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

[Ramkumar Ramachandra via llvm-commits]

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@@ -1803,7 +1811,7 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
           Value *V = IL.first;
           return !V || V == FrontV;
         })) {
-      SplatLeafs.insert(FrontV);
+      ConstLeafs.insert(FrontV);
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artagnon wrote:

Isn't this an ugly special case? Can't you handle constant splats in the cost code?

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