Index: lib/CodeGen/GlobalISel/Legalizer.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Legalizer.cpp
+++ lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -161,7 +161,7 @@
   // convergence for performance reasons.
   bool Changed = false;
   MachineBasicBlock::iterator NextMI;
-  for (auto &MBB : MF)
+  for (auto &MBB : MF) {
     for (auto MI = MBB.begin(); MI != MBB.end(); MI = NextMI) {
       // Get the next Instruction before we try to legalize, because there's a
       // good chance MI will be deleted.
@@ -171,18 +171,21 @@
       // and are assumed to be legal.
       if (!isPreISelGenericOpcode(MI->getOpcode()))
         continue;
+      SmallVector<MachineInstr *, 4> DebugReportList LLVM_ATTRIBUTE_UNUSED;
       SmallVector<MachineInstr *, 4> WorkList;
-      Helper.MIRBuilder.recordInsertions(
-          [&](MachineInstr *MI) { WorkList.push_back(MI); });
+      Helper.MIRBuilder.recordInsertions([&](MachineInstr *MI) {
+        DEBUG(DebugReportList.push_back(MI));
+        WorkList.push_back(MI);
+      });
       WorkList.push_back(&*MI);
 
+      bool Changed = false;
       LegalizerHelper::LegalizeResult Res;
       unsigned Idx = 0;
       do {
         Res = Helper.legalizeInstrStep(*WorkList[Idx]);
         // Error out if we couldn't legalize this instruction. We may want to
-        // fall
-        // back to DAG ISel instead in the future.
+        // fall back to DAG ISel instead in the future.
         if (Res == LegalizerHelper::UnableToLegalize) {
           Helper.MIRBuilder.stopRecordingInsertions();
           if (Res == LegalizerHelper::UnableToLegalize) {
@@ -194,10 +197,17 @@
         }
         Changed |= Res == LegalizerHelper::Legalized;
         ++Idx;
+
+#ifndef NDEBUG
+        for (const auto *MI : DebugReportList)
+          DEBUG(dbgs() << ".. .. New MI: "; MI->print(dbgs()));
+        DebugReportList.clear();
+#endif
       } while (Idx < WorkList.size());
 
       Helper.MIRBuilder.stopRecordingInsertions();
     }
+  }
 
   MachineRegisterInfo &MRI = MF.getRegInfo();
   const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
Index: lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -35,24 +35,34 @@
 
 LegalizerHelper::LegalizeResult
 LegalizerHelper::legalizeInstrStep(MachineInstr &MI) {
+  DEBUG(dbgs() << "Legalizing: "; MI.print(dbgs()));
+
   auto Action = LI.getAction(MI, MRI);
   switch (std::get<0>(Action)) {
   case LegalizerInfo::Legal:
+    DEBUG(dbgs() << ".. Already legal\n");
     return AlreadyLegal;
   case LegalizerInfo::Libcall:
+    DEBUG(dbgs() << ".. Convert to libcall\n");
     return libcall(MI);
   case LegalizerInfo::NarrowScalar:
+    DEBUG(dbgs() << ".. Narrow scalar\n");
     return narrowScalar(MI, std::get<1>(Action), std::get<2>(Action));
   case LegalizerInfo::WidenScalar:
+    DEBUG(dbgs() << ".. Widen scalar\n");
     return widenScalar(MI, std::get<1>(Action), std::get<2>(Action));
   case LegalizerInfo::Lower:
+    DEBUG(dbgs() << ".. Lower\n");
     return lower(MI, std::get<1>(Action), std::get<2>(Action));
   case LegalizerInfo::FewerElements:
+    DEBUG(dbgs() << ".. Reduce number of elements\n");
     return fewerElementsVector(MI, std::get<1>(Action), std::get<2>(Action));
   case LegalizerInfo::Custom:
+    DEBUG(dbgs() << ".. Custom legalization\n");
     return LI.legalizeCustom(MI, MRI, MIRBuilder) ? Legalized
                                                   : UnableToLegalize;
   default:
+    DEBUG(dbgs() << ".. Unable to legalize\n");
     return UnableToLegalize;
   }
 }