[llvm] [VPlan] Introduce chainUsesScalarValues (PR #158377)

[Ramkumar Ramachandra via llvm-commits]

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@@ -30,6 +30,30 @@ bool vputils::onlyScalarValuesUsed(const VPValue *Def) {
                 [Def](const VPUser *U) { return U->usesScalars(Def); });
 }
 
+bool vputils::chainUsesScalarValues(const VPValue *Root) {
+  SmallVector<std::pair<const VPValue *, const VPUser *>> Worklist;
+  for (const VPUser *V : Root->users())
+    Worklist.emplace_back(Root, V);
+  while (!Worklist.empty()) {
+    auto [Op, U] = Worklist.pop_back_val();
+    if (isa<VPWidenRecipe, VPWidenCastRecipe, VPWidenCallRecipe,
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artagnon wrote:

Certain widens cannot be narrowed as they don't have an underlying value that would permit us to turn them into single-scalars: for example, VPWidenCast and VPWidenIntrinsic. All the usesScalars are correct as they are written; I think you meant onlyScalarvaluesUsed in VPlanUtils, which is used by more than just optimizations: it is also used in the execution of recipes, and chainUsesScalars is a version that's suitable for optimizations like narrowToSingleScalars.

https://github.com/llvm/llvm-project/pull/158377