[llvm] [VPlan] Only use selectVectorizationFactor for cross-check (NFCI). (PR #103033)

[via llvm-commits]

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@@ -7300,7 +7283,20 @@ ElementCount LoopVectorizationPlanner::computeBestVF() {
         ProfitableVFs.push_back(CurrentFactor);
     }
   }
-  return BestFactor.Width;
+
+#ifndef NDEBUG
+  // Select the optimal vectorization factor according to the legacy cost-model.
+  // This is now only used to verify the decisions by the new VPlan-based
+  // cost-model and will be retired once the VPlan-based cost-model is
+  // stabilized.
+  VectorizationFactor LegacyVF = selectVectorizationFactor();
+  assert(BestFactor.Width == LegacyVF.Width &&
+         " VPlan cost model and legacy cost model disagreed");
+  assert((BestFactor.Width.isScalar() || BestFactor.ScalarCost > 0) &&
+         "when vectorizing, the scalar cost must be non-zero.");
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ayalz wrote:

nit:
```suggestion
         "when vectorizing, the scalar cost must be computed.");
```
zero cost represents uncomputed cost.

Perhaps worth having `InstructionCost::IsComputed()` instead of checking zero, distinct from IsValid()?

https://github.com/llvm/llvm-project/pull/103033