Index: docs/ClangCommandLineReference.rst
===================================================================
--- docs/ClangCommandLineReference.rst
+++ docs/ClangCommandLineReference.rst
@@ -2296,6 +2296,10 @@
 
 Reserve the x18 register (AArch64 only)
 
+.. option:: -ffixed-x20
+
+Reserve the x20 register (AArch64 only)
+
 .. option:: -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769
 
 Workaround Cortex-A53 erratum 835769 (AArch64 only)
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -2004,6 +2004,8 @@
   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 ffixed_x20 : Flag<["-"], "ffixed-x20">, Group<m_aarch64_Features_Group>,
+  HelpText<"Reserve the x20 register (AArch64 only)">;
 
 def msimd128 : Flag<["-"], "msimd128">, Group<m_wasm_Features_Group>;
 def mno_simd128 : Flag<["-"], "mno-simd128">, Group<m_wasm_Features_Group>;
Index: lib/Driver/ToolChains/Arch/AArch64.cpp
===================================================================
--- lib/Driver/ToolChains/Arch/AArch64.cpp
+++ lib/Driver/ToolChains/Arch/AArch64.cpp
@@ -198,6 +198,9 @@
   if (Args.hasArg(options::OPT_ffixed_x18))
     Features.push_back("+reserve-x18");
 
+  if (Args.hasArg(options::OPT_ffixed_x20))
+    Features.push_back("+reserve-x20");
+
   if (Args.hasArg(options::OPT_mno_neg_immediates))
     Features.push_back("+no-neg-immediates");
 }
Index: test/Driver/aarch64-fixed-x20.c
===================================================================
--- test/Driver/aarch64-fixed-x20.c
+++ test/Driver/aarch64-fixed-x20.c
@@ -0,0 +1,4 @@
+// RUN: %clang -target aarch64-none-gnu -ffixed-x20 -### %s 2> %t
+// RUN: FileCheck --check-prefix=CHECK-FIXED-X20 < %t %s
+
+// CHECK-FIXED-X20: "-target-feature" "+reserve-x20"