[Bolt] Fix integer division bug in computeEdgeWeights fallback weights

[noclowns]

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 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 division to floating-point to ensure correct fallback weights calculation.

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Slava Gurevich (noclowns)

Changes

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 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 calculation.

---

Full diff: https://github.com/llvm/llvm-project/pull/152880.diff

1 Files Affected:

• (modified) bolt/lib/Passes/MCF.cpp (+1-1)

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);