[llvm] [RISCV][GISEL] Legalize G_VAARG through expansion. (PR #73065)

[Matt Arsenault via llvm-commits]

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@@ -7868,6 +7870,63 @@ LegalizerHelper::lowerVectorReduction(MachineInstr &MI) {
   return UnableToLegalize;
 }
 
+static Type *getTypeForLLT(LLT Ty, LLVMContext &C);
+
+LegalizerHelper::LegalizeResult LegalizerHelper::lowerVAArg(MachineInstr &MI) {
+  Observer.changingInstr(MI);
+  MachineFunction &MF = *MI.getMF();
+  const DataLayout &DL = MIRBuilder.getDataLayout();
+  LLVMContext &Ctx = MF.getFunction().getContext();
+  Register ListPtr = MI.getOperand(1).getReg();
+  LLT PtrTy = MRI.getType(ListPtr);
+
+  // LstPtr is a pointer to the head of the list. Get the address
+  // of the head of the list.
+  Align PtrAlignment = Align(DL.getABITypeAlign(getTypeForLLT(PtrTy, Ctx)));
+  MachineMemOperand *PtrLoadMMO =
+      MF.getMachineMemOperand(MachinePointerInfo::getUnknownStack(MF),
+                              MachineMemOperand::MOLoad, PtrTy, PtrAlignment);
+  auto VAList = MIRBuilder.buildLoad(PtrTy, ListPtr, *PtrLoadMMO).getReg(0);
+
+  const Align A(MI.getOperand(2).getImm());
+  LLT PtrTyAsScalarTy = LLT::scalar(PtrTy.getSizeInBits());
+  if (A > TLI.getMinStackArgumentAlignment()) {
+    Register AlignAmt =
+        MIRBuilder.buildConstant(PtrTyAsScalarTy, A.value() - 1).getReg(0);
+    auto AddDst = MIRBuilder.buildPtrAdd(PtrTy, HeadOfList, AlignAmt);
+    auto AndDst = MIRBuilder.buildMaskLowPtrBits(PtrTy, AddDst, Log2(A));
+    VAList = AndDst.getReg(0);
+  }
+
+  // Increment the pointer, VAList, to the next vaarg
+  // The list should be bumped by the size of element in the current head of
+  // list.
+  Register Dst = MI.getOperand(0).getReg();
+  LLT Ty = MRI.getType(Dst);
+  Register IncAmt =
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arsenm wrote:

auto IncAmt and drop the .getReg(0) at the end 

https://github.com/llvm/llvm-project/pull/73065