[llvm] [LV] Improve code in selectInterleaveCount (NFC) (PR #128002)

[David Sherwood via llvm-commits]

================
@@ -4998,51 +4997,53 @@ LoopVectorizationCostModel::selectInterleaveCount(ElementCount VF,
   }
 
   unsigned EstimatedVF = getEstimatedRuntimeVF(VF, VScaleForTuning);
-  unsigned KnownTC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
-  if (KnownTC > 0) {
-    // At least one iteration must be scalar when this constraint holds. So the
-    // maximum available iterations for interleaving is one less.
-    unsigned AvailableTC =
-        requiresScalarEpilogue(VF.isVector()) ? KnownTC - 1 : KnownTC;
-
-    // If trip count is known we select between two prospective ICs, where
-    // 1) the aggressive IC is capped by the trip count divided by VF
-    // 2) the conservative IC is capped by the trip count divided by (VF * 2)
-    // The final IC is selected in a way that the epilogue loop trip count is
-    // minimized while maximizing the IC itself, so that we either run the
-    // vector loop at least once if it generates a small epilogue loop, or else
-    // we run the vector loop at least twice.
-
-    unsigned InterleaveCountUB = bit_floor(
-        std::max(1u, std::min(AvailableTC / EstimatedVF, MaxInterleaveCount)));
-    unsigned InterleaveCountLB = bit_floor(std::max(
-        1u, std::min(AvailableTC / (EstimatedVF * 2), MaxInterleaveCount)));
-    MaxInterleaveCount = InterleaveCountLB;
-
-    if (InterleaveCountUB != InterleaveCountLB) {
-      unsigned TailTripCountUB =
-          (AvailableTC % (EstimatedVF * InterleaveCountUB));
-      unsigned TailTripCountLB =
-          (AvailableTC % (EstimatedVF * InterleaveCountLB));
-      // If both produce same scalar tail, maximize the IC to do the same work
-      // in fewer vector loop iterations
-      if (TailTripCountUB == TailTripCountLB)
-        MaxInterleaveCount = InterleaveCountUB;
-    }
-  } else if (BestKnownTC && *BestKnownTC > 0) {
+
+  // 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)) {
     // At least one iteration must be scalar when this constraint holds. So the
     // maximum available iterations for interleaving is one less.
     unsigned AvailableTC = requiresScalarEpilogue(VF.isVector())
                                ? (*BestKnownTC) - 1
----------------
david-arm wrote:

Is there a problem here because `BestKnownTC` can be 0, and you'll end up with AvailableTC = -1? There is a similar concern for an else condition below. You could solve both problems by rewriting the code above to be:

```
  auto BestKnownTC = getSmallBestKnownTC(PSE, TheLoop);
  if (BestKnownTC && *BestKnownTC > 0)
```

or presumably given the rules around overloaded operators for std::opt you can also just do

```
  auto BestKnownTC = getSmallBestKnownTC(PSE, TheLoop);
  if (BestKnownTC > 0)
```
?

https://github.com/llvm/llvm-project/pull/128002