[AArch64] Introduce option to force placement of the frame record on top of the stack frame

[loicottet]

Summary

This option is needed for the LLVM backend for GraalVM Native Image, which needs the frame pointer to be located at a fixed distance from the top of the stack frame in order to be able to crawl the call stack. When it is set, it uses the Darwin behaviour on every target, which already enforces this frame record placement.

llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp

 
 using namespace llvm;
 
 #define GET_CC_REGISTER_LISTS
 #include "AArch64GenCallingConv.inc"
 #define GET_REGINFO_TARGET_DESC
 #include "AArch64GenRegisterInfo.inc"
 
+static cl::opt<bool>
+    FrameRecordOnTop("aarch64-frame-record-on-top",
+                     cl::desc("place the frame record on top of the frame"),
+                     cl::init(false), cl::Hidden);
+
 AArch64RegisterInfo::AArch64RegisterInfo(const Triple &TT)
     : AArch64GenRegisterInfo(AArch64::LR), TT(TT) {
   AArch64_MC::initLLVMToCVRegMapping(this);
 }
 
 /// Return whether the register needs a CFI entry. Not all unwinders may know
 /// about SVE registers, so we assume the lowest common denominator, i.e. the
 /// callee-saves required by the base ABI. For the SVE registers z8-z15 only the
AArch64RegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   if (MF->getFunction().getCallingConv() == CallingConv::PreserveAll)
     return CSR_AArch64_RT_AllRegs_SaveList;
   if (MF->getFunction().getCallingConv() == CallingConv::Win64)
     // This is for OSes other than Windows; Windows is a separate case further
     // above.
     return CSR_AArch64_AAPCS_X18_SaveList;
   if (MF->getInfo<AArch64FunctionInfo>()->isSVECC())
     return CSR_AArch64_SVE_AAPCS_SaveList;
+  if (FrameRecordOnTop)
+    return CSR_Darwin_AArch64_AAPCS_SaveList;
   return CSR_AArch64_AAPCS_SaveList;
 }
 
 const MCPhysReg *
 AArch64RegisterInfo::getDarwinCalleeSavedRegs(const MachineFunction *MF) const {
   assert(MF && "Invalid MachineFunction pointer.");
   assert(MF->getSubtarget<AArch64Subtarget>().isTargetDarwin() &&
          "Invalid subtarget for getDarwinCalleeSavedRegs");