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

[Andrei Safronov via llvm-commits]

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@@ -378,6 +381,56 @@ SDValue XtensaTargetLowering::LowerFormalArguments(
     }
   }
 
+  if (IsVarArg) {
+    unsigned Idx = CCInfo.getFirstUnallocated(IntRegs);
+    unsigned ArgRegsNum = std::size(IntRegs);
+    const TargetRegisterClass *RC = &Xtensa::ARRegClass;
+    MachineFrameInfo &MFI = MF.getFrameInfo();
+    MachineRegisterInfo &RegInfo = MF.getRegInfo();
+    unsigned RegSize = 4;
+    MVT RegTy = MVT::i32;
+
+    XtensaFI->setVarArgsFirstGPR(Idx + 2); // 2 - number of a2 register
+
+    XtensaFI->setVarArgsOnStackFrameIndex(
+        MFI.CreateFixedObject(4, 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 (ArgRegsNum == Idx) {
+      VaArgOffset = CCInfo.getStackSize();
+      VarArgsSaveSize = 0;
+    } else {
+      VarArgsSaveSize = RegSize * (ArgRegsNum - 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->setVarArgsInRegsFrameIndex(FI);
+
+      // Copy the integer registers that may have been used for passing varargs
+      // to the vararg save area.
+      for (unsigned I = Idx; I < ArgRegsNum; ++I, VaArgOffset += RegSize) {
+        const Register Reg = RegInfo.createVirtualRegister(RC);
+        RegInfo.addLiveIn(IntRegs[I], Reg);
+
+        SDValue ArgValue = DAG.getCopyFromReg(Chain, DL, Reg, RegTy);
+        FI = MFI.CreateFixedObject(RegSize, VaArgOffset, true);
+        SDValue PtrOff =
+            DAG.getFrameIndex(FI, getPointerTy(DAG.getDataLayout()));
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andreisfr wrote:

Thank you for comment. Fixed.

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