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

[via llvm-commits]

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@@ -312,15 +311,19 @@ InstructionCost VPRecipeBase::cost(ElementCount VF, VPCostContext &Ctx) {
 InstructionCost VPRecipeBase::computeCost(ElementCount VF,
                                           VPCostContext &Ctx) const {
   // Compute the cost for the recipe falling back to the legacy cost model using
-  // the underlying instruction. If there is no underlying instruction, returns
-  // 0.
+  // the underlying instruction. If there is no underlying instruction or the
+  // cost is computed by the recipe's computeCost, returns
+  // InstructionCost::getMax. It is used as  a sentinel value to detect recipes
+  // without underlying instructions for which no forced target instruction cost
+  // should be applied.
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ayalz wrote:

```suggestion
  // without underlying instructions for which forced target instruction cost
  // should not be applied.
```

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