Index: llvm/trunk/lib/CodeGen/MIRParser/MIRParser.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MIRParser.cpp
+++ llvm/trunk/lib/CodeGen/MIRParser/MIRParser.cpp
@@ -156,6 +156,9 @@
   if (In.error())
     return true;
   auto FunctionName = MF->Name;
+  if (Functions.find(FunctionName) != Functions.end())
+    return error(Twine("redefinition of machine function '") + FunctionName +
+                 "'");
   Functions.insert(std::make_pair(FunctionName, std::move(MF)));
   return false;
 }
Index: llvm/trunk/test/CodeGen/MIR/machine-function-redefinition-error.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/machine-function-redefinition-error.mir
+++ llvm/trunk/test/CodeGen/MIR/machine-function-redefinition-error.mir
@@ -0,0 +1,10 @@
+# RUN: not llc -start-after branch-folder -stop-after branch-folder -o /dev/null %s 2>&1 | FileCheck %s
+# This test ensures that the machine function errors are reported correctly.
+
+---
+name:            foo
+...
+---
+# CHECK: redefinition of machine function 'foo'
+name:            foo
+...