[llvm] [WIP][SelectionDAG] Add support for the 3-way comparison intrinsics [US]CMP (PR #91871)

[Nikita Popov via llvm-commits]

================
@@ -1327,6 +1344,31 @@ void DAGTypeLegalizer::SplitVecRes_TernaryOp(SDNode *N, SDValue &Lo,
                    {Op0Hi, Op1Hi, Op2Hi, MaskHi, EVLHi}, Flags);
 }
 
+void DAGTypeLegalizer::SplitVecRes_CMP(SDNode *N, SDValue &Lo, SDValue &Hi) {
+  LLVMContext &Ctxt = *DAG.getContext();
+  SDLoc dl(N);
+
+  SDValue LHS = N->getOperand(0);
+  SDValue RHS = N->getOperand(1);
+  EVT SplitOpVT = LHS.getValueType().getHalfNumVectorElementsVT(Ctxt);
+  EVT SplitResVT = N->getValueType(0).getHalfNumVectorElementsVT(Ctxt);
+
+  SDValue LHSLo = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, SplitOpVT, LHS,
+                              DAG.getVectorIdxConstant(0, dl));
+  SDValue LHSHi = DAG.getNode(
+      ISD::EXTRACT_SUBVECTOR, dl, SplitOpVT, LHS,
+      DAG.getVectorIdxConstant(SplitOpVT.getVectorNumElements(), dl));
----------------
nikic wrote:

I think you're looking for `DAG.SplitVector()` here.

https://github.com/llvm/llvm-project/pull/91871