[llvm] [SLP]Add cost estimation for gather node reshuffling (PR #115201)

[Simon Pilgrim via llvm-commits]

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@@ -4846,8 +4846,21 @@ getShuffleCost(const TargetTransformInfo &TTI, TTI::ShuffleKind Kind,
                TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput,
                int Index = 0, VectorType *SubTp = nullptr,
                ArrayRef<const Value *> Args = {}) {
-  if (Kind != TTI::SK_PermuteTwoSrc)
+  if (Kind != TTI::SK_PermuteTwoSrc) {
+    int SplatIdx = PoisonMaskElem;
+    if (!Mask.empty() && all_of(Mask, [&](int Idx) {
+          if (Idx == PoisonMaskElem)
+            return true;
+          if (SplatIdx == PoisonMaskElem) {
+            SplatIdx = Idx;
+            return true;
+          }
+          return SplatIdx == Idx;
+        }))
+      return TTI.getShuffleCost(TTI::SK_Broadcast, Tp, Mask, CostKind, Index,
+                                SubTp, Args);
     return TTI.getShuffleCost(Kind, Tp, Mask, CostKind, Index, SubTp, Args);
+  }
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RKSimon wrote:

Can we not just improve improveShuffleKindFromMask to handle this?

https://github.com/llvm/llvm-project/pull/115201