[lld][BP] Fix nondeterministic function order by using MapVector (#179841)

ellishg · web-flow · commit 9f4e8bca0394 · 2026-02-06T09:49:37.000-08:00
There are two places where the nondeterministic iteration order of `DenseMap` (the original type of `startupSectionIdxUNs`) causes the output function order to be nondeterministic. https://github.com/llvm/llvm-project/blob/16e2e7f59134e63810228c9a0dc990bfd96f9a1f/lld/include/lld/Common/BPSectionOrdererBase.inc#L240-L245 https://github.com/llvm/llvm-project/blob/16e2e7f59134e63810228c9a0dc990bfd96f9a1f/lld/include/lld/Common/BPSectionOrdererBase.inc#L267-L268 The fix is to use `MapVector` whose iteration order is guaranteed to be deterministic. To test, I built a large binary several times and observed this value no longer changes. https://github.com/llvm/llvm-project/blob/16e2e7f59134e63810228c9a0dc990bfd96f9a1f/lld/include/lld/Common/BPSectionOrdererBase.inc#L410-L411 It seems that this regresses linktime by a few seconds, but I believe the tradeoff is worthwhile.
diff --git a/lld/include/lld/Common/BPSectionOrdererBase.inc b/lld/include/lld/Common/BPSectionOrdererBase.inc
@@ -161,7 +161,7 @@ auto BPOrderer<D>::computeOrder(
     sectionToIdx.try_emplace(isec, i);
 
   BPFunctionNode::UtilityNodeT maxUN = 0;
-  DenseMap<unsigned, UtilityNodes> startupSectionIdxUNs;
+  MapVector<unsigned, UtilityNodes> startupSectionIdxUNs;
   // Used to define the initial order for startup functions.
   DenseMap<unsigned, size_t> sectionIdxToTimestamp;
   std::unique_ptr<InstrProfReader> reader;
@@ -177,7 +177,7 @@ auto BPOrderer<D>::computeOrder(
     }
     auto &traces = reader->getTemporalProfTraces();
 
-    DenseMap<unsigned, BPFunctionNode::UtilityNodeT> sectionIdxToFirstUN;
+    MapVector<unsigned, BPFunctionNode::UtilityNodeT> sectionIdxToFirstUN;
     for (size_t traceIdx = 0; traceIdx < traces.size(); traceIdx++) {
       uint64_t currentSize = 0, cutoffSize = 1;
       size_t cutoffTimestamp = 1;
