Index: docs/ClangCommandLineReference.rst
===================================================================
--- docs/ClangCommandLineReference.rst
+++ docs/ClangCommandLineReference.rst
@@ -2073,7 +2073,7 @@
 -------
 .. option:: -ffixed-x18
 
-Reserve the x18 register (AArch64 only)
+Reserve the x18 register (AArch64 only). It is on by default for AArch64 Windows target.
 
 .. option:: -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769
 
Index: lib/Basic/Targets.cpp
===================================================================
--- lib/Basic/Targets.cpp
+++ lib/Basic/Targets.cpp
@@ -6694,6 +6694,16 @@
   BuiltinVaListKind getBuiltinVaListKind() const override {
     return TargetInfo::CharPtrBuiltinVaList;
   }
+
+  bool handleTargetFeatures(std::vector<std::string> &Features,
+                            DiagnosticsEngine &Diags) override {
+    // Register x18 is reserved for AArch64. We make this the default for
+    // AArch64 Windows target.
+    WindowsTargetInfo<AArch64leTargetInfo>::handleTargetFeatures(Features,
+                                                                 Diags);
+    Features.push_back("+reserve-x18");
+    return true;
+  }
 };
 
 class AArch64beTargetInfo : public AArch64TargetInfo {
Index: test/Driver/coff-aarch64-fixed-x18.c
===================================================================
--- /dev/null
+++ test/Driver/coff-aarch64-fixed-x18.c
@@ -0,0 +1,5 @@
+// RUN: %clang -target aarch64-windows -emit-llvm -S %s -o - | FileCheck %s
+
+// CHECK: "target-features"{{.*}}+reserve-x18
+
+void foo() {}