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

[Nikita Popov via llvm-commits]

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@@ -6273,6 +6382,32 @@ SDValue DAGTypeLegalizer::WidenVecOp_EXTEND(SDNode *N) {
   }
 }
 
+SDValue DAGTypeLegalizer::WidenVecOp_CMP(SDNode *N) {
+  SDLoc dl(N);
+
+  EVT OpVT = N->getOperand(0).getValueType();
+  EVT ResVT = N->getValueType(0);
+  SDValue LHS = GetWidenedVector(N->getOperand(0));
+  SDValue RHS = GetWidenedVector(N->getOperand(1));
+
+  // 1. EXTRACT_SUBVECTOR
+  // 2. SIGN_EXTEND/ZERO_EXTEND
+  // 3. CMP
+  LHS = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, OpVT, LHS,
+                    DAG.getVectorIdxConstant(0, dl));
+  RHS = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, OpVT, RHS,
+                    DAG.getVectorIdxConstant(0, dl));
+
+  // At this point the result type is guaranteed to be valid, so we can use it
+  // as the operand type by extending it appropriately
+  ISD::NodeType ExtendOpcode =
+      N->getOpcode() == ISD::SCMP ? ISD::SIGN_EXTEND : ISD::ZERO_EXTEND;
+  LHS = DAG.getNode(ExtendOpcode, dl, ResVT, LHS);
+  RHS = DAG.getNode(ExtendOpcode, dl, ResVT, RHS);
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nikic wrote:

I'm confused by the WidenVecOp/WidenVecRes implementations. The widening here is in the sense of increasing the number of vector elements, not the size of the element type, so why are extends relevant here?

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