diff --git a/lib/CodeGen/LiveIntervalUnion.cpp b/lib/CodeGen/LiveIntervalUnion.cpp
index ec502cd..dccffbb 100644
--- a/lib/CodeGen/LiveIntervalUnion.cpp
+++ b/lib/CodeGen/LiveIntervalUnion.cpp
@@ -15,6 +15,7 @@
 
 #define DEBUG_TYPE "regalloc"
 #include "LiveIntervalUnion.h"
+#include "llvm/ADT/SparseBitVector.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
@@ -73,12 +74,10 @@
 #ifndef NDEBUG
     // check for overlap (inductively)
     if (segPos != segments_.begin()) {
-      SegmentIter prevPos = segPos;
-      --prevPos;
-      assert(prevPos->end <= segment.start && "overlapping segments" );
+      assert(llvm::prior(segPos)->end <= segment.start &&
+             "overlapping segments" );
     }
-    SegmentIter nextPos = segPos;
-    ++nextPos;
+    SegmentIter nextPos = llvm::next(segPos);
     if (nextPos != segments_.end())
       assert(segment.end <= nextPos->start && "overlapping segments" );
 #endif // NDEBUG
@@ -98,6 +97,49 @@
   }
 }
 
+raw_ostream& llvm::operator<<(raw_ostream& os, const LiveSegment &ls) {
+  return os << '[' << ls.start << ',' << ls.end << ':' <<
+    ls.liveVirtReg->reg << ")";
+}
+
+void LiveSegment::dump() const {
+  dbgs() << *this << "\n";
+}
+
+void
+LiveIntervalUnion::print(raw_ostream &os,
+                         const AbstractRegisterDescription *rdesc) const {
+  os << "LIU ";
+  if (rdesc != NULL)
+    os << rdesc->getName(repReg_);
+  else {
+    os << repReg_;
+  }
+  for (LiveSegments::const_iterator segI = segments_.begin(),
+         segEnd = segments_.end(); segI != segEnd; ++segI) {
+    dbgs() << " " << *segI;
+  }
+  os << "\n";
+}
+
+void LiveIntervalUnion::dump(const AbstractRegisterDescription *rdesc) const {
+  print(dbgs(), rdesc);
+}
+
+#ifndef NDEBUG
+// Verify the live intervals in this union and add them to the visited set.
+void LiveIntervalUnion::verify(LvrBitSet& visitedVRegs) {
+  SegmentIter segI = segments_.begin();
+  SegmentIter segEnd = segments_.end();
+  if (segI == segEnd) return;
+  visitedVRegs.set(segI->liveVirtReg->reg);
+  for (++segI; segI != segEnd; ++segI) {
+    visitedVRegs.set(segI->liveVirtReg->reg);
+    assert(llvm::prior(segI)->end <= segI->start && "overlapping segments" );
+  }
+}
+#endif //!NDEBUG
+
 // Private interface accessed by Query.
 //
 // Find a pair of segments that intersect, one in the live virtual register
diff --git a/lib/CodeGen/LiveIntervalUnion.h b/lib/CodeGen/LiveIntervalUnion.h
index cb07653..74499f6 100644
--- a/lib/CodeGen/LiveIntervalUnion.h
+++ b/lib/CodeGen/LiveIntervalUnion.h
@@ -23,6 +23,12 @@
 
 namespace llvm {
 
+#ifndef NDEBUG
+// forward declaration
+template <unsigned Element> class SparseBitVector;
+typedef SparseBitVector<128> LvrBitSet;
+#endif
+
 /// 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
@@ -51,6 +57,9 @@
 
   // Order segments by starting point only--we expect them to be disjoint.
   bool operator<(const LiveSegment &ls) const { return start < ls.start; }
+
+  void dump() const;
+  void print(raw_ostream &os) const;
 };
 
 inline bool operator<(SlotIndex V, const LiveSegment &ls) {
@@ -74,9 +83,9 @@
 class AbstractRegisterDescription {
 public:
   virtual const char *getName(unsigned reg) const = 0;
-  virtual ~AbstractRegisterDescription() { }
+  virtual ~AbstractRegisterDescription() {}
 };
-  
+
 /// 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
@@ -133,6 +142,16 @@
   // Remove a live virtual register's segments from this union.
   void extract(const LiveInterval &lvr);
 
+  void dump(const AbstractRegisterDescription *regInfo) const;
+
+  // If tri != NULL, use it to decode repReg_
+  void print(raw_ostream &os, const AbstractRegisterDescription *rdesc) const;
+  
+#ifndef NDEBUG
+  // Verify the live intervals in this union and add them to the visited set.
+  void verify(LvrBitSet& visitedVRegs);
+#endif
+
   /// Cache a single interference test result in the form of two intersecting
   /// segments. This allows efficiently iterating over the interferences. The
   /// iteration logic is handled by LiveIntervalUnion::Query which may
diff --git a/lib/CodeGen/RegAllocBase.h b/lib/CodeGen/RegAllocBase.h
index f4ca972..fa595fe 100644
--- a/lib/CodeGen/RegAllocBase.h
+++ b/lib/CodeGen/RegAllocBase.h
@@ -128,6 +128,11 @@
   // exists, return the interfering register, which may be preg or an alias.
   unsigned checkPhysRegInterference(LiveInterval& lvr, unsigned preg);
 
+#ifndef NDEBUG
+  // Verify each LiveIntervalUnion.
+  void verify();
+#endif
+  
   // Helper that spills all live virtual registers currently unified under preg
   // that interfere with the most recently queried lvr.
   void spillInterferences(unsigned preg,
diff --git a/lib/CodeGen/RegAllocBasic.cpp b/lib/CodeGen/RegAllocBasic.cpp
index a2f9ea2..cd77d13 100644
--- a/lib/CodeGen/RegAllocBasic.cpp
+++ b/lib/CodeGen/RegAllocBasic.cpp
@@ -34,6 +34,9 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Target/TargetRegisterInfo.h"
+#ifndef NDEBUG
+#include "llvm/ADT/SparseBitVector.h"
+#endif
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
@@ -46,6 +49,19 @@
 static RegisterRegAlloc basicRegAlloc("basic", "basic register allocator",
                                       createBasicRegisterAllocator);
 
+// Temporary verification option until we can put verification inside
+// MachineVerifier.
+static cl::opt<bool>
+VerifyRegAlloc("verify-regalloc",
+               cl::desc("Verify live intervals before renaming"));
+
+class PhysicalRegisterDescription : public AbstractRegisterDescription {
+  const TargetRegisterInfo *tri_;
+public:
+  PhysicalRegisterDescription(const TargetRegisterInfo *tri): tri_(tri) {}
+  virtual const char *getName(unsigned reg) const { return tri_->getName(reg); }
+};
+
 namespace {
 
 /// RABasic provides a minimal implementation of the basic register allocation
@@ -153,6 +169,40 @@
   RegAllocBase::releaseMemory();
 }
 
+#ifndef NDEBUG
+// Verify each LiveIntervalUnion.
+void RegAllocBase::verify() {
+  LvrBitSet visitedVRegs;
+  OwningArrayPtr<LvrBitSet> unionVRegs(new LvrBitSet[physReg2liu_.numRegs()]);
+  // Verify disjoint unions.
+  for (unsigned preg = 0; preg < physReg2liu_.numRegs(); ++preg) {
+    DEBUG(PhysicalRegisterDescription prd(tri_); physReg2liu_[preg].dump(&prd));
+    LvrBitSet &vregs = unionVRegs[preg];
+    physReg2liu_[preg].verify(vregs);
+    // Union + intersection test could be done efficiently in one pass, but
+    // don't add a method to SparseBitVector unless we really need it.
+    assert(!visitedVRegs.intersects(vregs) && "vreg in multiple unions");
+    visitedVRegs |= vregs;
+  }
+  // Verify vreg coverage.
+  for (LiveIntervals::iterator liItr = lis_->begin(), liEnd = lis_->end();
+       liItr != liEnd; ++liItr) {
+    unsigned reg = liItr->first;
+    LiveInterval &li = *liItr->second;
+    if (li.empty() ) continue;
+    if (TargetRegisterInfo::isPhysicalRegister(reg)) continue;
+    if (!vrm_->hasPhys(reg)) continue; // spilled?
+    unsigned preg = vrm_->getPhys(reg);
+    if (!unionVRegs[preg].test(reg)) {
+      dbgs() << "LiveVirtReg " << reg << " not in union " <<
+        tri_->getName(preg) << "\n";
+      llvm_unreachable("unallocated live vreg");
+    }
+  }
+  // FIXME: I'm not sure how to verify spilled intervals.
+}
+#endif //!NDEBUG
+
 //===----------------------------------------------------------------------===//
 //                         RegAllocBase Implementation
 //===----------------------------------------------------------------------===//
@@ -222,6 +272,7 @@
        liItr != liEnd; ++liItr) {
     unsigned reg = liItr->first;
     LiveInterval &li = *liItr->second;
+    if (li.empty()) continue;
     if (TargetRegisterInfo::isPhysicalRegister(reg)) {
       physReg2liu_[reg].unify(li);
     }
@@ -243,13 +294,14 @@
     unsigned availablePhysReg = selectOrSplit(*lvr, splitLVRs);
     if (availablePhysReg) {
       DEBUG(dbgs() << "allocating: " << tri_->getName(availablePhysReg) <<
-            " " << lvr << '\n');
+            " " << *lvr << '\n');
       assert(!vrm_->hasPhys(lvr->reg) && "duplicate vreg in interval unions");
       vrm_->assignVirt2Phys(lvr->reg, availablePhysReg);
       physReg2liu_[availablePhysReg].unify(*lvr);
     }
     for (LVRVec::iterator lvrI = splitLVRs.begin(), lvrEnd = splitLVRs.end();
          lvrI != lvrEnd; ++lvrI) {
+      if ((*lvrI)->empty()) continue;
       DEBUG(dbgs() << "queuing new interval: " << **lvrI << "\n");
       assert(TargetRegisterInfo::isVirtualRegister((*lvrI)->reg) &&
              "expect split value in virtual register");
@@ -274,26 +326,32 @@
   return 0;
 }
 
-// Spill all live virtual registers currently unified under preg that interfere
-// with lvr.
+// Spill or split all live virtual registers currently unified under preg that
+// interfere with lvr. The newly spilled or split live intervals are returned by
+// appending them to splitLVRs.
 void RABasic::spillInterferences(unsigned preg,
                                  SmallVectorImpl<LiveInterval*> &splitLVRs) {
   SmallPtrSet<LiveInterval*, 8> spilledLVRs;
   LiveIntervalUnion::Query &query = queries_[preg];
+  // Record each interference before mutating either the union or live
+  // intervals.
   LiveIntervalUnion::InterferenceResult ir = query.firstInterference();
   assert(query.isInterference(ir) && "expect interference");
   do {
-    LiveInterval *lvr = ir.liuSegPos()->liveVirtReg;
-    if (!spilledLVRs.insert(lvr)) continue;
-    // Spill the previously allocated lvr.
-    SmallVector<LiveInterval*, 1> spillIs; // ignored
-    spiller_->spill(lvr, splitLVRs, spillIs);
+    spilledLVRs.insert(ir.liuSegPos()->liveVirtReg);
   } while (query.nextInterference(ir));
   for (SmallPtrSetIterator<LiveInterval*> lvrI = spilledLVRs.begin(),
          lvrEnd = spilledLVRs.end();
        lvrI != lvrEnd; ++lvrI ) {
+    LiveInterval& lvr = **lvrI;
+    // Spill the previously allocated lvr.
+    DEBUG(dbgs() << "extracting from " << preg << " " << lvr << '\n');
     // Deallocate the interfering lvr by removing it from the preg union.
-    physReg2liu_[preg].extract(**lvrI);
+    // Live intervals may not be in a union during modification.
+    physReg2liu_[preg].extract(lvr);
+    // Spill the extracted interval.
+    SmallVector<LiveInterval*, 8> spillIs;
+    spiller_->spill(&lvr, splitLVRs, spillIs);
   }
   // After extracting segments, the query's results are invalid.
   query.clear();
@@ -399,6 +457,24 @@
   // optional HTML output
   DEBUG(rmf_->renderMachineFunction("After basic register allocation.", vrm_));
 
+  // FIXME: Verification currently must run before VirtRegRewriter. We should
+  // make the rewriter a separate pass and override verifyAnalysis instead. When
+  // that happens, verification naturally falls under VerifyMachineCode.
+#ifndef NDEBUG
+  if (VerifyRegAlloc) {
+    // Verify accuracy of LiveIntervals. The standard machine code verifier
+    // ensures that each LiveIntervals covers all uses of the virtual reg.
+
+    // FIXME: MachineVerifier is currently broken when using the standard
+    // spiller. Enable it for InlineSpiller only.
+    // mf_->verify(this);
+    
+    // Verify that LiveIntervals are partitioned into unions and disjoint within
+    // the unions.
+    verify();
+  }
+#endif // !NDEBUG
+  
   // Run rewriter
   std::auto_ptr<VirtRegRewriter> rewriter(createVirtRegRewriter());
   rewriter->runOnMachineFunction(*mf_, *vrm_, lis_);