[llvm] [ISel] Introduce llvm.clmul intrinsic (PR #168731)

[Piotr Fusik via llvm-commits]

================
@@ -8302,6 +8302,54 @@ SDValue TargetLowering::expandFunnelShift(SDNode *Node,
   return DAG.getNode(ISD::OR, DL, VT, ShX, ShY);
 }
 
+SDValue TargetLowering::expandCLMUL(SDNode *Node, SelectionDAG &DAG) const {
+  SDLoc DL(Node);
+  EVT VT = Node->getValueType(0);
+  SDValue X = Node->getOperand(0);
+  SDValue Y = Node->getOperand(1);
+  unsigned BW = VT.getScalarSizeInBits();
+  unsigned Opcode = Node->getOpcode();
+
+  switch (Opcode) {
+  case ISD::CLMUL: {
+    SDValue Res = DAG.getConstant(0, DL, VT);
+    for (unsigned I = 0; I < BW; ++I) {
+      SDValue Mask = DAG.getConstant(APInt::getOneBitSet(BW, I), DL, VT);
+      SDValue YMasked = DAG.getNode(ISD::AND, DL, VT, Y, Mask);
+      SDValue Mul = DAG.getNode(ISD::MUL, DL, VT, X, YMasked);
+      Res = DAG.getNode(ISD::XOR, DL, VT, Res, Mul);
+    }
+    return Res;
+  }
+  case ISD::CLMULR:
+  case ISD::CLMULH: {
+    EVT ExtVT =
+        VT.changeElementType(EVT::getIntegerVT(*DAG.getContext(), 2 * BW));
+    // For example, ExtVT = i64 based operations aren't legal on a 32-bit
+    // target; use bitreverse-based lowering in this case.
+    if (!isOperationLegalOrCustom(ISD::ZERO_EXTEND, ExtVT) ||
+        !isOperationLegalOrCustom(ISD::SRL, ExtVT)) {
+      SDValue XRev = DAG.getNode(ISD::BITREVERSE, DL, VT, X);
+      SDValue YRev = DAG.getNode(ISD::BITREVERSE, DL, VT, Y);
+      SDValue ClMul = DAG.getNode(ISD::CLMUL, DL, VT, XRev, YRev);
+      SDValue Res = DAG.getNode(ISD::BITREVERSE, DL, VT, ClMul);
+      if (Opcode == ISD::CLMULR)
----------------
pfusik wrote:

```suggestion
      if (Opcode == ISD::CLMULH)
```

https://github.com/llvm/llvm-project/pull/168731