[llvm] [VPlan] Thread branch weights into VPlan and use them for cost (NFCI) (PR #209309)

[Luke Lau via llvm-commits]

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@@ -2514,9 +2530,26 @@ uint64_t LoopVectorizationCostModel::getPredBlockCostDivisor(
   uint64_t BBFreq = getBFI().getBlockFreq(BB).getFrequency();
   assert(HeaderFreq >= BBFreq &&
          "Header has smaller block freq than dominated BB?");
+  // Treat a zero frequency (e.g. a block BFI considers unreachable) as
+  // always-executed, avoiding a division by zero.
+  if (BBFreq == 0)
+    return 1;
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lukel97 wrote:

Just to check, is this needed now because we eagerly compute block frequencies during construction, before unreachable blocks have been deleted?

https://github.com/llvm/llvm-project/pull/209309