[llvm] [RISCV][GlobalISel] Legalize Scalable Vector Loads and Stores (PR #84965)

[Matt Arsenault via llvm-commits]

================
@@ -649,6 +680,61 @@ bool RISCVLegalizerInfo::legalizeExt(MachineInstr &MI,
   return true;
 }
 
+bool RISCVLegalizerInfo::legalizeLoadStore(MachineInstr &MI,
+                                           MachineIRBuilder &MIB) const {
+  MachineRegisterInfo &MRI = *MIB.getMRI();
+  MachineFunction *MF = MI.getParent()->getParent();
+  const DataLayout &DL = MIB.getDataLayout();
+  LLVMContext &Ctx = MF->getFunction().getContext();
+
+  Register DstReg = MI.getOperand(0).getReg();
+  Register PtrReg = MI.getOperand(1).getReg();
+  LLT DataTy = MRI.getType(DstReg);
+  assert(DataTy.isVector() && "Expect vector load.");
+  assert(STI.hasVInstructions() &&
+         (DataTy.getScalarSizeInBits() != 64 || STI.hasVInstructionsI64()) &&
+         (DataTy.getElementCount().getKnownMinValue() != 1 ||
+          STI.getELen() == 64) &&
+         "Load type must be legal integer or floating point vector.");
+
+  assert(MI.hasOneMemOperand() &&
+         "Load instructions only have one MemOperand.");
+  MachineMemOperand *MMO = *MI.memoperands_begin();
+  Align Alignment = MMO->getAlign();
+
+  const auto *TLI = STI.getTargetLowering();
+  EVT VT = EVT::getEVT(getTypeForLLT(DataTy, Ctx));
+
+  if (TLI->allowsMemoryAccessForAlignment(Ctx, DL, VT, *MMO))
+    return true;
+
+  unsigned EltSizeBits = DataTy.getScalarSizeInBits();
+  assert((EltSizeBits == 16 || EltSizeBits == 32 || EltSizeBits == 64) &&
+         "Unexpected unaligned RVV load type");
+
+  // Calculate the new vector type with i8 elements
+  unsigned NumElements =
+      DataTy.getElementCount().getKnownMinValue() * (EltSizeBits / 8);
+  LLT NewDataTy = LLT::scalable_vector(NumElements, 8);
+
+  MachinePointerInfo PI = MMO->getPointerInfo();
+  MachineMemOperand *NewMMO =
+      MF->getMachineMemOperand(PI, MMO->getFlags(), NewDataTy, Alignment);
+
+  if (isa<GLoad>(MI)) {
+    auto NewLoad = MIB.buildLoad(NewDataTy, PtrReg, *NewMMO);
+    MIB.buildBitcast(DstReg, NewLoad);
----------------
arsenm wrote:

There is already a bitcast load/store action. 

https://github.com/llvm/llvm-project/pull/84965