[llvm] [LoongArch] Make rotl/rotr custom for lsx/lasx (PR #161154)

[via llvm-commits]

================
@@ -827,6 +834,59 @@ SDValue LoongArchTargetLowering::lowerPREFETCH(SDValue Op,
   return Op;
 }
 
+SDValue LoongArchTargetLowering::lowerRotate(SDValue Op,
+                                             SelectionDAG &DAG) const {
+  MVT VT = Op.getSimpleValueType();
+  if (!VT.isVector())
+    return Op;
+
+  SDLoc DL(Op);
+  SDValue R = Op.getOperand(0);
+  SDValue Amt = Op.getOperand(1);
+  unsigned Opcode = Op.getOpcode();
+  unsigned EltSizeInBits = VT.getScalarSizeInBits();
+
+  auto checkCstSplat = [](SDValue V, APInt &CstSplatValue) {
+    if (V.getOpcode() != ISD::BUILD_VECTOR)
+      return false;
+    if (SDValue SplatValue =
+            cast<BuildVectorSDNode>(V.getNode())->getSplatValue()) {
+      if (auto *C = dyn_cast<ConstantSDNode>(SplatValue)) {
+        CstSplatValue = C->getAPIntValue();
+        return true;
+      }
+    }
+    return false;
+  };
+
+  // Check for constant splat rotation amount.
+  APInt CstSplatValue;
+  bool IsCstSplat = checkCstSplat(Amt, CstSplatValue);
+  bool isROTL = Opcode == ISD::ROTL;
+
+  // Check for splat rotate by zero.
+  if (IsCstSplat && CstSplatValue.urem(EltSizeInBits) == 0)
+    return R;
+
+  // LoongArch tagets always prefers ISD::ROTR.
+  if (isROTL) {
+    SDValue Zero = DAG.getConstant(0, DL, VT);
+    return DAG.getNode(ISD::ROTR, DL, VT, R,
+                       DAG.getNode(ISD::SUB, DL, VT, Zero, Amt));
+  }
+
+  // Rotate by a immediate.
+  if (IsCstSplat) {
+    // ISD::ROTR: Attemp to rotate by a positive immediate.
+    SDValue Bits = DAG.getConstant(EltSizeInBits, DL, VT);
+    if (SDValue Urem =
+            DAG.FoldConstantArithmetic(ISD::UREM, DL, VT, {Amt, Bits}))
+      return DAG.getNode(Op.getOpcode(), DL, VT, R, Urem);
----------------
zhaoqi5 wrote:

```suggestion
      return DAG.getNode(Opcode, DL, VT, R, Urem);
```

https://github.com/llvm/llvm-project/pull/161154