[llvm] [Xtensa] Implement vararg support. (PR #117126)

[Matt Arsenault via llvm-commits]

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@@ -378,6 +393,68 @@ SDValue XtensaTargetLowering::LowerFormalArguments(
     }
   }
 
+  if (IsVarArg) {
+    static const MCPhysReg XtensaArgRegs[6] = {
+        Xtensa::A2, Xtensa::A3, Xtensa::A4, Xtensa::A5, Xtensa::A6, Xtensa::A7};
+    ArrayRef<MCPhysReg> ArgRegs = ArrayRef(XtensaArgRegs);
+    unsigned Idx = CCInfo.getFirstUnallocated(ArgRegs);
+    const TargetRegisterClass *RC = &Xtensa::ARRegClass;
+    MachineFrameInfo &MFI = MF.getFrameInfo();
+    MachineRegisterInfo &RegInfo = MF.getRegInfo();
+    unsigned RegSize = 4;
+    MVT RegTy = MVT::getIntegerVT(RegSize * 8);
+
+    XtensaFI->setVarArgsFirstGPR(Idx + 2); // 2 - number of a2 register
+
+    XtensaFI->setVarArgsStackOffset(MFI.CreateFixedObject(
+        PtrVT.getSizeInBits() / 8, CCInfo.getStackSize(), true));
+
+    // Offset of the first variable argument from stack pointer, and size of
+    // the vararg save area. For now, the varargs save area is either zero or
+    // large enough to hold a0-a7.
+    int VaArgOffset, VarArgsSaveSize;
+
+    // If all registers are allocated, then all varargs must be passed on the
+    // stack and we don't need to save any argregs.
+    if (ArgRegs.size() == Idx) {
+      VaArgOffset = CCInfo.getStackSize();
+      VarArgsSaveSize = 0;
+    } else {
+      VarArgsSaveSize = RegSize * (ArgRegs.size() - Idx);
+      VaArgOffset = -VarArgsSaveSize;
+    }
+
+    // Record the frame index of the first variable argument
+    // which is a value necessary to VASTART.
+    int FI = MFI.CreateFixedObject(RegSize, VaArgOffset, true);
+    XtensaFI->setVarArgsFrameIndex(FI);
+
+    // Copy the integer registers that may have been used for passing varargs
+    // to the vararg save area.
+    for (unsigned I = Idx; I < ArgRegs.size(); ++I, VaArgOffset += RegSize) {
+      const unsigned Reg = RegInfo.createVirtualRegister(RC);
+      unsigned FrameReg = Subtarget.getRegisterInfo()->getFrameRegister(MF);
+
+      // Argument passed in FrameReg we save in A8 (in emitPrologue),
+      // so load argument from A8
+      if (ArgRegs[I] == FrameReg) {
+        RegInfo.addLiveIn(Xtensa::A8, Reg);
+      } else {
+        RegInfo.addLiveIn(ArgRegs[I], Reg);
+      }
+
+      SDValue ArgValue = DAG.getCopyFromReg(Chain, DL, Reg, RegTy);
+      FI = MFI.CreateFixedObject(RegSize, VaArgOffset, true);
+      SDValue PtrOff = DAG.getFrameIndex(FI, getPointerTy(DAG.getDataLayout()));
+      SDValue Store = DAG.getStore(Chain, DL, ArgValue, PtrOff,
+                                   MachinePointerInfo::getFixedStack(MF, FI));
+      cast<StoreSDNode>(Store.getNode())
+          ->getMemOperand()
+          ->setValue((Value *)nullptr);
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arsenm wrote:

You shouldn't be adjusting this after the creation of the store 

https://github.com/llvm/llvm-project/pull/117126