Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td	(revision 226573)
+++ include/clang/Driver/Options.td	(working copy)
@@ -1214,6 +1214,8 @@ def mfix_cortex_a53_835769 : Flag<["-"], "mfix-cor
 def mno_fix_cortex_a53_835769 : Flag<["-"], "mno-fix-cortex-a53-835769">,
   Group<m_aarch64_Features_Group>,
   HelpText<"Don't workaround Cortex-A53 erratum 835769 (AArch64 only)">;
+def ffixed_x18 : Flag<["-"], "ffixed-x18">, Group<m_aarch64_Features_Group>,
+  HelpText<"Reserve the x18 register (AArch64 only)">;
 
 def mvsx : Flag<["-"], "mvsx">, Group<m_ppc_Features_Group>;
 def mno_vsx : Flag<["-"], "mno-vsx">, Group<m_ppc_Features_Group>;
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp	(revision 226573)
+++ lib/Driver/Tools.cpp	(working copy)
@@ -958,6 +958,11 @@ void Clang::AddAArch64TargetArgs(const ArgList &Ar
     if (A->getOption().matches(options::OPT_mno_global_merge))
       CmdArgs.push_back("-mno-global-merge");
   }
+
+  if (Args.hasArg(options::OPT_ffixed_x18)) {
+    CmdArgs.push_back("-backend-option");
+    CmdArgs.push_back("-aarch64-reserve-x18");
+  }
 }
 
 // Get CPU and ABI names. They are not independent
Index: test/Driver/aarch64-fixed-x18.c
===================================================================
--- test/Driver/aarch64-fixed-x18.c	(revision 0)
+++ test/Driver/aarch64-fixed-x18.c	(working copy)
@@ -0,0 +1,4 @@
+// RUN: %clang -target aarch64-none-gnu -ffixed-x18 -### %s 2> %t
+// RUN: FileCheck --check-prefix=CHECK-FIXED-X18 < %t %s
+
+// CHECK-FIXED-X18: "-backend-option" "-aarch64-reserve-x18"