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

[via llvm-commits]

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@@ -298,9 +293,13 @@ InstructionCost VPRecipeBase::cost(ElementCount VF, VPCostContext &Ctx) {
     return 0;
 
   InstructionCost RecipeCost = computeCost(VF, Ctx);
-  if (UI && ForceTargetInstructionCost.getNumOccurrences() > 0 &&
-      RecipeCost.isValid())
+  if (ForceTargetInstructionCost.getNumOccurrences() > 0 &&
+      (RecipeCost.isValid() && RecipeCost != InstructionCost::getMax()))
     RecipeCost = InstructionCost(ForceTargetInstructionCost);
+  // Max cost 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:

Would be good to indicate why such a sentinel is needed, rather than detecting the absence of an underlying instruction by directly checking UI, as done now?

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