diff --git a/llvm/lib/TargetParser/AArch64TargetParser.cpp b/llvm/lib/TargetParser/AArch64TargetParser.cpp
--- a/llvm/lib/TargetParser/AArch64TargetParser.cpp
+++ b/llvm/lib/TargetParser/AArch64TargetParser.cpp
@@ -97,7 +97,7 @@
 
 bool AArch64::isX18ReservedByDefault(const Triple &TT) {
   return TT.isAndroid() || TT.isOSDarwin() || TT.isOSFuchsia() ||
-         TT.isOSWindows() || TT.isOHOSFamily();
+         TT.isOSWindows() || TT.isOHOSFamily() || TT.isOSUnknown();
 }
 
 // Allows partial match, ex. "v8a" matches "armv8a".