[llvm] [SLP]: Introduce and use getDataFlowCost (PR #112999)

[Matt Arsenault via llvm-commits]

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@@ -9044,6 +9044,51 @@ static SmallVector<Type *> buildIntrinsicArgTypes(const CallInst *CI,
   return ArgTys;
 }
 
+// The cost model may determine that vectorizing and eliminating a series of
+// ExtractElements is beneficial. However, if the input vector is a function
+// argument, the calling convention may require extractions in the geneerated
+// code. In this scenario, vectorizaino would then not eliminate the
+// ExtractElement sequence, but would add additional vectorization code.
+// getCCCostFromScalars does the proper accounting for this.
+static unsigned getCCCostFromScalars(ArrayRef<Value *> &Scalars,
+                                     unsigned ScalarSize,
+                                     TargetTransformInfo *TTI) {
+  SetVector<Value *> ArgRoots;
+  for (unsigned I = 0; I < ScalarSize; I++) {
+    auto *Scalar = Scalars[I];
+    if (!Scalar)
+      continue;
+    auto *EE = dyn_cast<ExtractElementInst>(Scalar);
+    if (!EE)
+      continue;
+
+    auto *Vec = EE->getOperand(0);
+    if (!Vec->getType()->isVectorTy())
+      continue;
+
+    auto F = EE->getFunction();
+    auto FoundIt = find_if(
+        F->args(), [&Vec](Argument &I) { return Vec == cast<Value>(&I); });
+
+    if (FoundIt == F->arg_end())
+      continue;
+
+    if (!ArgRoots.contains(Vec))
+      ArgRoots.insert(Vec);
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arsenm wrote:

You can just insert into the set, you don't need to precheck contains 

https://github.com/llvm/llvm-project/pull/112999