[Xtensa] Change using of Frame Pointer.

Do not use Frame Pointer by default.
Also improve storing function argument from a7 register to a8 register.
Corrected funnel shift test.

Closes llvm#19

Author: andreisfr

clang/lib/Driver/ToolChains/Clang.cpp

@@ -539,6 +539,7 @@ static bool useFramePointerForTargetByDefault(const ArgList &Args,
   case llvm::Triple::riscv64:
   case llvm::Triple::amdgcn:
   case llvm::Triple::r600:
+  case llvm::Triple::xtensa:
     return !areOptimizationsEnabled(Args);
   default:
     break;

llvm/lib/Target/Xtensa/XtensaFrameLowering.cpp

@@ -14,6 +14,7 @@
 
 #include "XtensaFrameLowering.h"
 #include "XtensaInstrInfo.h"
+#include "XtensaMachineFunctionInfo.h"
 #include "XtensaSubtarget.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
@@ -99,6 +100,7 @@ void XtensaFrameLowering::emitPrologue(MachineFunction &MF,
   unsigned FP = RegInfo->getFrameRegister(MF);
   MachineModuleInfo &MMI = MF.getMMI();
   const MCRegisterInfo *MRI = MMI.getContext().getRegisterInfo();
+  XtensaFunctionInfo *XtensaFI = MF.getInfo<XtensaFunctionInfo>();
 
   // First, compute final stack size.
   uint64_t StackSize = MFI.getStackSize();
@@ -132,9 +134,11 @@ void XtensaFrameLowering::emitPrologue(MachineFunction &MF,
 
     // Store FP register in A8, because FP may be used to pass function
     // arguments
-    BuildMI(MBB, MBBI, dl, TII.get(Xtensa::OR), Xtensa::A8)
-        .addReg(FP)
-        .addReg(FP);
+    if (XtensaFI->isSaveFrameRegister()) {
+      BuildMI(MBB, MBBI, dl, TII.get(Xtensa::OR), Xtensa::A8)
+          .addReg(FP)
+          .addReg(FP);
+    }
 
     // if framepointer enabled, set it to point to the stack pointer.
     if (hasFP(MF)) {

llvm/lib/Target/Xtensa/XtensaISelLowering.cpp

@@ -778,6 +778,7 @@ SDValue XtensaTargetLowering::LowerFormalArguments(
       // so load argument from A8
       if (Subtarget.isWinABI() && (VA.getLocReg() == FrameReg)) {
         Reg = MF.addLiveIn(Xtensa::A8, RC);
+        XtensaFI->setSaveFrameRegister();
       } else {
         Reg = MF.addLiveIn(VA.getLocReg(), RC);
       }
@@ -872,6 +873,7 @@ SDValue XtensaTargetLowering::LowerFormalArguments(
       // so load argument from A8
       if (ArgRegs[I] == FrameReg) {
         RegInfo.addLiveIn(Xtensa::A8, Reg);
+        XtensaFI->setSaveFrameRegister();
       } else {
         RegInfo.addLiveIn(ArgRegs[I], Reg);
       }

llvm/lib/Target/Xtensa/XtensaMachineFunctionInfo.h

@@ -27,6 +27,7 @@ class XtensaFunctionInfo : public MachineFunctionInfo {
   unsigned VarArgsFirstGPR;
   int VarArgsStackOffset;
   unsigned VarArgsFrameIndex;
+  bool SaveFrameRegister = false;
 
 public:
   explicit XtensaFunctionInfo(MachineFunction &MF)
@@ -45,6 +46,9 @@ class XtensaFunctionInfo : public MachineFunctionInfo {
   unsigned getVarArgsFrameIndex() const { return VarArgsFrameIndex; }
   void setVarArgsFrameIndex(unsigned FI) { VarArgsFrameIndex = FI; }
 
+  bool isSaveFrameRegister() const { return SaveFrameRegister; }
+  void setSaveFrameRegister() { SaveFrameRegister = true; }
+
   // TODO: large frame size definition should be specified more precisely
   bool isLargeFrame() {
     return (MF.getFrameInfo().estimateStackSize(MF) > 512) ? true : false;