[llvm] [VPlan] Add vputils::recipesOnly that returns a downcasted range (NFC) (PR #222770)

[Ramkumar Ramachandra via llvm-commits]

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@@ -166,6 +166,15 @@ inline VPRecipeBase *findRecipe(VPValue *Start, PredT Pred) {
   return nullptr;
 }
 
+/// Returns an iterator range over \p Range which only includes \p RecipeTy
+/// recipes. The accesses are casted to \p RecipeTy.
+template <typename RecipeTy, typename T> auto recipesOnly(T &&Range) {
+  auto Filter = make_filter_range(std::forward<T>(Range),
+                                  [](auto &R) { return isa<RecipeTy>(R); });
+  return map_range(Filter,
+                   [](auto &R) -> RecipeTy & { return cast<RecipeTy>(R); });
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artagnon wrote:

> I don't think we're calling to_vector anywhere.

Right, I meant that a hypothetical?

> map_range just lazily calls the function on each element without any extra storage IIUC.

Yep, I think so: same story with make_filter_range -- it's a lot of template magic, and from memory, chaining too many of them would result in a compiler crash, due to a hard maximum-depth-of-templates-limit?

https://github.com/llvm/llvm-project/pull/222770