[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);
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arsenm wrote:

Just use MVT::i32 directly? 

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