[llvm] [VPlan] Return Max from computeCost without underlying instrs. (PR #109708)

[via llvm-commits]

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@@ -844,7 +844,9 @@ class VPRecipeBase : public ilist_node_with_parent<VPRecipeBase, VPBasicBlock>,
 protected:
   /// Compute the cost of this recipe either using a recipe's specialized
   /// implementation or using the legacy cost model and the underlying
-  /// instructions.
+  /// instructions. Returns InstructionCost::max() if the cost of this recipe
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ayalz wrote:

> Alternatively computeCost could return an optional cost.

Perhaps better to return no cost at all, as in optional, when cost should be ignored, rather than a max sentinel. The latter requires additional attention to remain unmodified, preventing (mistaken) updates.

https://github.com/llvm/llvm-project/pull/109708