Index: lib/Target/AArch64/AArch64.td
===================================================================
--- lib/Target/AArch64/AArch64.td
+++ lib/Target/AArch64/AArch64.td
@@ -40,6 +40,10 @@
 def FeatureZCZeroing : SubtargetFeature<"zcz", "HasZeroCycleZeroing", "true",
                                         "Has zero-cycle zeroing instructions">;
 
+def FeatureReserveX18 : SubtargetFeature<"reserve-x18", "ReserveX18", "true",
+                                         "Reserve X18, making it unavailable "
+                                         "as a GPR">;
+
 //===----------------------------------------------------------------------===//
 // Architectures.
 //
Index: lib/Target/AArch64/AArch64RegisterInfo.cpp
===================================================================
--- lib/Target/AArch64/AArch64RegisterInfo.cpp
+++ lib/Target/AArch64/AArch64RegisterInfo.cpp
@@ -34,10 +34,6 @@
 #define GET_REGINFO_TARGET_DESC
 #include "AArch64GenRegisterInfo.inc"
 
-static cl::opt<bool>
-ReserveX18("aarch64-reserve-x18", cl::Hidden,
-          cl::desc("Reserve X18, making it unavailable as GPR"));
-
 AArch64RegisterInfo::AArch64RegisterInfo(const Triple &TT)
     : AArch64GenRegisterInfo(AArch64::LR), TT(TT) {}
 
@@ -104,7 +100,7 @@
     Reserved.set(AArch64::W29);
   }
 
-  if (TT.isOSDarwin() || ReserveX18) {
+  if (TT.isOSDarwin() || MF.getSubtarget<AArch64Subtarget>().isX18Reserved()) {
     Reserved.set(AArch64::X18); // Platform register
     Reserved.set(AArch64::W18);
   }
@@ -131,7 +127,8 @@
     return true;
   case AArch64::X18:
   case AArch64::W18:
-    return TT.isOSDarwin() || ReserveX18;
+    return TT.isOSDarwin() ||
+           MF.getSubtarget<AArch64Subtarget>().isX18Reserved();
   case AArch64::FP:
   case AArch64::W29:
     return TFI->hasFP(MF) || TT.isOSDarwin();
@@ -402,9 +399,11 @@
   case AArch64::GPR32commonRegClassID:
   case AArch64::GPR64commonRegClassID:
     return 32 - 1                                // XZR/SP
-           - (TFI->hasFP(MF) || TT.isOSDarwin()) // FP
-           - (TT.isOSDarwin() || ReserveX18) // X18 reserved as platform register
-           - hasBasePointer(MF);           // X19
+      - (TFI->hasFP(MF) || TT.isOSDarwin()) // FP
+      - (TT.isOSDarwin() ||
+         MF.getSubtarget<AArch64Subtarget>()
+             .isX18Reserved()) // X18 reserved as platform register
+      - hasBasePointer(MF);    // X19
   case AArch64::FPR8RegClassID:
   case AArch64::FPR16RegClassID:
   case AArch64::FPR32RegClassID:
Index: lib/Target/AArch64/AArch64Subtarget.h
===================================================================
--- lib/Target/AArch64/AArch64Subtarget.h
+++ lib/Target/AArch64/AArch64Subtarget.h
@@ -51,6 +51,9 @@
   // HasZeroCycleZeroing - Has zero-cycle zeroing instructions.
   bool HasZeroCycleZeroing;
 
+  // ReserveX18 - X18 is not available as a general purpose register.
+  bool ReserveX18;
+
   bool IsLittle;
 
   /// CPUString - String name of used CPU.
@@ -101,6 +104,7 @@
 
   bool hasZeroCycleZeroing() const { return HasZeroCycleZeroing; }
 
+  bool isX18Reserved() const { return ReserveX18; }
   bool hasFPARMv8() const { return HasFPARMv8; }
   bool hasNEON() const { return HasNEON; }
   bool hasCrypto() const { return HasCrypto; }
Index: test/CodeGen/AArch64/arm64-platform-reg.ll
===================================================================
--- test/CodeGen/AArch64/arm64-platform-reg.ll
+++ test/CodeGen/AArch64/arm64-platform-reg.ll
@@ -1,5 +1,5 @@
 ; RUN: llc -mtriple=arm64-apple-ios -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE-X18
-; RUN: llc -mtriple=arm64-freebsd-gnu -aarch64-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
 
 ; x18 is reserved as a platform register on Darwin but not on other