Index: test/Index/print-mangled-name.cpp
===================================================================
--- test/Index/print-mangled-name.cpp
+++ test/Index/print-mangled-name.cpp
@@ -29,3 +29,8 @@
 // ITANIUM: mangled=_Z3foo1SRS_
 // MACHO: mangled=__Z3foo1SRS_
 // MICROSOFT: mangled=?foo@@YAHUS
+
+extern "C" int foo(int);
+// ITANIUM: mangled=
+// MACHO: mangled=
+// MICROSOFT: mangled=
Index: tools/libclang/CIndex.cpp
===================================================================
--- tools/libclang/CIndex.cpp
+++ tools/libclang/CIndex.cpp
@@ -3891,6 +3891,10 @@
   ASTContext &Ctx = ND->getASTContext();
   std::unique_ptr<MangleContext> MC(Ctx.createMangleContext());
 
+  // Don't mangle if we don't need to.
+  if (!MC->shouldMangleCXXName(ND))
+    return cxstring::createEmpty();
+
   std::string FrontendBuf;
   llvm::raw_string_ostream FrontendBufOS(FrontendBuf);
   MC->mangleName(ND, FrontendBufOS);