[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[Simon Pilgrim via llvm-commits]

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@@ -8327,6 +8597,48 @@ void BoUpSLP::buildTree_rec(ArrayRef<Value *> VL, unsigned Depth,
            S.getMainOp()) &&
        !all_of(VL, isVectorLikeInstWithConstOps)) ||
       NotProfitableForVectorization(VL)) {
+    if (!S) {
+      Instruction *MainOp = nullptr;
+      Instruction *AltOp = nullptr;
+      for (Value *V : VL) {
+        if (isa<PoisonValue>(V))
+          continue;
+        auto *I = dyn_cast<Instruction>(V);
+        if (!I) {
+          MainOp = AltOp = nullptr;
+          break;
+        }
+        if (!MainOp) {
+          MainOp = I;
+          continue;
+        }
+        if (MainOp->getOpcode() == I->getOpcode()) {
+          if (I->getParent() != MainOp->getParent()) {
+            MainOp = AltOp = nullptr;
+            break;
+          }
+          continue;
+        }
+        if (!AltOp) {
+          AltOp = I;
+          continue;
+        }
+        if (AltOp->getOpcode() == I->getOpcode()) {
+          if (I->getParent() != AltOp->getParent()) {
+            MainOp = AltOp = nullptr;
+            break;
+          }
+          continue;
+        }
+        MainOp = AltOp = nullptr;
+        break;
+      }
+      // Last chance to try to vectorize alternate node.
+      constexpr unsigned SmallNodeSize = 4;
----------------
RKSimon wrote:

Pull out these duplicate thresholds?

https://github.com/llvm/llvm-project/pull/123360