[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[Simon Pilgrim via llvm-commits]

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@@ -6314,6 +6414,51 @@ void BoUpSLP::reorderBottomToTop(bool IgnoreReorder) {
       return Data1.first->Idx > Data2.first->Idx;
     });
     for (auto &Data : UsersVec) {
+      if (Data.first->State == TreeEntry::SplitVectorize) {
+        assert(
+            Data.second.size() <= 2 &&
+            "Expected not greater than 2 operands for split vectorize node.");
+        if (any_of(Data.second, [](const auto &Op) {
+              return Op.second->UserTreeIndices.size() != 1;
+            }))
+          continue;
+        // Update orders in user split vectorize nodes.
+        for (const auto &P : Data.first->CombinedEntriesWithIndices) {
+          TreeEntry &OpTE = *VectorizableTree[P.first].get();
+          if (OpTE.isGather() || OpTE.ReorderIndices.empty())
+            continue;
+          SmallVector<int> Mask;
+          inversePermutation(OpTE.ReorderIndices, Mask);
+          SmallVector<int> MaskOrder(OpTE.ReorderIndices.size(),
+                                     PoisonMaskElem);
+          unsigned E = OpTE.ReorderIndices.size();
+          transform(OpTE.ReorderIndices, MaskOrder.begin(), [E](unsigned I) {
+            return I < E ? static_cast<int>(I) : PoisonMaskElem;
+          });
+          SmallVector<int> NewMask(Data.first->getVectorFactor());
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RKSimon wrote:

Duplicate of UpdateSplitUserNode ?

https://github.com/llvm/llvm-project/pull/123360