Index: cfe/trunk/lib/Sema/SemaChecking.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaChecking.cpp
+++ cfe/trunk/lib/Sema/SemaChecking.cpp
@@ -929,6 +929,7 @@
     break;
   case Builtin::BI__va_start: {
     switch (Context.getTargetInfo().getTriple().getArch()) {
+    case llvm::Triple::aarch64:
     case llvm::Triple::arm:
     case llvm::Triple::thumb:
       if (SemaBuiltinVAStartARMMicrosoft(TheCall))
Index: cfe/trunk/test/SemaCXX/microsoft-varargs.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/microsoft-varargs.cpp
+++ cfe/trunk/test/SemaCXX/microsoft-varargs.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -triple thumbv7-windows -fms-compatibility -fsyntax-only %s -verify
+// RUN: %clang_cc1 -triple aarch64-windows -fms-compatibility -fsyntax-only %s -verify
 // expected-no-diagnostics
 
 extern "C" {