[AArch64] Support reserving x20 register

Register x20 is a callee-saved register which may be used for other
purposes in certain contexts, for example to hold special variables
within the kernel. This change adds support for reserving this register
both to frontend and backend to make this register usable for these
purposes.

Differential Revision: https://reviews.llvm.org/D46552

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@334531 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ClangCommandLineReference.rst

@@ -2296,6 +2296,10 @@ AARCH64
 
 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)

include/clang/Driver/Options.td

@@ -2004,6 +2004,8 @@ def mno_fix_cortex_a53_835769 : Flag<["-"], "mno-fix-cortex-a53-835769">,
   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>;

lib/Driver/ToolChains/Arch/AArch64.cpp

@@ -198,6 +198,9 @@ void aarch64::getAArch64TargetFeatures(const Driver &D, const ArgList &Args,
   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");
 }

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"