Index: lib/CodeGen/MIRParser/MIRParser.cpp
===================================================================
--- lib/CodeGen/MIRParser/MIRParser.cpp
+++ lib/CodeGen/MIRParser/MIRParser.cpp
@@ -173,6 +173,9 @@
   Functions.insert(std::make_pair(FunctionName, std::move(MF)));
   if (NoLLVMIR)
     createDummyFunction(FunctionName, M);
+  else if (!M.getFunction(FunctionName))
+    return error(Twine("function '") + FunctionName +
+                 "' isn't defined in the provided LLVM IR");
   return false;
 }
 
Index: test/CodeGen/MIR/machine-function-missing-function.mir
===================================================================
--- /dev/null
+++ test/CodeGen/MIR/machine-function-missing-function.mir
@@ -0,0 +1,19 @@
+# RUN: not llc -start-after branch-folder -stop-after branch-folder -o /dev/null %s 2>&1 | FileCheck %s
+# This test ensures that an error is reported when the mir file has LLVM IR and
+# one of the machine functions has a name that doesn't match any function in
+# the LLVM IR.
+
+--- |
+
+  define i32 @foo() {
+    ret i32 0
+  }
+
+...
+---
+name:            foo
+...
+---
+# CHECK: function 'faa' isn't defined in the provided LLVM IR
+name:            faa
+...