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

[Craig Topper via llvm-commits]

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@@ -7865,6 +7867,71 @@ 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);
+  Register HeadOfList = MRI.createGenericVirtualRegister(PtrTy);
+  Register VAList =
+      MIRBuilder.buildLoad(HeadOfList, ListPtr, *PtrLoadMMO).getReg(0);
+
+  const MaybeAlign MA(MI.getOperand(2).getImm());
+  LLT PtrTyAsScalarTy = LLT::scalar(PtrTy.getSizeInBits());
+  if (MA && *MA > TLI.getMinStackArgumentAlignment()) {
+    Register AlignAmt =
+        MIRBuilder.buildConstant(PtrTyAsScalarTy, MA->value() - 1).getReg(0);
+    Register AddDst = MRI.createGenericVirtualRegister(PtrTy);
+    MIRBuilder.buildPtrAdd(AddDst, HeadOfList, AlignAmt);
+    Register Mask =
+        MIRBuilder.buildConstant(PtrTyAsScalarTy, -(int64_t)MA->value())
+            .getReg(0);
+    Register AndDst = MRI.createGenericVirtualRegister(PtrTy);
+    VAList = MIRBuilder.buildPtrMask(AndDst, AddDst, Mask).getReg(0);
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topperc wrote:

Can we use `MIRBuilder.buildMaskLowPtrBits`?

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