Index: cfe/trunk/include/clang/Basic/BuiltinsAArch64.def
===================================================================
--- cfe/trunk/include/clang/Basic/BuiltinsAArch64.def
+++ cfe/trunk/include/clang/Basic/BuiltinsAArch64.def
@@ -94,6 +94,7 @@
 TARGET_HEADER_BUILTIN(_BitScanForward64, "UcUNi*ULLi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 TARGET_HEADER_BUILTIN(_BitScanReverse64, "UcUNi*ULLi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 
+TARGET_HEADER_BUILTIN(_InterlockedAdd,           "LiLiD*Li",    "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 TARGET_HEADER_BUILTIN(_InterlockedAnd64,         "LLiLLiD*LLi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 TARGET_HEADER_BUILTIN(_InterlockedDecrement64,   "LLiLLiD*",    "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 TARGET_HEADER_BUILTIN(_InterlockedExchange64,    "LLiLLiD*LLi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
Index: cfe/trunk/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGBuiltin.cpp
+++ cfe/trunk/lib/CodeGen/CGBuiltin.cpp
@@ -8517,6 +8517,15 @@
     return EmitMSVCBuiltinExpr(MSVCIntrin::_InterlockedDecrement, E);
   case AArch64::BI_InterlockedIncrement64:
     return EmitMSVCBuiltinExpr(MSVCIntrin::_InterlockedIncrement, E);
+
+  case AArch64::BI_InterlockedAdd: {
+    Value *Arg0 = EmitScalarExpr(E->getArg(0));
+    Value *Arg1 = EmitScalarExpr(E->getArg(1));
+    AtomicRMWInst *RMWI = Builder.CreateAtomicRMW(
+      AtomicRMWInst::Add, Arg0, Arg1,
+      llvm::AtomicOrdering::SequentiallyConsistent);
+    return Builder.CreateAdd(RMWI, Arg1);
+  }
   }
 }
 
Index: cfe/trunk/lib/Headers/intrin.h
===================================================================
--- cfe/trunk/lib/Headers/intrin.h
+++ cfe/trunk/lib/Headers/intrin.h
@@ -869,6 +869,7 @@
 \*----------------------------------------------------------------------------*/
 #if defined(__aarch64__)
 unsigned __int64 __getReg(int);
+long _InterlockedAdd(long volatile *Addend, long Value);
 #endif
 
 /*----------------------------------------------------------------------------*\
Index: cfe/trunk/test/CodeGen/arm64-microsoft-intrinsics.c
===================================================================
--- cfe/trunk/test/CodeGen/arm64-microsoft-intrinsics.c
+++ cfe/trunk/test/CodeGen/arm64-microsoft-intrinsics.c
@@ -4,6 +4,16 @@
 // RUN: not %clang_cc1 -triple arm64-linux -Werror -S -o /dev/null %s 2>&1 \
 // RUN:    | FileCheck %s -check-prefix CHECK-LINUX
 
+long test_InterlockedAdd(long volatile *Addend, long Value) {
+  return _InterlockedAdd(Addend, Value);
+}
+
+// CHECK-LABEL: define {{.*}} i32 @test_InterlockedAdd(i32* %Addend, i32 %Value) {{.*}} {
+// CHECK-MSVC: %[[OLDVAL:[0-9]+]] = atomicrmw add i32* %1, i32 %2 seq_cst
+// CHECK-MSVC: %[[NEWVAL:[0-9]+]] = add i32 %[[OLDVAL:[0-9]+]], %2
+// CHECK-MSVC: ret i32 %[[NEWVAL:[0-9]+]]
+// CHECK-LINUX: error: implicit declaration of function '_InterlockedAdd'
+
 void check__dmb(void) {
   __dmb(0);
 }