Index: include/llvm/CodeGen/LiveIntervalAnalysis.h
===================================================================
--- include/llvm/CodeGen/LiveIntervalAnalysis.h
+++ include/llvm/CodeGen/LiveIntervalAnalysis.h
@@ -436,6 +436,10 @@
     /// have any segments or value numbers.
     void constructMainRangeFromSubranges(LiveInterval &LI);
 
+    /// Simple check to verify that segments/SlotIndices are valid in
+    /// LiveIntervals including SubRanges
+    bool verifySegments(void) const;
+
   private:
     /// Compute live intervals for all virtual registers.
     void computeVirtRegs();
Index: lib/CodeGen/LiveIntervalAnalysis.cpp
===================================================================
--- lib/CodeGen/LiveIntervalAnalysis.cpp
+++ lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -1596,3 +1596,59 @@
   LRCalc->reset(MF, getSlotIndexes(), DomTree, &getVNInfoAllocator());
   LRCalc->constructMainRangeFromSubranges(LI);
 }
+
+bool LiveIntervals::verifySegments(void) const {
+  // Simple verification that ensures that all the Intervals and SubRanges refer
+  // to valid slots.
+  // This is intended to be used for testing purposes - usually after
+  // encountering code that provokes a SubRange join assertion
+
+  bool Result = true;
+
+  for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
+    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
+    if (hasInterval(Reg)) {
+      const LiveInterval &Interval = getInterval(Reg);
+      for (const LiveInterval::Segment &S : Interval.segments) {
+        MachineInstr *MI = nullptr;
+        if (!S.start.isBlock()) {
+          MI = getInstructionFromIndex(S.start);
+          if (!MI) {
+            DEBUG(dbgs() << "verifySegments " << Interval << " : bad segment start = " << S << "\n");
+            Result = false;
+          }
+        }
+        if (!S.end.isBlock()) {
+          MI = getInstructionFromIndex(S.end);
+          if (!MI) {
+            DEBUG(dbgs() << "verifySegments " << Interval << " : bad segment end = " << S << "\n");
+            Result = false;
+          }
+        }
+      }
+
+      // Check SubRanges
+      for (const LiveInterval::SubRange &SR : Interval.subranges()) {
+        for (const LiveInterval::Segment &S : SR.segments) {
+          MachineInstr *MI = nullptr;
+          if (!S.start.isBlock()) {
+            MI = getInstructionFromIndex(S.start);
+            if (!MI) {
+              DEBUG(dbgs() << "verifySegments " << Interval << " : SubRange : " << SR << " : bad segment start = " << S << "\n");
+              Result = false;
+            }
+          }
+          if (!S.end.isBlock()) {
+            MI = getInstructionFromIndex(S.end);
+            if (!MI) {
+              DEBUG(dbgs() << "verifySegments " << Interval << " : SubRange : " << SR << " : bad segment end = " << S << "\n");
+              Result = false;
+            }
+          }
+        }
+      }
+    }
+  }
+  return Result;
+}
+
Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -95,6 +95,11 @@
          cl::desc("Verify machine instrs before and after register coalescing"),
          cl::Hidden);
 
+static cl::opt<bool>
+VerifyLiveIntervals("verify-live-intervals",
+         cl::desc("Verify LiveIntervals by inspecting all Segments for invalid slot indices"),
+         cl::Hidden);
+
 namespace {
 
   class RegisterCoalescer : public MachineFunctionPass,
@@ -3399,6 +3404,15 @@
     }
   }
 
+  // Verify that the Segments in the LiveIntervals are still valid
+  // This check looks for Segments within Intervals that start or end with
+  // invalid slot indices
+  // It is often more helpful for debug to insert this check after each joinCopy
+  // to pinpoint the precise cause of the error
+  if (VerifyLiveIntervals)
+    // Check that LiveIntervals are still valid after joinCopy
+    assert(LIS->verifySegments() && "joinCopy has resulted in invalid Segments in liveIntervals");
+
   DEBUG(dump());
   if (VerifyCoalescing)
     MF->verify(this, "After register coalescing");