CodeGen/LiveIntervalAnalysis.h"
-#include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/ADT/OwningPtr.h"
-#include <vector>
-#include <queue>
 
 namespace llvm {
 
+template<typename T> class SmallVectorImpl;
+class TargetRegisterInfo;
 class VirtRegMap;
+class LiveIntervals;
+
+// Heuristic that determines the priority of assigning virtual to physical
+// registers. The main impact of the heuristic is expected to be compile time.
+// The default is to simply compare spill weights.
+struct LessSpillWeightPriority
+  : public std::binary_function<LiveInterval,LiveInterval, bool> {
+  bool operator()(const LiveInterval *left, const LiveInterval *right) const {
+    return left->weight < right->weight;
+  }
+};
+
+// Forward declare a priority queue of live virtual registers. If an
+// implementation needs to prioritize by anything other than spill weight, then
+// this will become an abstract base class with virtual calls to push/get.
+class LiveVirtRegQueue;
 
 /// RegAllocBase provides the register allocation driver and interface that can
 /// be extended to add interesting heuristics.
@@ -58,9 +70,6 @@
 /// standard comparator.
 class RegAllocBase {
 protected:
-  typedef SmallVector<LiveInterval*, 4> LiveVirtRegs;
-  typedef LiveVirtRegs::iterator LVRIter;
-  
   // Array of LiveIntervalUnions indexed by physical register.
   class LIUArray {
     unsigned nRegs_;
@@ -92,18 +101,20 @@
   // A RegAlloc pass should call this before allocatePhysRegs.
   void init(const TargetRegisterInfo &tri, VirtRegMap &vrm, LiveIntervals &lis);
 
-  // The top-level driver. Specialize with the comparator that determines the
-  // priority of assigning live virtual registers. The output is a VirtRegMap
-  // that us updated with physical register assignments.
-  template<typename LICompare>
-  void allocatePhysRegs(LICompare liCompare);
+  // The top-level driver. The output is a VirtRegMap that us updated with
+  // physical register assignments.
+  //
+  // If an implementation wants to override the LiveInterval comparator, we
+  // should modify this interface to allow passing in an instance derived from
+  // LiveVirtRegQueue.
+  void allocatePhysRegs();
 
   // A RegAlloc pass should override this to provide the allocation heuristics.
-  // Each call must guarantee forward progess by returning an available
-  // PhysReg or new set of split LiveVirtRegs. It is up to the splitter to
+  // Each call must guarantee forward progess by returning an available PhysReg
+  // or new set of split live virtual registers. It is up to the splitter to
   // converge quickly toward fully spilled live ranges.
   virtual unsigned selectOrSplit(LiveInterval &lvr,
-                                 LiveVirtRegs &splitLVRs) = 0;
+                                 SmallVectorImpl<LiveInterval*> &splitLVRs) = 0;
 
   // A RegAlloc pass should call this when PassManager releases its memory.
   virtual void releaseMemory();
@@ -113,69 +124,9 @@
   bool checkPhysRegInterference(LiveIntervalUnion::Query &query, unsigned preg);
   
 private:
-  template<typename PQ>
-  void seedLiveVirtRegs(PQ &lvrQ);
+  void seedLiveVirtRegs(LiveVirtRegQueue &lvrQ);
 };
 
-// Heuristic that determines the priority of assigning virtual to physical
-// registers. The main impact of the heuristic is expected to be compile time.
-// The default is to simply compare spill weights.
-struct LessSpillWeightPriority
-  : public std::binary_function<LiveInterval,LiveInterval, bool> {
-  bool operator()(const LiveInterval *left, const LiveInterval *right) const {
-    return left->weight < right->weight;
-  }
-};
-
-// Visit all the live virtual registers. If they are already assigned to a
-// physical register, unify them with the corresponding LiveIntervalUnion,
-// otherwise push them on the priority queue for later assignment.
-template<typename PQ>
-void RegAllocBase::seedLiveVirtRegs(PQ &lvrQ) {
-  for (LiveIntervals::iterator liItr = lis_->begin(), liEnd = lis_->end();
-       liItr != liEnd; ++liItr) {
-    unsigned reg = liItr->first;
-    LiveInterval &li = *liItr->second;
-    if (TargetRegisterInfo::isPhysicalRegister(reg)) {
-      physReg2liu_[reg].unify(li);
-    }
-    else {
-      lvrQ.push(&li);
-    }
-  }
-}
-
-// Top-level driver to manage the queue of unassigned LiveVirtRegs and call the
-// selectOrSplit implementation.
-template<typename LICompare>
-void RegAllocBase::allocatePhysRegs(LICompare liCompare) {
-  typedef std::priority_queue
-    <LiveInterval*, std::vector<LiveInterval*>, LICompare> LiveVirtRegQueue;
-
-  LiveVirtRegQueue lvrQ(liCompare);
-  seedLiveVirtRegs(lvrQ);
-  while (!lvrQ.empty()) {
-    LiveInterval *lvr = lvrQ.top();
-    lvrQ.pop();
-    LiveVirtRegs splitLVRs;
-    unsigned availablePhysReg = selectOrSplit(*lvr, splitLVRs);
-    if (availablePhysReg) {
-      assert(splitLVRs.empty() && "inconsistent splitting");
-      assert(!vrm_->hasPhys(lvr->reg) && "duplicate vreg in interval unions");
-      vrm_->assignVirt2Phys(lvr->reg, availablePhysReg);
-      physReg2liu_[availablePhysReg].unify(*lvr);
-    }
-    else {
-      for (LVRIter lvrI = splitLVRs.begin(), lvrEnd = splitLVRs.end();
-           lvrI != lvrEnd; ++lvrI ) {
-        assert(TargetRegisterInfo::isVirtualRegister((*lvrI)->reg) &&
-               "expect split value in virtual register");
-        lvrQ.push(*lvrI);
-      }
-    }
-  }
-}
-
 } // end namespace llvm
 
 #endif // !defined(LLVM_CODEGEN_REGALLOCBASE)
diff --git a/lib/CodeGen/RegAllocBasic.cpp b/lib/CodeGen/RegAllocBasic.cpp
index ce5f9cf..83999d9 100644
--- a/lib/CodeGen/RegAllocBasic.cpp
+++ b/lib/CodeGen/RegAllocBasic.cpp
@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "regalloc"
+#include "LiveIntervalUnion.h"
 #include "RegAllocBase.h"
 #include "RenderMachineFunction.h"
 #include "Spiller.h"
@@ -33,6 +34,14 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 
+#include "VirtRegMap.h"
+#include "llvm/CodeGen/LiveIntervalAnalysis.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+
+
+#include <vector>
+#include <queue>
+
 using namespace llvm;
 
 static RegisterRegAlloc basicRegAlloc("basic", "basic register allocator",
@@ -72,7 +81,8 @@
 
   virtual void releaseMemory();
 
-  virtual unsigned selectOrSplit(LiveInterval &lvr, LiveVirtRegs &splitLVRs);
+  virtual unsigned selectOrSplit(LiveInterval &lvr,
+                                 SmallVectorImpl<LiveInterval*> &splitLVRs);
 
   /// Perform register allocation.
   virtual bool runOnMachineFunction(MachineFunction &mf);
@@ -101,7 +111,7 @@
 #endif
 INITIALIZE_PASS_END(RABasic, "basic-regalloc",
                     "Basic Register Allocator", false, false)
-#endif // INITIALIZE_PASS
+#endif // disable INITIALIZE_PASS
 
 RABasic::RABasic(): MachineFunctionPass(ID) {
   initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
@@ -168,6 +178,79 @@
   physReg2liu_.clear();
 }
 
+namespace llvm {
+/// This class defines a queue of live virtual registers prioritized by spill
+/// weight. The heaviest vreg is popped first.
+///
+/// Currently, this is trivial wrapper that gives us an opaque type in the
+/// header, but we may later give it a virtual interface for register allocators
+/// to override the priority queue comparator.
+class LiveVirtRegQueue {
+  typedef std::priority_queue
+    <LiveInterval*, std::vector<LiveInterval*>, LessSpillWeightPriority> PQ;
+  PQ pq_;
+  
+public:
+  // Is the queue empty?
+  bool empty() { return pq_.empty(); }
+  
+  // Get the highest priority lvr (top + pop)
+  LiveInterval *get() {
+    LiveInterval *lvr = pq_.top();
+    pq_.pop();
+    return lvr;
+  }
+  // Add this lvr to the queue
+  void push(LiveInterval *lvr) {
+    pq_.push(lvr);
+  }
+};
+} // end namespace llvm
+
+// Visit all the live virtual registers. If they are already assigned to a
+// physical register, unify them with the corresponding LiveIntervalUnion,
+// otherwise push them on the priority queue for later assignment.
+void RegAllocBase::seedLiveVirtRegs(LiveVirtRegQueue &lvrQ) {
+  for (LiveIntervals::iterator liItr = lis_->begin(), liEnd = lis_->end();
+       liItr != liEnd; ++liItr) {
+    unsigned reg = liItr->first;
+    LiveInterval &li = *liItr->second;
+    if (TargetRegisterInfo::isPhysicalRegister(reg)) {
+      physReg2liu_[reg].unify(li);
+    }
+    else {
+      lvrQ.push(&li);
+    }
+  }
+}
+
+// Top-level driver to manage the queue of unassigned LiveVirtRegs and call the
+// selectOrSplit implementation.
+void RegAllocBase::allocatePhysRegs() {
+  LiveVirtRegQueue lvrQ;
+  seedLiveVirtRegs(lvrQ);
+  while (!lvrQ.empty()) {
+    LiveInterval *lvr = lvrQ.get();
+    typedef SmallVector<LiveInterval*, 4> LVRVec;
+    LVRVec splitLVRs;
+    unsigned availablePhysReg = selectOrSplit(*lvr, splitLVRs);
+    if (availablePhysReg) {
+      assert(splitLVRs.empty() && "inconsistent splitting");
+      assert(!vrm_->hasPhys(lvr->reg) && "duplicate vreg in interval unions");
+      vrm_->assignVirt2Phys(lvr->reg, availablePhysReg);
+      physReg2liu_[availablePhysReg].unify(*lvr);
+    }
+    else {
+      for (LVRVec::iterator lvrI = splitLVRs.begin(), lvrEnd = splitLVRs.end();
+           lvrI != lvrEnd; ++lvrI) {
+        assert(TargetRegisterInfo::isVirtualRegister((*lvrI)->reg) &&
+               "expect split value in virtual register");
+        lvrQ.push(*lvrI);
+      }
+    }
+  }
+}
+
 // Check if this live virtual reg interferes with a physical register. If not,
 // then check for interference on each register that aliases with the physical
 // register.
@@ -201,7 +284,8 @@
 // available register. So, the number of interference tests in the worst case is
 // |vregs| * |machineregs|. And since the number of interference tests is
 // minimal, there is no value in caching them.
-unsigned RABasic::selectOrSplit(LiveInterval &lvr, LiveVirtRegs &splitLVRs) {
+unsigned RABasic::selectOrSplit(LiveInterval &lvr,
+                                SmallVectorImpl<LiveInterval*> &splitLVRs) {
   // Check for an available reg in this class. 
   const TargetRegisterClass *trc = mri_->getRegClass(lvr.reg);
   for (TargetRegisterClass::iterator trcI = trc->allocation_order_begin(*mf_),
@@ -238,7 +322,7 @@
 
   spiller_.reset(createSpiller(*this, *mf_, *vrm_));
   
-  allocatePhysRegs(LessSpillWeightPriority());
+  allocatePhysRegs();
 
   // Diagnostic output before rewriting
   DEBUG(dbgs() << "Post alloc VirtRegMap:\n" << *vrm_ << "\n");
@@ -249,6 +333,9 @@
   // Run rewriter
   std::auto_ptr<VirtRegRewriter> rewriter(createVirtRegRewriter());
   rewriter->runOnMachineFunction(*mf_, *vrm_, lis_);
+
+  // The pass output is in VirtRegMap. Release all the transient data.
+  releaseMemory();
   
   return true;
 }