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

[Nikita Popov via llvm-commits]

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@@ -4426,6 +4511,56 @@ SDValue DAGTypeLegalizer::WidenVecRes_Binary(SDNode *N) {
                      {InOp1, InOp2, Mask, N->getOperand(3)}, N->getFlags());
 }
 
+SDValue DAGTypeLegalizer::WidenVecRes_CMP(SDNode *N) {
+  LLVMContext &Ctxt = *DAG.getContext();
+  SDLoc dl(N);
+
+  SDValue LHS = N->getOperand(0);
+  SDValue RHS = N->getOperand(1);
+  EVT OpVT = LHS.getValueType();
+  EVT TransformedOpVT = TLI.getTypeToTransformTo(Ctxt, OpVT);
+  if (TransformedOpVT.isVector() &&
+      TransformedOpVT.getVectorNumElements() > OpVT.getVectorNumElements()) {
+    LHS = GetWidenedVector(LHS);
+    RHS = GetWidenedVector(RHS);
+  }
+
+  EVT WidenResVT = TLI.getTypeToTransformTo(Ctxt, N->getValueType(0));
+  ElementCount WidenResEC = WidenResVT.getVectorElementCount();
+  EVT WidenResElementVT = WidenResVT.getVectorElementType();
+
+  // At this point we know that the type of operands should be legal,
----------------
nikic wrote:

This doesn't sound right. The result types are legalized first, so we don't know at this point that the operands are legal.

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