Index: lib/CodeGen/GlobalISel/InstructionSelect.cpp
===================================================================
--- lib/CodeGen/GlobalISel/InstructionSelect.cpp
+++ lib/CodeGen/GlobalISel/InstructionSelect.cpp
@@ -57,15 +57,39 @@
 #endif
 
   for (MachineBasicBlock *MBB : post_order(&MF)) {
-    for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),
-                                             End = MBB->rend();
-         MII != End;) {
-      MachineInstr &MI = *MII++;
-      DEBUG(dbgs() << "Selecting: " << MI << '\n');
+    if (MBB->empty())
+      continue;
+
+    // Select instructions in reverse block order. We permit erasing so have
+    // to resort to manually iterating and recognizing the begin (rend) case.
+    bool ReachedBegin = false;
+    for (auto MII = std::prev(MBB->end()), Begin = MBB->begin();
+         !ReachedBegin;) {
+      // Keep track of the insertion range for debug printing.
+      const auto AfterIt = std::next(MII);
+
+      // Select this instruction.
+      MachineInstr &MI = *MII;
+
+      // And have our iterator point to the next instruction, if there is one.
+      if (MII == Begin)
+        ReachedBegin = true;
+      else
+        --MII;
+
+      DEBUG(dbgs() << "Selecting: \n" << MI);
+
       if (!ISel->select(MI))
         reportSelectionError(MI, "Cannot select");
-      // FIXME: It would be nice to dump all inserted instructions.  It's not
-      // obvious how, esp. considering select() can insert after MI.
+
+      // Dump the range of instructions that MI expanded into.
+      DEBUG({
+        auto InsertedBegin = ReachedBegin ? MBB->begin() : std::next(MII);
+        dbgs() << "Into:\n";
+        for (auto &InsertedMI : make_range(InsertedBegin, AfterIt))
+          dbgs() << InsertedMI;
+        dbgs() << '\n';
+      });
     }
   }