[llvm] [SLP] Relax store chain user limit to 2 (PR #215700)

[Alexey Bataev via llvm-commits]

================
@@ -28679,13 +28679,39 @@ SLPVectorizerPass::vectorizeStoreChainImpl(ArrayRef<Value *> Chain, BoUpSLP &R,
     // only merges the stores, while the scalars remain live for the other users
     // and all the lanes are gathered back. A single outside use may still be a
     // part of the larger vectorizable graph, same for the values, fed by the
-    // loads, where the vector loads may pay off the gathering.
+    // loads, where the vector loads may pay off the gathering. Two outside uses
+    // may still be part of the same tree when they are local carry deps
+    // (cmp/add in the same block); reject 3+ outside users or 2 non-local
+    // outside users.
+    auto IsBenignOutsideUser = [](Instruction *I, User *U) {
+      auto *UI = dyn_cast<Instruction>(U);
+      if (!UI || UI->getParent() != I->getParent())
+        return false;
+      return isa<CmpInst, BinaryOperator, SelectInst>(UI);
+    };
+    auto HasTooManyOutsideUsers = [&](Instruction *I) {
+      // Cap the walk: more than one store use + two outside users is enough to
+      // reject, so skip values with clearly too many uses.
+      if (I->hasNUsesOrMore(Stores.size() + 3))
----------------
alexey-bataev wrote:

```suggestion
      if (I->hasNUsesOrMore(UsesLimit))
```

https://github.com/llvm/llvm-project/pull/215700