[llvm] [SelectionDAG] Improve type legalisation for PARTIAL_REDUCE_MLA (PR #130935)

[Benjamin Maxwell via llvm-commits]

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@@ -3220,8 +3220,26 @@ void DAGTypeLegalizer::SplitVecRes_VP_REVERSE(SDNode *N, SDValue &Lo,
 void DAGTypeLegalizer::SplitVecRes_PARTIAL_REDUCE_MLA(SDNode *N, SDValue &Lo,
                                                       SDValue &Hi) {
   SDLoc DL(N);
-  SDValue Expanded = TLI.expandPartialReduceMLA(N, DAG);
-  std::tie(Lo, Hi) = DAG.SplitVector(Expanded, DL);
+  SDValue Acc = N->getOperand(0);
+  SDValue Input1 = N->getOperand(1);
+
+  // If the node has not gone through the DAG combine, then do not attempt to
+  // legalise, just expand.
+  if (!TLI.isPartialReduceMLALegal(Acc.getValueType(), Input1.getValueType())) {
+    SDValue Expanded = TLI.expandPartialReduceMLA(N, DAG);
+    std::tie(Lo, Hi) = DAG.SplitVector(Expanded, DL);
+    return;
+  }
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MacDue wrote:

So is this to disable the splitting for all the pairs marked `Custom`? 

https://github.com/llvm/llvm-project/pull/130935