Index: include/llvm/CodeGen/SlotIndexes.h
===================================================================
--- include/llvm/CodeGen/SlotIndexes.h
+++ include/llvm/CodeGen/SlotIndexes.h
@@ -122,10 +122,6 @@
       return lie.getPointer();
     }
 
-    unsigned getIndex() const {
-      return listEntry()->getIndex() | getSlot();
-    }
-
     /// Returns the slot for this SlotIndex.
     Slot getSlot() const {
       return static_cast<Slot>(lie.getInt());
@@ -147,6 +143,10 @@
              "Attempt to construct index with 0 pointer.");
     }
 
+    unsigned getIndex() const {
+      return listEntry()->getIndex() | getSlot();
+    }
+
     /// Returns true if this is a valid index. Invalid indices do
     /// not point into an index table, and cannot be compared.
     bool isValid() const {
Index: lib/CodeGen/LiveDebugVariables.cpp
===================================================================
--- lib/CodeGen/LiveDebugVariables.cpp
+++ lib/CodeGen/LiveDebugVariables.cpp
@@ -164,7 +164,7 @@
 
   /// Set of interval start indexes that have been trimmed to the
   /// lexical scope.
-  SmallSet<SlotIndex, 2> trimmedDefs;
+  SmallDenseMap<unsigned, SlotIndex, 2> trimmedDefs;
 
   /// insertDebugValue - Insert a DBG_VALUE into MBB at Idx for LocNo.
   void insertDebugValue(MachineBasicBlock *MBB, SlotIndex StartIdx,
@@ -812,10 +812,10 @@
       return;
 
     if (I.start() < RStart) {
+      // Remember that this interval was trimmed.
+      trimmedDefs.insert({RStart.getIndex(), I.start()});
       // Interval start overlaps range - trim to the scope range.
       I.setStartUnchecked(RStart);
-      // Remember that this interval was trimmed.
-      trimmedDefs.insert(RStart);
     }
 
     // The end of a lexical scope range is the last instruction in the
@@ -1210,8 +1210,9 @@
     // If the interval start was trimmed to the lexical scope insert the
     // DBG_VALUE at the previous index (otherwise it appears after the
     // first instruction in the range).
-    if (trimmedDefs.count(Start))
-      Start = Start.getPrevIndex();
+    auto trimmedDef = trimmedDefs.find(Start.getIndex());
+    if (trimmedDef != trimmedDefs.end())
+      Start = trimmedDef->second;
 
     DEBUG(dbgs() << "\t[" << Start << ';' << Stop << "):" << Loc.locNo());
     MachineFunction::iterator MBB = LIS.getMBBFromIndex(Start)->getIterator();