Index: llvm/trunk/lib/Demangle/Demangle.cpp
===================================================================
--- llvm/trunk/lib/Demangle/Demangle.cpp
+++ llvm/trunk/lib/Demangle/Demangle.cpp
@@ -13,9 +13,15 @@
 
 #include "llvm/Demangle/Demangle.h"
 
+static bool isItaniumEncoding(const std::string &MangledName) {
+  size_t Pos = MangledName.find_first_not_of('_');
+  // A valid Itanium encoding requires 1-4 leading underscores, followed by 'Z'.
+  return Pos > 0 && Pos <= 4 && MangledName[Pos] == 'Z';
+}
+
 std::string llvm::demangle(const std::string &MangledName) {
   char *Demangled;
-  if (MangledName.compare(0, 2, "_Z") == 0)
+  if (isItaniumEncoding(MangledName))
     Demangled = itaniumDemangle(MangledName.c_str(), nullptr, nullptr, nullptr);
   else
     Demangled =
Index: llvm/trunk/unittests/Demangle/DemangleTest.cpp
===================================================================
--- llvm/trunk/unittests/Demangle/DemangleTest.cpp
+++ llvm/trunk/unittests/Demangle/DemangleTest.cpp
@@ -13,7 +13,13 @@
 using namespace llvm;
 
 TEST(Demangle, demangleTest) {
+  EXPECT_EQ(demangle("_"), "_");
   EXPECT_EQ(demangle("_Z3fooi"), "foo(int)");
+  EXPECT_EQ(demangle("__Z3fooi"), "foo(int)");
+  EXPECT_EQ(demangle("___Z3fooi_block_invoke"),
+            "invocation function for block in foo(int)");
+  EXPECT_EQ(demangle("____Z3fooi_block_invoke"),
+            "invocation function for block in foo(int)");
   EXPECT_EQ(demangle("?foo@@YAXH@Z"), "void __cdecl foo(int)");
   EXPECT_EQ(demangle("foo"), "foo");
 }