[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;
+  auto *VecTy = getWidenedType(ScalarTy, VF);
+  InstructionCost ScalarsCost =
+      ::getScalarizationOverhead(TTI, ScalarTy, VecTy, APInt::getAllOnes(VF),
+                                 /*Insert=*/true, /*Extract=*/false, CostKind);
+  if (!Mask.empty())
+    ScalarsCost +=
+        getShuffleCost(TTI, TTI::SK_PermuteSingleSrc, VecTy, Mask, CostKind);
+  return ExtractCost < UserScalarsCost + ScalarsCost;
----------------
gbossu wrote:

I think that function would be more readable if it had a single `return` statement. E.g. something like:
```
return UserExtractCost < UserScalarsCost + OperandScalarizationCost;
```

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