Index: llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h
@@ -120,6 +120,8 @@
 
   void setMF(MachineFunction &MF);
 
+  void verify();
+
   /// Records a newly created inst in a list and lazily insert it to the CSEMap.
   /// Sometimes, this method might be called with a partially constructed
   /// MachineInstr,
Index: llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h
+++ llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h
@@ -113,5 +113,16 @@
   ~RAIIDelegateInstaller();
 };
 
+/// A simple RAII based Observer installer.
+/// Use this in a scope to install the Observer to the MachineFunction and reset
+/// it at the end of the scope.
+class RAIIMFObserverInstaller {
+  MachineFunction &MF;
+
+public:
+  RAIIMFObserverInstaller(MachineFunction &MF, GISelChangeObserver &Observer);
+  ~RAIIMFObserverInstaller();
+};
+
 } // namespace llvm
 #endif
Index: llvm/include/llvm/CodeGen/MachineFunction.h
===================================================================
--- llvm/include/llvm/CodeGen/MachineFunction.h
+++ llvm/include/llvm/CodeGen/MachineFunction.h
@@ -71,6 +71,7 @@
 class TargetSubtargetInfo;
 struct WasmEHFuncInfo;
 struct WinEHFuncInfo;
+class GISelChangeObserver;
 
 template <> struct ilist_alloc_traits<MachineBasicBlock> {
   void deleteNode(MachineBasicBlock *MBB);
@@ -395,6 +396,7 @@
 
 private:
   Delegate *TheDelegate = nullptr;
+  GISelChangeObserver *Observer = nullptr;
 
   using CallSiteInfoMap = DenseMap<const MachineInstr *, CallSiteInfo>;
   /// Map a call instruction to call site arguments forwarding info.
@@ -439,6 +441,10 @@
     TheDelegate = delegate;
   }
 
+  void setObserver(GISelChangeObserver *O) { Observer = O; }
+
+  GISelChangeObserver *getObserver() const { return Observer; }
+
   MachineModuleInfo &getMMI() const { return MMI; }
   MCContext &getContext() const { return Ctx; }
 
Index: llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp
+++ llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp
@@ -255,6 +255,34 @@
 #endif
 }
 
+void GISelCSEInfo::verify() {
+#ifndef NDEBUG
+  handleRecordedInsts();
+  // For each instruction in map from MI -> UMI,
+  // Profile(MI) and make sure UMI is found for that profile.
+  for (auto &It : InstrMapping) {
+    FoldingSetNodeID TmpID;
+    GISelInstProfileBuilder(TmpID, *MRI).addNodeID(It.first);
+    void *InsertPos;
+    UniqueMachineInstr *FoundNode =
+        CSEMap.FindNodeOrInsertPos(TmpID, InsertPos);
+    if (FoundNode != It.second)
+      LLVM_DEBUG(dbgs() << "Instr mismatch :" << *It.first;);
+    assert(FoundNode == It.second && "CSEMap mismatch. InstrMapping has MIs "
+                                     "without corresponding Nodes in CSEMap");
+  }
+
+  // For every node in the CSEMap, make sure that the InstrMapping
+  // points to it.
+  for (auto It = CSEMap.begin(), End = CSEMap.end(); It != End; ++It) {
+    const UniqueMachineInstr &UMI = *It;
+    assert(InstrMapping.count(UMI.MI) && "Node in CSE without InstrMapping");
+    assert(InstrMapping[UMI.MI] == &UMI &&
+           "Node in CSE with incorrect InstrMapping");
+  }
+#endif
+}
+
 void GISelCSEInfo::print() {
   LLVM_DEBUG(for (auto &It
                   : OpcodeHitTable) {
@@ -364,6 +392,7 @@
 GISelCSEAnalysisWrapper::get(std::unique_ptr<CSEConfigBase> CSEOpt,
                              bool Recompute) {
   if (!AlreadyComputed || Recompute) {
+    Info.releaseMemory();
     Info.setCSEConfig(std::move(CSEOpt));
     Info.analyze(*MF);
     AlreadyComputed = true;
Index: llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
+++ llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
@@ -38,3 +38,11 @@
 }
 
 RAIIDelegateInstaller::~RAIIDelegateInstaller() { MF.resetDelegate(Delegate); }
+
+RAIIMFObserverInstaller::RAIIMFObserverInstaller(MachineFunction &MF,
+                                                 GISelChangeObserver &Observer)
+    : MF(MF) {
+  MF.setObserver(&Observer);
+}
+
+RAIIMFObserverInstaller::~RAIIMFObserverInstaller() { MF.setObserver(nullptr); }
Index: llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -2269,6 +2269,7 @@
     WrapperObserver.addObserver(&Verifier);
 #endif // ifndef NDEBUG
     RAIIDelegateInstaller DelInstall(*MF, &WrapperObserver);
+    RAIIMFObserverInstaller ObsInstall(*MF, WrapperObserver);
     for (const BasicBlock *BB : RPOT) {
       MachineBasicBlock &MBB = getMBB(*BB);
       // Set the insertion point of all the following translations to
Index: llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
+++ llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -199,6 +199,7 @@
   // Now install the observer as the delegate to MF.
   // This will keep all the observers notified about new insertions/deletions.
   RAIIDelegateInstaller DelInstall(MF, &WrapperObserver);
+  RAIIMFObserverInstaller ObsInstall(MF, WrapperObserver);
   LegalizerHelper Helper(MF, WrapperObserver, *MIRBuilder.get());
   const LegalizerInfo &LInfo(Helper.getLegalizerInfo());
   LegalizationArtifactCombiner ArtCombiner(*MIRBuilder.get(), MF.getRegInfo(),
@@ -294,6 +295,8 @@
     reportGISelFailure(MF, TPC, MORE, R);
     return false;
   }
+  if (CSEInfo)
+    CSEInfo->verify();
 
   return Changed;
 }
Index: llvm/lib/CodeGen/GlobalISel/Utils.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/Utils.cpp
+++ llvm/lib/CodeGen/GlobalISel/Utils.cpp
@@ -12,6 +12,7 @@
 #include "llvm/CodeGen/GlobalISel/Utils.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/CodeGen/GlobalISel/GISelChangeObserver.h"
 #include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
@@ -63,6 +64,15 @@
               TII.get(TargetOpcode::COPY), Reg)
           .addReg(ConstrainedReg);
     }
+  } else {
+    if (GISelChangeObserver *Observer = MF.getObserver()) {
+      if (!RegMO.isDef()) {
+        MachineInstr *RegDef = MRI.getVRegDef(Reg);
+        Observer->changedInstr(*RegDef);
+      }
+      Observer->changingAllUsesOfReg(MRI, Reg);
+      Observer->finishedChangingAllUsesOfReg();
+    }
   }
   return ConstrainedReg;
 }