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

[David Green via llvm-commits]

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@@ -1878,6 +1893,31 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
   if (NumVisited <= 1)
     return false;
 
+  LLVM_DEBUG(dbgs() << "Found a set of shuffles that can be removed:\n");
+  InstructionCost OldShuffleCost;
+  for (auto *I : VisitedShuffles) {
+    InstructionCost C = TTI.getInstructionCost(I, TTI::TCK_RecipThroughput);
+    LLVM_DEBUG(dbgs() << C << *I << "\n");
+    OldShuffleCost += C;
+  }
+  LLVM_DEBUG(dbgs() << "  total cost " << OldShuffleCost << "\n");
+  SmallVector<int, 16> ExtractMask(Ty->getNumElements());
+  std::iota(ExtractMask.begin(), ExtractMask.end(), 0);
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davemgreen wrote:

I'm not sure what you mean. The iota fills it with an identity mask.

I've tried to rename the Mask name at least, that name was a copy-pasted from elsewhere.

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