Index: llvm/lib/CodeGen/MachineVerifier.cpp
===================================================================
--- llvm/lib/CodeGen/MachineVerifier.cpp
+++ llvm/lib/CodeGen/MachineVerifier.cpp
@@ -263,6 +263,7 @@
     void verifyStackFrame();
 
     void verifySlotIndexes() const;
+    void verifyDebugLocations(const MachineFunction &MF);
     void verifyProperties(const MachineFunction &MF);
   };
 
@@ -322,6 +323,40 @@
   }
 }
 
+void MachineVerifier::verifyDebugLocations(const MachineFunction &MF) {
+  // We're only guaranteed monotonicity at -O0. Optimizations
+  // may shuffle instructions around.
+  if (MF.getTarget().getOptLevel() == CodeGenOpt::None &&
+      MF.getTarget().Options.EnableGlobalISel)
+    return;
+  for (const MachineBasicBlock &MBB : MF) {
+    for (const MachineInstr &MI : MBB.instrs()) {
+      // Reset the line every time we enter a new MBB.
+      unsigned LastLine = 0;
+
+      // Debug instructions don't generate code, so we
+      // can skip them for verification's sake.
+      if (MI.isDebugInstr())
+        continue;
+
+      // Also skip instructions without a debug location
+      // associated.
+      auto DL = MI.getDebugLoc();
+      if (!DL)
+        continue;
+      unsigned CurLine = DL.getLine();
+
+      // If the line associated with this instruction isn't
+      // line zero, and it's smaller than the last one we visited,
+      // trigger an error.
+      if (CurLine != 0 && CurLine < LastLine)
+        report("Non-monotonically increasing debug location within "
+               "a single MachineBasicBlock", &MI);
+      LastLine = CurLine;
+    }
+  }
+}
+
 void MachineVerifier::verifyProperties(const MachineFunction &MF) {
   // If a pass has introduced virtual registers without clearing the
   // NoVRegs property (or set it without allocating the vregs)
@@ -372,6 +407,8 @@
 
   verifyProperties(MF);
 
+  verifyDebugLocations(MF);
+
   visitMachineFunctionBefore();
   for (const MachineBasicBlock &MBB : MF) {
     visitMachineBasicBlockBefore(&MBB);