diff --git a/llvm/lib/CodeGen/MachineDebugify.cpp b/llvm/lib/CodeGen/MachineDebugify.cpp
--- a/llvm/lib/CodeGen/MachineDebugify.cpp
+++ b/llvm/lib/CodeGen/MachineDebugify.cpp
@@ -153,10 +153,15 @@
       NMD->setOperand(Idx, MDNode::get(Ctx, ValueAsMetadata::getConstant(
                                                 ConstantInt::get(Int32Ty, N))));
     };
+    auto getDebugifyOperand = [&](unsigned Idx) {
+      return mdconst::extract<ConstantInt>(NMD->getOperand(Idx)->getOperand(0))
+          ->getZExtValue();
+    };
     // Set number of lines.
     setDebugifyOperand(0, NextLine - 1);
     // Set number of variables.
-    setDebugifyOperand(1, VarSet.size());
+    auto OldNumVars = getDebugifyOperand(1);
+    setDebugifyOperand(1, OldNumVars + VarSet.size());
   }
 
   return true;
@@ -166,6 +171,9 @@
 /// legacy module pass manager.
 struct DebugifyMachineModule : public ModulePass {
   bool runOnModule(Module &M) override {
+    // We will insert new debugify metadata, so erasing the old one.
+    assert(!M.getNamedMetadata("llvm.mir.debugify") &&
+           "llvm.mir.debugify metadata already exists! Strip it first");
     MachineModuleInfo &MMI =
         getAnalysis<MachineModuleInfoWrapperPass>().getMMI();
     return applyDebugifyMetadata(
diff --git a/llvm/test/CodeGen/Generic/MIRDebugify/multifunction-module.mir b/llvm/test/CodeGen/Generic/MIRDebugify/multifunction-module.mir
--- a/llvm/test/CodeGen/Generic/MIRDebugify/multifunction-module.mir
+++ b/llvm/test/CodeGen/Generic/MIRDebugify/multifunction-module.mir
@@ -1,6 +1,7 @@
-# RUN: llc -run-pass=mir-debugify,mir-check-debugify %s
+# RUN: llc -run-pass=mir-debugify,mir-check-debugify -o - %s 2>&1 | FileCheck %s
 
-# XFAIL: *
+# CHECK: Machine IR debug info check: PASS
+# CHECK-NOT: Assertion `Var <= NumVars && "Unexpected name for DILocalVariable"'
 
 --- |
   define i32 @foo(i32 %a0, i32 %a1, i32 %a2, i32 %a3) {