[llvm] [LLVM][LV] Improve UF calculation for vscale based scalar loops. (PR #146102)

[Paul Walker via llvm-commits]

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@@ -4813,16 +4839,19 @@ LoopVectorizationCostModel::selectInterleaveCount(VPlan &Plan, ElementCount VF,
       MaxInterleaveCount = ForceTargetMaxVectorInterleaveFactor;
   }
 
-  unsigned EstimatedVF = getEstimatedRuntimeVF(VF, VScaleForTuning);
-
   // Try to get the exact trip count, or an estimate based on profiling data or
-  // ConstantMax from PSE, failing that.
-  if (auto BestKnownTC = getSmallBestKnownTC(PSE, TheLoop)) {
+  // ConstantMax from PSE, failing that. For fixed length VFs treat a scalable
+  // trip count as if unknown.
+  if (auto BestKnownTC = getSmallBestKnownTC(PSE, TheLoop);
+      BestKnownTC && (BestKnownTC->isFixed() || VF.isScalable())) {
+    // Re-evaluate trip counts and VFs to be in the same numerical space.
+    unsigned AvailableTC = getEstimatedRuntimeVF(*BestKnownTC, VScaleForTuning);
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paulwalker-arm wrote:

I've renamed the function `estimateElementCount` to better represent the action.

https://github.com/llvm/llvm-project/pull/146102