[llvm] [SLP] Compute a shuffle mask for getGatherCost (PR #85330)

[Alexey Bataev via llvm-commits]

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@@ -10526,6 +10527,7 @@ InstructionCost BoUpSLP::getGatherCost(ArrayRef<Value *> VL,
     if (!UniqueElements.insert(V).second) {
       DuplicateNonConst = true;
       ShuffledElements.setBit(I);
+      ShuffleMask[I] = I;
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alexey-bataev wrote:

This does not look correct. You generated identity mask here, which is not. Need to record the position of the first unique value and then use it here as index.

https://github.com/llvm/llvm-project/pull/85330