Index: cfe/trunk/lib/CodeGen/TargetInfo.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/TargetInfo.cpp
+++ cfe/trunk/lib/CodeGen/TargetInfo.cpp
@@ -4785,7 +4785,8 @@
 public:
   enum ABIKind {
     AAPCS = 0,
-    DarwinPCS
+    DarwinPCS,
+    Win64
   };
 
 private:
@@ -4823,10 +4824,14 @@
 
   Address EmitVAArg(CodeGenFunction &CGF, Address VAListAddr,
                     QualType Ty) const override {
-    return isDarwinPCS() ? EmitDarwinVAArg(VAListAddr, Ty, CGF)
-                         : EmitAAPCSVAArg(VAListAddr, Ty, CGF);
+    return Kind == Win64 ? EmitMSVAArg(CGF, VAListAddr, Ty)
+                         : isDarwinPCS() ? EmitDarwinVAArg(VAListAddr, Ty, CGF)
+                                         : EmitAAPCSVAArg(VAListAddr, Ty, CGF);
   }
 
+  Address EmitMSVAArg(CodeGenFunction &CGF, Address VAListAddr,
+                      QualType Ty) const override;
+
   bool shouldPassIndirectlyForSwift(CharUnits totalSize,
                                     ArrayRef<llvm::Type*> scalars,
                                     bool asReturnValue) const override {
@@ -5332,6 +5337,14 @@
                           TyInfo, SlotSize, /*AllowHigherAlign*/ true);
 }
 
+Address AArch64ABIInfo::EmitMSVAArg(CodeGenFunction &CGF, Address VAListAddr,
+                                    QualType Ty) const {
+  return emitVoidPtrVAArg(CGF, VAListAddr, Ty, /*indirect*/ false,
+                          CGF.getContext().getTypeInfoInChars(Ty),
+                          CharUnits::fromQuantity(8),
+                          /*allowHigherAlign*/ false);
+}
+
 //===----------------------------------------------------------------------===//
 // ARM ABI Implementation
 //===----------------------------------------------------------------------===//
@@ -8494,6 +8507,8 @@
     AArch64ABIInfo::ABIKind Kind = AArch64ABIInfo::AAPCS;
     if (getTarget().getABI() == "darwinpcs")
       Kind = AArch64ABIInfo::DarwinPCS;
+    else if (Triple.isOSWindows())
+      Kind = AArch64ABIInfo::Win64;
 
     return SetCGInfo(new AArch64TargetCodeGenInfo(Types, Kind));
   }
Index: cfe/trunk/test/CodeGen/aarch64-varargs-ms.c
===================================================================
--- cfe/trunk/test/CodeGen/aarch64-varargs-ms.c
+++ cfe/trunk/test/CodeGen/aarch64-varargs-ms.c
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -triple arm64-windows-msvc -emit-llvm -o - %s | FileCheck --check-prefix=CHECK %s
+
+#include <stdarg.h>
+
+int simple_int(va_list ap) {
+// CHECK-LABEL: define i32 @simple_int
+  return va_arg(ap, int);
+// CHECK: [[ADDR:%[a-z_0-9]+]] = bitcast i8* %argp.cur to i32*
+// CHECK: [[RESULT:%[a-z_0-9]+]] = load i32, i32* [[ADDR]]
+// CHECK: ret i32 [[RESULT]]
+}