diff --git a/llvm/include/llvm/CodeGen/LiveIntervalUnion.h b/llvm/include/llvm/CodeGen/LiveIntervalUnion.h
--- a/llvm/include/llvm/CodeGen/LiveIntervalUnion.h
+++ b/llvm/include/llvm/CodeGen/LiveIntervalUnion.h
@@ -114,7 +114,7 @@
     const LiveRange *LR = nullptr;
     LiveRange::const_iterator LRI;  ///< current position in LR
     ConstSegmentIter LiveUnionI;    ///< current position in LiveUnion
-    SmallVector<LiveInterval*,4> InterferingVRegs;
+    Optional<SmallVector<LiveInterval *, 4>> InterferingVRegs;
     bool CheckedFirstInterference = false;
     bool SeenAllInterferences = false;
     unsigned Tag = 0;
@@ -124,7 +124,7 @@
                const LiveIntervalUnion &NewLiveUnion) {
       LiveUnion = &NewLiveUnion;
       LR = &NewLR;
-      InterferingVRegs.clear();
+      InterferingVRegs = None;
       CheckedFirstInterference = false;
       SeenAllInterferences = false;
       Tag = NewLiveUnion.getTag();
@@ -164,7 +164,7 @@
 
     // Vector generated by collectInterferingVRegs.
     const SmallVectorImpl<LiveInterval*> &interferingVRegs() const {
-      return InterferingVRegs;
+      return *InterferingVRegs;
     }
   };
 
diff --git a/llvm/lib/CodeGen/LiveIntervalUnion.cpp b/llvm/lib/CodeGen/LiveIntervalUnion.cpp
--- a/llvm/lib/CodeGen/LiveIntervalUnion.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalUnion.cpp
@@ -112,7 +112,7 @@
 // Scan the vector of interfering virtual registers in this union. Assume it's
 // quite small.
 bool LiveIntervalUnion::Query::isSeenInterference(LiveInterval *VirtReg) const {
-  return is_contained(InterferingVRegs, VirtReg);
+  return is_contained(*InterferingVRegs, VirtReg);
 }
 
 // Collect virtual registers in this union that interfere with this
@@ -126,9 +126,12 @@
 //
 unsigned LiveIntervalUnion::Query::
 collectInterferingVRegs(unsigned MaxInterferingRegs) {
+  if (!InterferingVRegs)
+    InterferingVRegs.emplace();
+
   // Fast path return if we already have the desired information.
-  if (SeenAllInterferences || InterferingVRegs.size() >= MaxInterferingRegs)
-    return InterferingVRegs.size();
+  if (SeenAllInterferences || InterferingVRegs->size() >= MaxInterferingRegs)
+    return InterferingVRegs->size();
 
   // Set up iterators on the first call.
   if (!CheckedFirstInterference) {
@@ -157,14 +160,14 @@
       LiveInterval *VReg = LiveUnionI.value();
       if (VReg != RecentReg && !isSeenInterference(VReg)) {
         RecentReg = VReg;
-        InterferingVRegs.push_back(VReg);
-        if (InterferingVRegs.size() >= MaxInterferingRegs)
-          return InterferingVRegs.size();
+        InterferingVRegs->push_back(VReg);
+        if (InterferingVRegs->size() >= MaxInterferingRegs)
+          return InterferingVRegs->size();
       }
       // This LiveUnion segment is no longer interesting.
       if (!(++LiveUnionI).valid()) {
         SeenAllInterferences = true;
-        return InterferingVRegs.size();
+        return InterferingVRegs->size();
       }
     }
 
@@ -185,7 +188,7 @@
     LiveUnionI.advanceTo(LRI->start);
   }
   SeenAllInterferences = true;
-  return InterferingVRegs.size();
+  return InterferingVRegs->size();
 }
 
 void LiveIntervalUnion::Array::init(LiveIntervalUnion::Allocator &Alloc,
diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -987,7 +987,7 @@
 
   for (MCRegUnitIterator Units(PhysReg, TRI); Units.isValid(); ++Units) {
     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
-
+    Q.collectInterferingVRegs();
     // Check if any interfering live range is heavier than MaxWeight.
     for (const LiveInterval *Intf : reverse(Q.interferingVRegs())) {
       // Check if interference overlast the segment in interest.