Index: include/llvm/Support/Compiler.h
===================================================================
--- include/llvm/Support/Compiler.h	(revision 127522)
+++ include/llvm/Support/Compiler.h	(working copy)
@@ -126,4 +126,19 @@
   decl
 #endif
 
+// LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
+// to an expression which states that it is undefined behavior for the
+// compiler to reach this point.
+#if defined(__clang__) || (__GNUC__ > 4) \
+ || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
+# define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
+#else
+#ifdef __cplusplus
+extern "C" LLVM_ATTRIBUTE_NORETURN void abort();
+#else
+extern LLVM_ATTRIBUTE_NORETURN void abort();
+#endif
+# define LLVM_BUILTIN_UNREACHABLE abort()
+#endif
+
 #endif
Index: include/llvm/Support/ErrorHandling.h
===================================================================
--- include/llvm/Support/ErrorHandling.h	(revision 127522)
+++ include/llvm/Support/ErrorHandling.h	(working copy)
@@ -97,7 +97,7 @@ namespace llvm {
 #define llvm_unreachable(msg) \
   ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
 #else
-#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
 #endif
 
 #endif