[llvm] [IR] Add llvm `clmul` intrinsic (PR #140301)

[Matt Arsenault via llvm-commits]

================
@@ -5417,6 +5422,35 @@ void DAGTypeLegalizer::ExpandIntRes_FunnelShift(SDNode *N, SDValue &Lo,
   Hi = DAG.getNode(Opc, DL, HalfVT, Select3, Select2, NewShAmt);
 }
 
+void DAGTypeLegalizer::ExpandIntRes_CLMUL(SDNode *N, SDValue &Lo,
+                                                SDValue &Hi) {
+  // Values numbered from least significant to most significant.
+  SDValue In1, In2, In3, In4;
+  GetExpandedInteger(N->getOperand(0), In3, In4);
+  GetExpandedInteger(N->getOperand(1), In1, In2);
+  EVT HalfVT = In1.getValueType();
+  SDLoc DL(N);
+  
+  // CLMUL is carryless so Lo is computed from the low half
+  Lo = DAG.getNode(ISD::CLMUL, DL, HalfVT, In1, In3);
+  // the high bits not included in CLMUL(A,B) can be computed by
+  // BITREVERSE(CLMUL(BITREVERSE(A), BITREVERSE(B))) >> 1
+  // Therefore we can compute the 2 hi/lo cross products
+  // and the the overflow of the low product
+  // and xor them together to compute HI
+  SDValue BitRevIn1 = DAG.getNode(ISD::BITREVERSE, DL, HalfVT, In1);
+  SDValue BitRevIn3 = DAG.getNode(ISD::BITREVERSE, DL, HalfVT, In3);
+  SDValue BitRevLoHi = DAG.getNode(ISD::CLMUL, DL, HalfVT, BitRevIn1, BitRevIn3);
+  SDValue LoHi = DAG.getNode(ISD::BITREVERSE, DL, HalfVT, BitRevLoHi);
+  SDValue One = DAG.getConstant(0, DL, HalfVT);
----------------
arsenm wrote:

The shift amount type can differ based on the shifted value, equivalently you can query what type to use for it 

https://github.com/llvm/llvm-project/pull/140301