[llvm] [LoopVectorize] Improve Vectorization of Low Trip Count Loops (PR #195823)

[Jack Styles via llvm-commits]

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@@ -6084,6 +6108,42 @@ LoopVectorizationPlanner::computeBestVF() {
     return {VectorizationFactor::Disabled(), nullptr};
   // If there is a single VPlan with a single VF, return it directly.
   VPlan &FirstPlan = *VPlans[0];
+  auto IsUnprofitableOneScalarTail =
+      [&](const VectorizationFactor &CurrentFactor, bool HasTail,
+          bool ForceVectorization, const ElementCount &ExactTC,
+          const VectorizationFactor &ScalarFactor,
+          const InstructionCost &ScalarCost) {
+        if (ForceVectorization || !HasTail || !ExactTC.isFixed() ||
+            CurrentFactor.Width.isScalable())
+          return false;
+
+        unsigned TC = ExactTC.getFixedValue();
+        if (TC == 0 || TC > TTI.getMinTripCountTailFoldingThreshold())
+          return false;
+
+        unsigned EstimatedWidth = estimateElementCount(
+            CurrentFactor.Width, Config.getVScaleForTuning());
+        if (TC != EstimatedWidth + 1)
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Stylie777 wrote:

Yes it should be considered if the user has set an IC. I've added this and a test to support this.

https://github.com/llvm/llvm-project/pull/195823