Index: llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
===================================================================
--- llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -2314,6 +2314,10 @@
   if (Reg == 0)
     return MatchOperand_NoMatch;
 
+  if (getSTI().hasFeature(AArch64::FeatureReserveX18) &&
+      (Reg == AArch64::X18 || Reg == AArch64::W18))
+    Error(Tok.getLoc(), "reserved register x18 is used");
+
   RegNum = Reg;
   Parser.Lex(); // Eat identifier token.
   return MatchOperand_Success;
Index: llvm/test/MC/AArch64/reserve-x18.s
===================================================================
--- /dev/null
+++ llvm/test/MC/AArch64/reserve-x18.s
@@ -0,0 +1,6 @@
+// RUN: echo "mov x18, 0" | llvm-mc --arch=arm64  | FileCheck %s --check-prefix=OK
+// RUN: echo "mov x18, 0" | not llvm-mc --arch=arm64 --mattr=reserve-x18 2>&1 | FileCheck %s --check-prefix=ERROR
+// RUN: echo "mov w18, 0" | not llvm-mc --arch=arm64 --mattr=reserve-x18 2>&1 | FileCheck %s --check-prefix=ERROR
+
+// OK: mov x18, #0 
+// ERROR: error: reserved register x18 is used