[llvm] [SLP] Check for extracts, being replaced by original scalars, for user nodes (PR #149572)

[Gaëtan Bossu via llvm-commits]

================
@@ -9151,6 +9165,93 @@ getVectorCallCosts(CallInst *CI, FixedVectorType *VecTy,
   return {IntrinsicCost, LibCost};
 }
 
+/// Check if extracts are cheaper than the original scalars.
+static bool
+areExtractsCheaperThanScalars(TargetTransformInfo &TTI, Type *UserScalarTy,
+                              VectorType *UserVecTy, const APInt &DemandedElts,
+                              const InstructionCost UserScalarsCost,
+                              Type *ScalarTy, unsigned VF, ArrayRef<int> Mask,
+                              InstructionCost UserEntryCost) {
+  constexpr TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
+  // If extracts are cheaper than the original scalars - success.
+  InstructionCost ExtractCost =
+      ::getScalarizationOverhead(TTI, UserScalarTy, UserVecTy, DemandedElts,
+                                 /*Insert=*/false, /*Extract=*/true, CostKind);
+  if (ExtractCost <= UserScalarsCost)
+    return true;
+  // The node is profitable for vectorization - success.
+  if (ExtractCost <= UserEntryCost)
+    return true;
----------------
gbossu wrote:

I understand the first `ExtractCost <= UserScalarsCost` check, because if the lanes are already extracted from the user `TreeEntry`, then there's no need to extract or scalarize operands.

But the check below is still a bit of a mystery to me:
```
  // The node is profitable for vectorization - success.
  if (ExtractCost <= UserEntryCost)
    return true;
```
Which node are we talking about? The user node has already been vectorized, and this is checking nothing about the to-be-created node, so I feel this check should be removed.

https://github.com/llvm/llvm-project/pull/149572