[Bolt] Fix integer division bug in computeEdgeWeights fallback weights #152880
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The fallback edge weight calculation in computeEdgeWeights incorrectly used integer division when computing weights as `1 / numChildren`. This caused zero weights for fallback cases when `numChildren > 1`, leading to incorrect edge weight assignments in the absence of profile data. This fix improves correctness and stability of edge weight computations by changing the divisor to floating-point to ensure correct fallback weights.
noclowns requested review from aaupov, ayermolo, maksfb, paschalis-mpeis, rafaelauler, yota9 and yozhu as code owners 
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| @llvm/pr-subscribers-bolt Author: Slava Gurevich (noclowns) ChangesThe fallback edge weight calculation in Bolt computeEdgeWeights() incorrectly used integer division when computing weights as This fix improves correctness and stability of edge weight computations by changing the divisor to floating-point to ensure correct fallback weights calculation. Full diff: https://github.com/llvm/llvm-project/pull/152880.diff 1 Files Affected:
diff --git a/bolt/lib/Passes/MCF.cpp b/bolt/lib/Passes/MCF.cpp index 4f3a964fd3230..38350efa2a730 100644 --- a/bolt/lib/Passes/MCF.cpp +++ b/bolt/lib/Passes/MCF.cpp @@ -153,7 +153,7 @@ void computeEdgeWeights(BinaryBasicBlock *BB, EdgeWeightMap &EdgeWeights) { E = GraphT::child_end(BB); CI != E; ++CI) { typename GraphT::NodeRef Child = *CI; - double Weight = 1 / (GraphT::child_end(BB) - GraphT::child_begin(BB)); + double Weight = 1.0 / (GraphT::child_end(BB) - GraphT::child_begin(BB)); if (TotalChildrenCount != 0.0) Weight = ChildrenExecCount[ChildIndex] / TotalChildrenCount; updateEdgeWeight<NodeT>(EdgeWeights, BB, Child, Weight); |
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| @aaupov could you please take a look when you get a chance? Thanks. |
maksfb reviewed Aug 27, 2025
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maksfb left a comment
maksfb left a comment There was a problem hiding this comment.
Thanks for the fix! Could you please add a test case?
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The fallback edge weight calculation in Bolt computeEdgeWeights() incorrectly used integer division when computing weights as
1 / numChildren. This caused zero weights for fallback cases whennumChildren > 1, leading to incorrect edge weight assignments in the absence of profile data.This fix improves correctness and stability of edge weight computations by changing the division to floating-point to ensure correct fallback weights calculation.