diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -274,6 +274,13 @@
 
     // Map of the preferred location for each value.
     DenseMap<ValueIDNum, LocIdx> ValueToLoc;
+
+    // Initialized the preferred-location map with illegal locations, to be
+    // filled in later.
+    for (auto &VLoc : VLocs)
+      if (VLoc.second.Kind == DbgValue::Def)
+        ValueToLoc.insert({VLoc.second.ID, LocIdx::MakeIllegalLoc()});
+
     ActiveMLocs.reserve(VLocs.size());
     ActiveVLocs.reserve(VLocs.size());
 
@@ -285,21 +292,20 @@
       ValueIDNum &VNum = MLocs[Idx.asU64()];
       VarLocs.push_back(VNum);
 
-      // Short-circuit unnecessary preferred location update.
-      if (VLocs.empty())
+      // Is there a variable that wants a location for this value? If not, skip.
+      auto VIt = ValueToLoc.find(VNum);
+      if (VIt == ValueToLoc.end())
         continue;
 
-      auto it = ValueToLoc.find(VNum);
+      LocIdx CurLoc = VIt->second;
       // In order of preference, pick:
       //  * Callee saved registers,
       //  * Other registers,
       //  * Spill slots.
-      if (it == ValueToLoc.end() || MTracker->isSpill(it->second) ||
-          (!isCalleeSaved(it->second) && isCalleeSaved(Idx.asU64()))) {
+      if (CurLoc.isIllegal() || MTracker->isSpill(CurLoc) ||
+          (!isCalleeSaved(CurLoc) && isCalleeSaved(Idx.asU64()))) {
         // Insert, or overwrite if insertion failed.
-        auto PrefLocRes = ValueToLoc.insert(std::make_pair(VNum, Idx));
-        if (!PrefLocRes.second)
-          PrefLocRes.first->second = Idx;
+        VIt->second = Idx;
       }
     }
 
@@ -314,7 +320,7 @@
       // If the value has no location, we can't make a variable location.
       const ValueIDNum &Num = Var.second.ID;
       auto ValuesPreferredLoc = ValueToLoc.find(Num);
-      if (ValuesPreferredLoc == ValueToLoc.end()) {
+      if (ValuesPreferredLoc->second.isIllegal()) {
         // If it's a def that occurs in this block, register it as a
         // use-before-def to be resolved as we step through the block.
         if (Num.getBlock() == (unsigned)MBB.getNumber() && !Num.isPHI())