[llvm] [DAGCombiner] Add generic DAG combine for ISD::PARTIAL_REDUCE_MLA (PR #127083)

[Benjamin Maxwell via llvm-commits]

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@@ -1639,6 +1639,25 @@ class TargetLoweringBase {
            getCondCodeAction(CC, VT) == Custom;
   }
 
+  /// Return how a PARTIAL_REDUCE_U/SMLA node with Acc type AccVT and Input type
+  /// InputVT should be treated. Either it's legal, needs to be promoted to a
+  /// larger size, needs to be expanded to some other code sequence, or the
+  /// target has a custom expander for it.
+  LegalizeAction getPartialReduceMLAAction(EVT AccVT, EVT InputVT) const {
+    unsigned AccI = (unsigned)AccVT.getSimpleVT().SimpleTy;
+    unsigned InputI = (unsigned)InputVT.getSimpleVT().SimpleTy;
+    assert(AccI < MVT::VALUETYPE_SIZE && InputI < MVT::VALUETYPE_SIZE &&
+           "Table isn't big enough!");
+    return PartialReduceMLAActions[AccI][InputI];
+  }
+
+  /// Return true if a PARTIAL_REDUCE_U/SMLA node with the specified types is
+  /// legal or custom for this target.
+  bool isPartialReduceMLALegalOrCustom(EVT AccVT, EVT InputVT) const {
+    return getPartialReduceMLAAction(AccVT, InputVT) == Legal ||
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MacDue wrote:

Should these changes (and setPartialReduceMLAAction) be moved to the PR that adds the lowering, as they're not really used in this PR?

https://github.com/llvm/llvm-project/pull/127083