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

[Gaëtan Bossu via llvm-commits]

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@@ -9567,14 +9564,34 @@ 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()) {
+    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;
+    }
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gbossu wrote:

Maybe outline in a `countVectorizedEntires()` function to make the code more concise? It could e.g. return a `struct { unsigned NumVectorized, NumExtracted; }`

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