[llvm] [VPlan] Record estimated branch probabilities on VPlan0 for cost modeling (PR #216172)

[Luke Lau via llvm-commits]

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@@ -5347,8 +5348,19 @@ void VPCostContext::invalidateWideningDecision(Instruction *I,
                          LoopVectorizationCostModel::CM_InvalidatedDecision, 0);
 }
 
-uint64_t VPCostContext::getPredBlockCostDivisor(BasicBlock *BB) const {
-  return CM.getPredBlockCostDivisor(CostKind, BB);
+uint64_t
+VPCostContext::getPredBlockCostDivisor(const VPRegionBlock *Region) const {
+  if (CostKind == TTI::TCK_CodeSize)
+    return 1;
+  std::optional<VPExecutionFrequency> Freq =
+      Region->getEntryBranchOnMask()->getExecutionFrequency();
+  if (!Freq)
+    return 1;
+  // A recorded frequency is neither zero nor always-executing, so the
+  // probability is non-zero and the division below is safe.
+  return divideNearest(
+      BranchProbability::getDenominator(),
+      vputils::getExecutionProbability(Freq->Freq).getNumerator());
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lukel97 wrote:

IIUC BlockFrequencyInfo is now only used for the legacy cost model + widening decisions? Given that we now have our own VPlan BFI (computeExecutionFrequencies) I presume we'll want to eventually drop the dependency on BFI. 

https://github.com/llvm/llvm-project/pull/216172