[llvm] [VPlan] Use BlockFrequencyInfo in getPredBlockCostDivisor (PR #158690)

[Luke Lau via llvm-commits]

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@@ -2886,6 +2884,23 @@ bool LoopVectorizationCostModel::isPredicatedInst(Instruction *I) const {
   }
 }
 
+unsigned LoopVectorizationCostModel::getPredBlockCostDivisor(
+    TargetTransformInfo::TargetCostKind CostKind, const BasicBlock *BB) const {
+  // If the block wasn't originally predicated then return early to avoid
+  // computing BlockFrequencyInfo unnecessarily.
+  if (!Legal->blockNeedsPredication(BB))
+    return 1;
+  if (CostKind == TTI::TCK_CodeSize)
+    return 1;
+
+  BlockFrequencyInfo *BFI = GetBFI();
+  uint64_t HeaderFreq = BFI->getBlockFreq(TheLoop->getHeader()).getFrequency();
+  uint64_t BBFreq = BFI->getBlockFreq(BB).getFrequency();
+  assert(HeaderFreq >= BBFreq &&
+         "Header has smaller block freq than dominated BB?");
+  return HeaderFreq / BBFreq;
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lukel97 wrote:

Exactly, as an aside I added that assert in because when I first started this PR there was a bug with the pass manager that meant the header sometimes had a smaller frequency, which led to divisions by zero. It was fixed in 4663d2521c65827ca22884e12a96ddd437377e31

https://github.com/llvm/llvm-project/pull/158690