Index: lib/CodeGen/GlobalISel/Legalizer.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Legalizer.cpp
+++ lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -173,10 +173,8 @@
       // and are assumed to be legal.
       if (!isPreISelGenericOpcode(MI->getOpcode()))
         continue;
-      unsigned NumNewInsns = 0;
       SmallVector<MachineInstr *, 4> WorkList;
       Helper.MIRBuilder.recordInsertions([&](MachineInstr *MI) {
-        ++NumNewInsns;
         WorkList.push_back(MI);
       });
       WorkList.push_back(&*MI);
@@ -185,30 +183,48 @@
       LegalizerHelper::LegalizeResult Res;
       unsigned Idx = 0;
       do {
-        Res = Helper.legalizeInstrStep(*WorkList[Idx]);
+        MachineInstr &CurrMI = *WorkList[Idx];
+#ifndef NDEBUG
+        // Record the instructions either side of the current instruction so we
+        // can iterate over its replacements (even after CurrMI has been erased).
+        // PrintFrom is set to MBB.end() if there is no instruction before CurrMI.
+        MachineBasicBlock::iterator PrintFrom = MBB.end();
+        if (CurrMI != MBB.begin()) {
+          PrintFrom = CurrMI;
+          --PrintFrom;
+        }
+        MachineBasicBlock::iterator PrintTo =
+            std::next(MachineBasicBlock::iterator(CurrMI));
+#endif // NDEBUG
+
+        Res = Helper.legalizeInstrStep(CurrMI);
         // Error out if we couldn't legalize this instruction. We may want to
         // fall back to DAG ISel instead in the future.
         if (Res == LegalizerHelper::UnableToLegalize) {
           Helper.MIRBuilder.stopRecordingInsertions();
           if (Res == LegalizerHelper::UnableToLegalize) {
             reportGISelFailure(MF, TPC, MORE, "gisel-legalize",
-                               "unable to legalize instruction",
-                               *WorkList[Idx]);
+                               "unable to legalize instruction", CurrMI);
             return false;
           }
         }
         Changed |= Res == LegalizerHelper::Legalized;
-        ++Idx;
 
 #ifndef NDEBUG
-        if (NumNewInsns) {
-          DEBUG(dbgs() << ".. .. Emitted " << NumNewInsns << " insns\n");
-          for (auto I = WorkList.end() - NumNewInsns, E = WorkList.end();
-               I != E; ++I)
-            DEBUG(dbgs() << ".. .. New MI: "; (*I)->print(dbgs()));
-          NumNewInsns = 0;
-        }
-#endif
+        // Now that the current instruction has been erased (if it was going to
+        // be) advancing PrintFrom will select it again if we didn't erase it,
+        // select PrintTo if we erased it but emitted zero instructions, or the
+        // first newly created instruction.
+        if (PrintFrom == MBB.end())
+          PrintFrom = MBB.begin();
+        else
+          ++PrintFrom;
+
+        for (auto I = PrintFrom, E = PrintTo; I != E; ++I)
+          DEBUG(dbgs() << ".. .. MI: "; I->print(dbgs()));
+#endif // NDEBUG
+
+        ++Idx;
       } while (Idx < WorkList.size());
 
       Helper.MIRBuilder.stopRecordingInsertions();