[lld][BP] Fix nondeterministic function order by using MapVector (#179841) 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. GitOrigin-RevId: 9f4e8bca0394972bed475bf3046a55744a5794a5
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