Index: lib/Target/AArch64/AArch64Subtarget.cpp
===================================================================
--- lib/Target/AArch64/AArch64Subtarget.cpp
+++ lib/Target/AArch64/AArch64Subtarget.cpp
@@ -171,7 +171,9 @@
 AArch64Subtarget::AArch64Subtarget(const Triple &TT, const std::string &CPU,
                                    const std::string &FS,
                                    const TargetMachine &TM, bool LittleEndian)
-    : AArch64GenSubtargetInfo(TT, CPU, FS), ReserveX18(TT.isOSDarwin()),
+    : AArch64GenSubtargetInfo(TT, CPU, FS),
+      ReserveX18(TT.isOSDarwin() ||
+                (TT.isOSWindows() && TT.isOSBinFormatCOFF())),
       IsLittle(LittleEndian), TargetTriple(TT), FrameLowering(),
       InstrInfo(initializeSubtargetDependencies(FS, CPU)), TSInfo(),
       TLInfo(TM, *this), GISel() {
Index: test/CodeGen/AArch64/arm64-platform-reg.ll
===================================================================
--- test/CodeGen/AArch64/arm64-platform-reg.ll
+++ test/CodeGen/AArch64/arm64-platform-reg.ll
@@ -1,6 +1,7 @@
 ; RUN: llc -mtriple=arm64-apple-ios -mattr=+reserve-x18 -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE-X18
 ; RUN: llc -mtriple=arm64-freebsd-gnu -mattr=+reserve-x18 -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE-X18
 ; RUN: llc -mtriple=arm64-linux-gnu -o - %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-windows -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE-X18-COFF
 
 ; x18 is reserved as a platform register on Darwin but not on other
 ; systems. Create loads of register pressure and make sure this is respected.
@@ -23,5 +24,8 @@
 ; CHECK-RESERVE-X18-NOT: ldr fp
 ; CHECK-RESERVE-X18-NOT: ldr x18
 ; CHECK-RESERVE-X18: ret
+
+; CHECK-RESERVE-X18-COFF-NOT: ldr x18
+; CHECK-RESERVE-X18-COFF-NOT: str x18
   ret void
 }