diff --git a/clang/include/clang/Basic/BuiltinsAArch64.def b/clang/include/clang/Basic/BuiltinsAArch64.def
--- a/clang/include/clang/Basic/BuiltinsAArch64.def
+++ b/clang/include/clang/Basic/BuiltinsAArch64.def
@@ -256,6 +256,11 @@
 TARGET_HEADER_BUILTIN(__readx18dword, "ULiULi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 TARGET_HEADER_BUILTIN(__readx18qword, "ULLiULi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
 
+TARGET_HEADER_BUILTIN(__writex18byte,  "vULiUc", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
+TARGET_HEADER_BUILTIN(__writex18word,  "vULiUs", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
+TARGET_HEADER_BUILTIN(__writex18dword, "vULiULi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
+TARGET_HEADER_BUILTIN(__writex18qword, "vULiULLi", "nh", "intrin.h", ALL_MS_LANGUAGES, "")
+
 #undef BUILTIN
 #undef LANGBUILTIN
 #undef TARGET_HEADER_BUILTIN
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -9976,6 +9976,31 @@
     return Load;
   }
 
+  if (BuiltinID == AArch64::BI__writex18byte ||
+      BuiltinID == AArch64::BI__writex18word ||
+      BuiltinID == AArch64::BI__writex18dword ||
+      BuiltinID == AArch64::BI__writex18qword) {
+    llvm::Type *IntTy = ConvertType(E->getArg(1)->getType());
+
+    // Read x18 as byte ptr, word ptr, dword ptr, or qword ptr
+    LLVMContext &Context = CGM.getLLVMContext();
+    llvm::Metadata *Ops[] = {llvm::MDString::get(Context, "x18")};
+    llvm::MDNode *RegName = llvm::MDNode::get(Context, Ops);
+    llvm::Value *Metadata = llvm::MetadataAsValue::get(Context, RegName);
+    llvm::Function *F =
+        CGM.getIntrinsic(llvm::Intrinsic::read_register, {Int64Ty});
+    llvm::Value *X18 = Builder.CreateCall(F, Metadata);
+    X18 = Builder.CreateIntToPtr(X18, llvm::PointerType::get(IntTy, 256));
+
+    // Store val at x18 + offset
+    Value *Offset = Builder.CreateZExt(EmitScalarExpr(E->getArg(0)), Int64Ty);
+    Value *Ptr = Builder.CreateGEP(IntTy, X18, Offset);
+    Value *Val = EmitScalarExpr(E->getArg(1));
+    StoreInst *Store = Builder.CreateAlignedStore(
+        Val, Ptr, getContext().getTypeAlignInChars(E->getType()));
+    return Store;
+  }
+
   // Handle MSVC intrinsics before argument evaluation to prevent double
   // evaluation.
   if (Optional<MSVCIntrin> MsvcIntId = translateAarch64ToMsvcIntrin(BuiltinID))
diff --git a/clang/lib/Headers/intrin.h b/clang/lib/Headers/intrin.h
--- a/clang/lib/Headers/intrin.h
+++ b/clang/lib/Headers/intrin.h
@@ -567,6 +567,11 @@
 unsigned short __readx18word(unsigned long offset);
 unsigned long __readx18dword(unsigned long offset);
 unsigned __int64 __readx18qword(unsigned long offset);
+
+void __writex18byte(unsigned long offset, unsigned char data);
+void __writex18word(unsigned long offset, unsigned short data);
+void __writex18dword(unsigned long offset, unsigned long data);
+void __writex18qword(unsigned long offset, unsigned __int64 data);
 #endif
 
 /*----------------------------------------------------------------------------*\
diff --git a/clang/test/CodeGen/arm64-microsoft-intrinsics.c b/clang/test/CodeGen/arm64-microsoft-intrinsics.c
--- a/clang/test/CodeGen/arm64-microsoft-intrinsics.c
+++ b/clang/test/CodeGen/arm64-microsoft-intrinsics.c
@@ -176,5 +176,69 @@
 // CHECK-MSVC: %[[RETVAL:.*]] = load i64, i64 addrspace(256)* %[[PTR]], align 8
 // CHECK-MSVC: ret i64 %[[RETVAL]]
 
+void check__writex18byte(unsigned long offset, unsigned char data) {
+  __writex18byte(offset, data);
+}
+
+// CHECK-MSVC: %[[DATA_ADDR:.*]] = alloca i8, align 1
+// CHECK-MSVC: %[[OFFSET_ADDR:.*]] = alloca i32, align 4
+// CHECK-MSVC: store i8 %data, i8* %[[DATA_ADDR]], align 1
+// CHECK-MSVC: store i32 %offset, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[X18:.*]] = call i64 @llvm.read_register.i64(metadata ![[MD2]])
+// CHECK-MSVC: %[[X18_AS_PTR:.*]] = inttoptr i64 %[[X18]] to i8 addrspace(256)*
+// CHECK-MSVC: %[[OFFSET:.*]] = load i32, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[ZEXT_OFFSET:.*]] = zext i32 %[[OFFSET]] to i64
+// CHECK-MSVC: %[[PTR:.*]] = getelementptr i8, i8 addrspace(256)* %[[X18_AS_PTR]], i64 %[[ZEXT_OFFSET]]
+// CHECK-MSVC: %[[DATA:.*]] = load i8, i8* %[[DATA_ADDR]], align 1
+// CHECK-MSVC: store i8 %[[DATA]], i8 addrspace(256)* %[[PTR]], align 1
+
+void check__writex18word(unsigned long offset, unsigned short data) {
+  __writex18word(offset, data);
+}
+
+// CHECK-MSVC: %[[DATA_ADDR:.*]] = alloca i16, align 2
+// CHECK-MSVC: %[[OFFSET_ADDR:.*]] = alloca i32, align 4
+// CHECK-MSVC: store i16 %data, i16* %[[DATA_ADDR]], align 2
+// CHECK-MSVC: store i32 %offset, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[X18:.*]] = call i64 @llvm.read_register.i64(metadata ![[MD2]])
+// CHECK-MSVC: %[[X18_AS_PTR:.*]] = inttoptr i64 %[[X18]] to i16 addrspace(256)*
+// CHECK-MSVC: %[[OFFSET:.*]] = load i32, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[ZEXT_OFFSET:.*]] = zext i32 %[[OFFSET]] to i64
+// CHECK-MSVC: %[[PTR:.*]] = getelementptr i16, i16 addrspace(256)* %[[X18_AS_PTR]], i64 %[[ZEXT_OFFSET]]
+// CHECK-MSVC: %[[DATA:.*]] = load i16, i16* %[[DATA_ADDR]], align 2
+// CHECK-MSVC: store i16 %[[DATA]], i16 addrspace(256)* %[[PTR]], align 1
+
+void check__writex18dword(unsigned long offset, unsigned long data) {
+  __writex18dword(offset, data);
+}
+
+// CHECK-MSVC: %[[DATA_ADDR:.*]] = alloca i32, align 4
+// CHECK-MSVC: %[[OFFSET_ADDR:.*]] = alloca i32, align 4
+// CHECK-MSVC: store i32 %data, i32* %[[DATA_ADDR]], align 4
+// CHECK-MSVC: store i32 %offset, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[X18:.*]] = call i64 @llvm.read_register.i64(metadata ![[MD2]])
+// CHECK-MSVC: %[[X18_AS_PTR:.*]] = inttoptr i64 %[[X18]] to i32 addrspace(256)*
+// CHECK-MSVC: %[[OFFSET:.*]] = load i32, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[ZEXT_OFFSET:.*]] = zext i32 %[[OFFSET]] to i64
+// CHECK-MSVC: %[[PTR:.*]] = getelementptr i32, i32 addrspace(256)* %[[X18_AS_PTR]], i64 %[[ZEXT_OFFSET]]
+// CHECK-MSVC: %[[DATA:.*]] = load i32, i32* %[[DATA_ADDR]], align 4
+// CHECK-MSVC: store i32 %[[DATA]], i32 addrspace(256)* %[[PTR]], align 1
+
+void check__writex18qword(unsigned long offset, unsigned __int64 data) {
+  __writex18qword(offset, data);
+}
+
+// CHECK-MSVC: %[[DATA_ADDR:.*]] = alloca i64, align 8
+// CHECK-MSVC: %[[OFFSET_ADDR:.*]] = alloca i32, align 4
+// CHECK-MSVC: store i64 %data, i64* %[[DATA_ADDR]], align 8
+// CHECK-MSVC: store i32 %offset, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[X18:.*]] = call i64 @llvm.read_register.i64(metadata ![[MD2]])
+// CHECK-MSVC: %[[X18_AS_PTR:.*]] = inttoptr i64 %[[X18]] to i64 addrspace(256)*
+// CHECK-MSVC: %[[OFFSET:.*]] = load i32, i32* %[[OFFSET_ADDR]], align 4
+// CHECK-MSVC: %[[ZEXT_OFFSET:.*]] = zext i32 %[[OFFSET]] to i64
+// CHECK-MSVC: %[[PTR:.*]] = getelementptr i64, i64 addrspace(256)* %[[X18_AS_PTR]], i64 %[[ZEXT_OFFSET]]
+// CHECK-MSVC: %[[DATA:.*]] = load i64, i64* %[[DATA_ADDR]], align 8
+// CHECK-MSVC: store i64 %[[DATA]], i64 addrspace(256)* %[[PTR]], align 1
+
 // CHECK-MSVC: ![[MD2]] = !{!"x18"}
 // CHECK-MSVC: ![[MD3]] = !{!"sp"}