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

[Simon Pilgrim via llvm-commits]

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@@ -259,6 +259,33 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
     return AddrExtractCost + MemoryOpCost + PackingCost + ConditionalCost;
   }
 
+  /// Checks if the provided mask \p is a splat mask, i.e. it contains only -1
+  /// or same non -1 index value and this index value contained at least twice.
+  /// So, mask <0, -1,-1, -1> is not considered splat (it is just identity),
+  /// same for <-1, 0, -1, -1> (just a slide), while <2, -1, 2, -1> is a splat
+  /// with \p Index=2.
+  static bool isSplatMask(ArrayRef<int> Mask, unsigned NumSrcElts, int &Index) {
+    // Check that the broadcast index meets at least twice.
+    bool IsCompared = false;
+    if (int SplatIdx = PoisonMaskElem;
+        all_of(enumerate(Mask), [&](const auto &P) {
+          if (P.value() == PoisonMaskElem)
+            return P.index() != Mask.size() - 1 || IsCompared;
+          if (static_cast<unsigned>(P.value()) >= NumSrcElts * 2)
+            return false;
+          if (SplatIdx == PoisonMaskElem) {
+            SplatIdx = P.value();
+            return P.index() != Mask.size() - 1;
+          }
+          IsCompared = true;
+          return SplatIdx == P.value();
+        })) {
+      Index = SplatIdx;
+      return true;
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RKSimon wrote:

This should only update Index and return true if SplatIdx != PoisonMaskElem

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