[llvm] LoopVectorize: Use a better heuristic for epilogue branch weights (PR #72589)

[Matthias Braun via llvm-commits]

================
@@ -8079,8 +8075,16 @@ EpilogueVectorizerEpilogueLoop::emitMinimumVectorEpilogueIterCountCheck(
 
   BranchInst &BI =
       *BranchInst::Create(Bypass, LoopVectorPreHeader, CheckMinIters);
-  if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator()))
-    setBranchWeights(BI, EpilogueMinItersBypassWeights);
+  if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator())) {
+    // Assume the tripcount for the epilogue loop is equally distributed between
+    // 0 and `VectorTripCount - 1` leaving us with a `VectorTripCount * 0.5`
+    // branch weight.
+    unsigned CombinedFactor = UF * VF.getKnownMinValue();
+    assert(CombinedFactor > 1 && "UF times VF should be bigger than one");
+    unsigned EstimatedTripCount = (CombinedFactor + 1) / 2;
+    uint32_t Weights[] = {1, EstimatedTripCount};
----------------
MatzeB wrote:

Thanks for catching this!

https://github.com/llvm/llvm-project/pull/72589