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

[Craig Topper via llvm-commits]

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@@ -5417,6 +5422,36 @@ 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 LL, LH, RL, RH;
+  GetExpandedInteger(N->getOperand(0), LL, LH);
+  GetExpandedInteger(N->getOperand(1), RL, RH);
+  EVT HalfVT = LL.getValueType();
+  SDLoc DL(N);
+  
+  // CLMUL is carryless so Lo is computed from the low half
+  Lo = DAG.getNode(ISD::CLMUL, DL, HalfVT, LL, RL);
+  // 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
+  // TODO: if the target supports a widening CLMUL or a CLMULH we should probably use that
+  SDValue BitRevLL = DAG.getNode(ISD::BITREVERSE, DL, HalfVT, LL);
+  SDValue BitRevRL = DAG.getNode(ISD::BITREVERSE, DL, HalfVT, RL);
+  SDValue BitRevLoHi = DAG.getNode(ISD::CLMUL, DL, HalfVT, BitRevLL, BitRevRL);
+  SDValue LoHi = DAG.getNode(ISD::BITREVERSE, DL, HalfVT, BitRevLoHi);
+  SDValue One = DAG.getShiftAmountConstant(1, HalfVT, DL);
+  Hi = DAG.getNode(ISD::SRL, DL, HalfVT, LoHi, One);
+  
+  SDValue HITMP = DAG.getNode(ISD::CLMUL, DL, HalfVT, LL, RH);
----------------
topperc wrote:

HITMP -> HiTmp

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