[llvm] [VPlan] Add VPPhiAccessors to provide interface for phi recipes (NFC) (PR #129388)

[Florian Hahn via llvm-commits]

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@@ -1166,6 +1166,30 @@ class VPIRInstruction : public VPRecipeBase {
   void extractLastLaneOfFirstOperand(VPBuilder &Builder);
 };
 
+/// Helper type to provide functions to access incoming values and blocks for
+/// phi-like recipes. RecipeTy must be a sub-class of VPRecipeBase.
+template <typename RecipeTy> class VPPhiAccessors {
+  /// Return a VPRecipeBase* to the current object.
+  const VPRecipeBase *getAsRecipe() const {
+    return static_cast<const RecipeTy *>(this);
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fhahn wrote:

Yep it seems so, problem is that w/o the template there's no inheritance relation here, and static_casts are rejected (Curiously Recurring Template Pattern). For some reason, we need to cast exactly to the type.

But the template parameter may cause problems in the future, so I updated `getAsRecipe` to be pure virtual to be implemented by the derived classes.  The single implementation could also be used for other trait classes in the future. WDYT?

https://github.com/llvm/llvm-project/pull/129388