[llvm] [SLP]Support revectorization of the previously vectorized scalars (PR #133091)

[Alexey Bataev via llvm-commits]

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@@ -9253,14 +9253,44 @@ void BoUpSLP::buildTree_rec(ArrayRef<Value *> VL, unsigned Depth,
   // We now know that this is a vector of instructions of the same type from
   // the same block.
 
-  // Check that none of the instructions in the bundle are already in the tree.
-  for (Value *V : VL) {
-    if ((!IsScatterVectorizeUserTE && !isa<Instruction>(V)) ||
-        doesNotNeedToBeScheduled(V))
-      continue;
-    if (isVectorized(V)) {
-      LLVM_DEBUG(dbgs() << "SLP: The instruction (" << *V
-                        << ") is already in tree.\n");
+  // Check that none of the instructions in the bundle are already in the tree
+  // and the node may be not profitable for the vectorization as the small
+  // alternate node.
+  if (S && S.isAltShuffle()) {
+    auto GetNumVectorizedExtracted = [&]() {
+      unsigned NumVectorized = 0;
+      unsigned NumExtracted = 0;
+      for (Value *V : VL) {
+        auto *I = dyn_cast<Instruction>(V);
+        if (!I || doesNotNeedToBeScheduled(V) ||
+            all_of(I->operands(), [&](const Use &U) {
+              return isa<ExtractElementInst>(U.get());
+            }))
+          continue;
+        if (isVectorized(V))
+          ++NumVectorized;
+        else if (!V->hasOneUser() && !areAllUsersVectorized(I, UserIgnoreList))
+          ++NumExtracted;
+      }
+      return std::make_pair(NumVectorized, NumExtracted);
+    };
+    auto [NumVectorized, NumExtracted] = GetNumVectorizedExtracted();
+    constexpr TTI::TargetCostKind Kind = TTI::TCK_RecipThroughput;
+    bool PreferScalarize = NumVectorized > 0 && VL.size() == 2;
+    if (NumVectorized > 0 && !PreferScalarize) {
+      // Rough cost estimation, if the vector code (+ potential extracts) is
+      // more profitable than the scalar + buildvector.
+      InstructionCost VectorizeCostEstimate =
+          getShuffleCost(*TTI, TTI::SK_PermuteTwoSrc,
+                         getWidenedType(VL.front()->getType(), VL.size()), {},
+                         Kind) +
+          NumExtracted;
+      InstructionCost ScalarizeCostEstimate = VL.size() - NumVectorized;
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alexey-bataev wrote:

What do you expect here?

https://github.com/llvm/llvm-project/pull/133091