Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -346,16 +346,20 @@
 }
 
 static bool isKnownZFlag(StringRef S) {
-  return S == "combreloc" || S == "copyreloc" || S == "defs" ||
-         S == "execstack" || S == "global" || S == "hazardplt" ||
-         S == "initfirst" || S == "interpose" ||
-         S == "keep-text-section-prefix" || S == "lazy" || S == "muldefs" ||
-         S == "nocombreloc" || S == "nocopyreloc" || S == "nodelete" ||
-         S == "nodlopen" || S == "noexecstack" ||
-         S == "nokeep-text-section-prefix" || S == "norelro" || S == "notext" ||
-         S == "now" || S == "origin" || S == "relro" || S == "retpolineplt" ||
-         S == "rodynamic" || S == "text" || S == "wxneeded" ||
-         S.startswith("max-page-size=") || S.startswith("stack-size=");
+  static llvm::SmallDenseSet<StringRef> KnownFlags {
+    "combreloc", "copyreloc", "defs",
+    "execstack", "global", "hazardplt",
+    "initfirst", "interpose", "keep-text-section-prefix",
+    "lazy", "muldefs", "nocombreloc",
+    "nocopyreloc", "nodelete", "nodlopen", 
+    "noexecstack", "nokeep-text-section-prefix", "norelro", 
+    "notext", "now", "origin",
+    "relro", "retpolineplt", "rodynamic",
+    "text", "wxneeded"
+  };
+   
+  return KnownFlags.count(S) || S.startswith("max-page-size=") ||
+         S.startswith("stack-size=");
 }
 
 // Report an error for an unknown -z option.
Index: test/ELF/invalid/zunknown-flag.test
===================================================================
--- test/ELF/invalid/zunknown-flag.test
+++ test/ELF/invalid/zunknown-flag.test
@@ -0,0 +1,3 @@
+# RUN: not ld.lld -z obviouslyunknownflag 2>&1 | FileCheck --check-prefix=ERR %s
+# ERR: error: unknown -z value: obviouslyunknownflag
+