Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -1741,6 +1741,25 @@
     Builder.CreateMemSet(Dest, ByteVal, SizeVal, false);
     return RValue::get(Dest.getPointer());
   }
+  case Builtin::BI__builtin_wmemcmp: {
+    // The MSVC runtime library does not provide a definition of wmemcmp, so we
+    // need to implement __builtin_wmemcmp using memcmp.
+    if (!getTarget().getTriple().isOSMSVCRT())
+      break;
+    llvm::Constant *Memcmp = CGM.CreateRuntimeFunction(
+        llvm::FunctionType::get(IntTy, {Int8PtrTy, Int8PtrTy, SizeTy},
+                                /*isVarArg=*/false),
+        "memcmp");
+    llvm::Value *Dst =
+        Builder.CreateBitOrPointerCast(EmitScalarExpr(E->getArg(0)), Int8PtrTy);
+    llvm::Value *Src =
+        Builder.CreateBitOrPointerCast(EmitScalarExpr(E->getArg(1)), Int8PtrTy);
+    llvm::Value *Size = Builder.CreateMul(
+        EmitScalarExpr(E->getArg(2)),
+        llvm::ConstantInt::get(
+            SizeTy, getContext().getTypeSize(getContext().WCharTy) / 8));
+    return RValue::get(Builder.CreateCall(Memcmp, {Dst, Src, Size}));
+  }
   case Builtin::BI__builtin_dwarf_cfa: {
     // The offset in bytes from the first argument to the CFA.
     //
Index: clang/test/CodeGen/wmemcmp.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/wmemcmp.c
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 %s -triple x86_64-pc-win32 -emit-llvm -o - | FileCheck %s
+
+typedef __SIZE_TYPE__ size_t;
+typedef __WCHAR_TYPE__ wchar_t;
+
+int wmemcmp_test(const wchar_t *s1, const wchar_t *s2, size_t n) {
+  // CHECK: [[S1:%.*]] = load
+  // CHECK: [[S1BC:%.*]] = bitcast i16* [[S1]] to i8*
+  // CHECK: [[S2:%.*]] = load
+  // CHECK: [[S2BC:%.*]] = bitcast i16* [[S2]] to i8*
+  // CHECK: [[N:%.*]] = load
+  // CHECK: [[N2:%.*]] = mul i64 [[N]], 2
+  // CHECK: [[RV:%.*]] = call i32 @memcmp(i8* [[S1BC]], i8* [[S2BC]], i64 [[N2]])
+  // CHECK: ret i32 [[RV]]
+  return __builtin_wmemcmp(s1, s2, n);
+}