Index: include/llvm/CodeGen/GlobalISel/GISelWorkList.h
===================================================================
--- include/llvm/CodeGen/GlobalISel/GISelWorkList.h
+++ include/llvm/CodeGen/GlobalISel/GISelWorkList.h
@@ -32,6 +32,10 @@
   SmallVector<MachineInstr *, N> Worklist;
   DenseMap<MachineInstr *, unsigned> WorklistMap;
 
+#ifndef NDEBUG
+  bool Finalized = true;
+#endif
+
 public:
   GISelWorkList() : WorklistMap(N) {}
 
@@ -39,16 +43,50 @@
 
   unsigned size() const { return WorklistMap.size(); }
 
+  // Since we don't know ahead of time how many instructions we're going to add
+  // to the worklist, and migrating densemap's elements is quite expensive
+  // everytime we resize, only insert to the smallvector (typically during the
+  // initial phase of populating lists). Before the worklist can be used,
+  // finalize should be called. Also assert with NDEBUG if list is ever used
+  // without finalizing. Note that unlike insert, we won't check for duplicates
+  // - so the ideal place to use this is during the initial prepopulating phase
+  // of most passes.
+  void deferred_insert(MachineInstr *I) {
+    Worklist.push_back(I);
+#ifndef NDEBUG
+    Finalized = false;
+#endif
+  }
+
+  // This should only be called when using deferred_insert.
+  // This asserts that the WorklistMap is empty, and then
+  // inserts all the elements in the Worklist into the map.
+  // It also asserts if there are any duplicate elements found.
+  void finalize() {
+    assert(WorklistMap.empty() && "Expecting empty worklistmap");
+    if (Worklist.size() > N)
+      WorklistMap.reserve(Worklist.size());
+    for (unsigned i = 0; i < Worklist.size(); ++i)
+      if (!WorklistMap.try_emplace(Worklist[i], i).second)
+        llvm_unreachable("Duplicate elements in the list");
+#ifndef NDEBUG
+    Finalized = true;
+#endif
+  }
+
   /// Add the specified instruction to the worklist if it isn't already in it.
   void insert(MachineInstr *I) {
+    assert(Finalized && "GISelWorkList used without finalizing");
     if (WorklistMap.try_emplace(I, Worklist.size()).second)
       Worklist.push_back(I);
   }
 
   /// Remove I from the worklist if it exists.
   void remove(const MachineInstr *I) {
+    assert((Finalized || WorklistMap.empty()) && "Neither finalized nor empty");
     auto It = WorklistMap.find(I);
-    if (It == WorklistMap.end()) return; // Not in worklist.
+    if (It == WorklistMap.end())
+      return; // Not in worklist.
 
     // Don't bother moving everything down, just null out the slot.
     Worklist[It->second] = nullptr;
@@ -62,6 +100,7 @@
   }
 
   MachineInstr *pop_back_val() {
+    assert(Finalized && "GISelWorkList used without finalizing");
     MachineInstr *I;
     do {
       I = Worklist.pop_back_val();
Index: lib/CodeGen/GlobalISel/Combiner.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Combiner.cpp
+++ lib/CodeGen/GlobalISel/Combiner.cpp
@@ -129,9 +129,10 @@
           CurMI->eraseFromParentAndMarkDBGValuesForRemoval();
           continue;
         }
-        WorkList.insert(CurMI);
+        WorkList.deferred_insert(CurMI);
       }
     }
+    WorkList.finalize();
     // Main Loop. Process the instructions here.
     while (!WorkList.empty()) {
       MachineInstr *CurrInst = WorkList.pop_back_val();
Index: lib/CodeGen/GlobalISel/Legalizer.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Legalizer.cpp
+++ lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -155,11 +155,13 @@
       if (!isPreISelGenericOpcode(MI.getOpcode()))
         continue;
       if (isArtifact(MI))
-        ArtifactList.insert(&MI);
+        ArtifactList.deferred_insert(&MI);
       else
-        InstList.insert(&MI);
+        InstList.deferred_insert(&MI);
     }
   }
+  ArtifactList.finalize();
+  InstList.finalize();
   std::unique_ptr<MachineIRBuilder> MIRBuilder;
   GISelCSEInfo *CSEInfo = nullptr;
   bool EnableCSE = EnableCSEInLegalizer.getNumOccurrences()