[llvm] [VectorCombine] Refine cost model and decision logic in foldSelectShuffle (PR #146694)

[David Green via llvm-commits]

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@@ -3297,15 +3338,80 @@ bool VectorCombine::foldSelectShuffle(Instruction &I, bool FromReduction) {
            TTI.getShuffleCost(TTI::SK_PermuteTwoSrc, VT, VT, Mask, CostKind);
   };
 
+  unsigned ElementSize = VT->getElementType()->getPrimitiveSizeInBits();
+  unsigned MaxVectorSize =
+      TTI.getRegisterBitWidth(TargetTransformInfo::RGK_FixedWidthVector);
+  unsigned MaxElementsInVector = MaxVectorSize / ElementSize;
+  // When there are multiple shufflevector operations on the same input,
+  // especially when the vector length is larger than the register size,
+  // identical shuffle patterns may occur across different groups of elements.
+  // To avoid overestimating the cost by counting these repeated shuffles more
+  // than once, we only account for unique shuffle patterns. This adjustment
+  // prevents inflated costs in the cost model for wide vectors split into
+  // several register-sized groups.
+  std::set<SmallVector<int, 4>> UniqueShuffles;
+  auto AddShuffleMaskAdjustedCost = [&](InstructionCost C, ArrayRef<int> Mask) {
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davemgreen wrote:

Is it worth moving this into TTI, so that it can be overridden by the target? A function that calculates the cost of multiple shuffles at the same time?

https://github.com/llvm/llvm-project/pull/146694