[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);
+  InstructionCost IdentityCost = TTI.getShuffleCost(
+      TTI::SK_PermuteSingleSrc, Ty, ExtractMask, TTI::TCK_RecipThroughput);
+  InstructionCost SplatCost = TTI.getShuffleCost(
+      TTI::SK_Broadcast, Ty, std::nullopt, TTI::TCK_RecipThroughput);
+  InstructionCost NewShuffleCost =
+      IdentityCost * IdentityLeafs.size() + SplatCost * SplatLeafs.size();
+  LLVM_DEBUG(dbgs() << "      vs     " << NewShuffleCost << " (" << IdentityCost
+                    << " * " << IdentityLeafs.size() << " + " << SplatCost
+                    << " * " << SplatLeafs.size() << ")\n");
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davemgreen wrote:

Whilst I appreciate succinct debug messages, it has been useful a couple of times already to be able to tell the different costs of splats vs identities on different platforms.

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