[llvm] [VectorCombine] Refine cost model and decision logic in foldSelectShuffle (PR #146694)

[David Green via llvm-commits]

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@@ -3055,6 +3055,47 @@ bool VectorCombine::foldCastFromReductions(Instruction &I) {
   return true;
 }
 
+/// Returns true if this ShuffleVectorInst eventually feeds into a
+/// vector reduction intrinsic (e.g., vector_reduce_add) by only following
+/// chains of shuffles and binary operators (in any combination/order).
+static bool feedsIntoVectorReduction(ShuffleVectorInst *SVI) {
+  SmallPtrSet<Instruction *, 8> Visited;
+  SmallVector<Instruction *, 4> WorkList;
+  bool FoundReduction = false;
+
+  WorkList.push_back(SVI);
+  while (!WorkList.empty()) {
+    Instruction *I = WorkList.pop_back_val();
+    for (User *U : I->users()) {
+      auto *UI = dyn_cast<Instruction>(U);
+      if (!UI || !Visited.insert(UI).second)
+        continue;
+      if (auto *II = dyn_cast<IntrinsicInst>(UI)) {
+        // More than one reduction reached
+        if (FoundReduction)
+          return false;
+        switch (II->getIntrinsicID()) {
+        case Intrinsic::vector_reduce_add:
+        case Intrinsic::vector_reduce_mul:
+        case Intrinsic::vector_reduce_and:
+        case Intrinsic::vector_reduce_or:
+        case Intrinsic::vector_reduce_xor:
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davemgreen wrote:

What about the other kinds?

https://github.com/llvm/llvm-project/pull/146694