[COFF, ARM64] Make +reserve-x18 the default

[mstorsjo]

Wouldn't it make more sense to set this by default within LLVM, where it's already enabled by default for darwin?

Currently there's this code there:

AArch64Subtarget::AArch64Subtarget(const Triple &TT, const std::string &CPU,
                                   const std::string &FS,
                                   const TargetMachine &TM, bool LittleEndian)
    : AArch64GenSubtargetInfo(TT, CPU, FS), ReserveX18(TT.isOSDarwin()),

[mgrang]

In D35529#812427, @mstorsjo wrote:

Wouldn't it make more sense to set this by default within LLVM, where it's already enabled by default for darwin?

Currently there's this code there:

AArch64Subtarget::AArch64Subtarget(const Triple &TT, const std::string &CPU,
                                   const std::string &FS,
                                   const TargetMachine &TM, bool LittleEndian)
    : AArch64GenSubtargetInfo(TT, CPU, FS), ReserveX18(TT.isOSDarwin()),

Thanks for pointing this out. This certainly seems a cleaner way/place to do this. Lemme give it a shot.

[mgrang]

I pushed a new patch D35531. This implements the logic in llvm AArch64SubTarget which seems to be the proper place to do this.
Once that patch is reviewed/accepted, I will abandon this one.

docs/ClangCommandLineReference.rst

 .. option:: -mx32
 
 .. option:: -mxgot, -mno-xgot
 
 AARCH64
 -------
 .. option:: -ffixed-x18
 
-Reserve the x18 register (AArch64 only)
+Reserve the x18 register (AArch64 only). It is on by default for AArch64 Windows target.
 
 .. option:: -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769
 
 Workaround Cortex-A53 erratum 835769 (AArch64 only)
 
 .. option:: -mgeneral-regs-only
 
 Generate code which only uses the general purpose registers (AArch64 only)

lib/Basic/Targets.cpp

void getTargetDefines(const LangOptions &Opts,
                         MacroBuilder &Builder) const override {
     WindowsTargetInfo::getTargetDefines(Opts, Builder);
     getVisualStudioDefines(Opts, Builder);
   }
 
   BuiltinVaListKind getBuiltinVaListKind() const override {
     return TargetInfo::CharPtrBuiltinVaList;
   }
+
+  bool handleTargetFeatures(std::vector<std::string> &Features,
+                            DiagnosticsEngine &Diags) override {
+    // Register x18 is reserved for AArch64. We make this the default for
+    // AArch64 Windows target.
+    WindowsTargetInfo<AArch64leTargetInfo>::handleTargetFeatures(Features,
+                                                                 Diags);
+    Features.push_back("+reserve-x18");
+    return true;
+  }
 };
 
 class AArch64beTargetInfo : public AArch64TargetInfo {
   void setDataLayout() override {
     assert(!getTriple().isOSBinFormatMachO());
     resetDataLayout("E-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128");
   }
 

test/Driver/coff-aarch64-fixed-x18.c

+// RUN: %clang -target aarch64-windows -emit-llvm -S %s -o - | FileCheck %s
+
+// CHECK: "target-features"{{.*}}+reserve-x18
+
+void foo() {}