[llvm] [RISCV] Implement EmitTargetCodeForMemset for Xqcilsm (PR #151555)

[Sudharsan Veeravalli via llvm-commits]

================
@@ -62,3 +64,102 @@ void RISCVSelectionDAGInfo::verifyTargetNode(const SelectionDAG &DAG,
   }
 #endif
 }
+
+SDValue RISCVSelectionDAGInfo::EmitTargetCodeForMemset(
+    SelectionDAG &DAG, const SDLoc &dl, SDValue Chain, SDValue Dst, SDValue Src,
+    SDValue Size, Align Alignment, bool isVolatile, bool AlwaysInline,
+    MachinePointerInfo DstPtrInfo) const {
+  const RISCVSubtarget &Subtarget =
+      DAG.getMachineFunction().getSubtarget<RISCVSubtarget>();
+  // We currently do this only for Xqcilsm
+  if (!Subtarget.hasVendorXqcilsm())
+    return SDValue();
+
+  // Do this only if we know the size at compile time.
+  ConstantSDNode *ConstantSize = dyn_cast<ConstantSDNode>(Size);
+  if (!ConstantSize)
+    return SDValue();
+
+  uint64_t NumberOfBytesToWrite = ConstantSize->getZExtValue();
+
+  // Do this only if it is word aligned and we write multiple of 4 bytes.
+  if (!((Alignment.value() & 3) == 0 && (NumberOfBytesToWrite & 3) == 0))
+    return SDValue();
+
+  SmallVector<SDValue, 8> OutChains;
+  SDValue SizeWords, OffsetSetwmi;
+  SDValue SrcValueReplicated = DAG.getNode(ISD::ZERO_EXTEND, dl, MVT::i32, Src);
+  int NumberOfWords = NumberOfBytesToWrite / 4;
+
+  // Helper for constructing the QC_SETWMI instruction
+  auto getSetwmiNode = [&](SDValue SizeWords, SDValue OffsetSetwmi) -> SDValue {
+    SDValue Ops[] = {Chain, SrcValueReplicated, Dst, SizeWords, OffsetSetwmi};
+    return DAG.getNode(RISCVISD::QC_SETWMI, dl, MVT::Other, Ops);
----------------
svs-quic wrote:

I took this from the `TH_SDD` and `TH_SWD` definitions. I think I should do something similar to this:

```
unsigned Opcode = (MemVT == MVT::i32) ? RISCVISD::TH_SWD : RISCVISD::TH_SDD;

SDValue Res = DAG.getMemIntrinsicNode(
    Opcode, SDLoc(LSNode1), DAG.getVTList(MVT::Other),
    {LSNode1->getChain(), LSNode1->getOperand(1), LSNode2->getOperand(1),
     BasePtr, DAG.getConstant(Imm, SDLoc(LSNode1), XLenVT)},
    NewMemVT, NewMMO);
```

https://github.com/llvm/llvm-project/pull/151555