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

[Florian Hahn via llvm-commits]

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@@ -3149,12 +3150,35 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
       // the trip count but the scalable factor does not, use the fixed-width
       // factor in preference to allow the generation of a non-predicated loop.
       if (EpilogueLoweringStatus == CM_EpilogueNotAllowedLowTripLoop &&
-          NoScalarEpilogueNeeded(MaxFactors.FixedVF.getFixedValue())) {
+          NoScalarEpilogueNeeded(MaxFactors.FixedVF.getFixedValue(),
+                                 EffectiveIC)) {
         LLVM_DEBUG(dbgs() << "LV: Picking a fixed-width so that no tail will "
                              "remain for any chosen VF.\n");
         MaxFactors.ScalableVF = ElementCount::getScalable(0);
         return MaxFactors;
       }
+      // Allow cases where the ExactTC == VF + 1. VF can be any power of
+      // 2 between 2 and MaxVF.
+      //
+      // This produces 1 vector iteration, and 1 scalar iteration with
+      // no remainder. Later passes will eliminate the loop and leave
+      // straight-line code as the both iteration counts are statically known.
+      ElementCount ExactTC = getSmallConstantTripCount(PSE.getSE(), TheLoop);
+      if (EpilogueLoweringStatus == CM_EpilogueNotAllowedLowTripLoop &&
+          ExactTC.getFixedValue() > 1) {
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fhahn wrote:

`getSmallConstantTripCount` can return a scalable value I think, for loops with a `vscale` based trip count. Can we ever reach this code path here with such loops?

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