[llvm] [VectorCombine] Add a cost model for shuffleToIdentity. (PR #93937)

[David Green via llvm-commits]

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@@ -1670,8 +1670,12 @@ bool VectorCombine::foldShuffleOfShuffles(Instruction &I) {
 
 using InstLane = std::pair<Value *, int>;
 
-static InstLane lookThroughShuffles(Value *V, int Lane) {
+static InstLane
+lookThroughShuffles(Value *V, int Lane,
+                    SmallPtrSetImpl<Instruction *> *VisitedShuffles) {
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davemgreen wrote:

I think you mean generateInstLaneVectorFromOperand? This one is only called with non-null. There is only one call to generateInstLaneVectorFromOperand with nullptr, so I opted to keep it explicit. It does not feel like there is a lot to gain from adding the default.

https://github.com/llvm/llvm-project/pull/93937