[PATCH] D144732: [Assignment Tracking] Elide a map copy in some cases

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

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Restructure `AssignmentTrackingLowering::join` to avoid a map copy in the case where `BB` has more than one pred.

We only need to perform a copy of a pred `LiveOut` if there's exactly one already-visited pred (`Result = PredLiveOut`). With more than one pred the result is built by calling `Result = join(std::move(Result), PredLiveOut)` for each subsequent pred, where `join` parameters are `const &`. i.e. with more than 1 pred we can avoid copying by referencing the first two pred `LiveOut`s in the first `join` and then using a move + reference for the rest.

This gives marginal gains in compile time for CTMark LTO-g builds (geomean reduction of 0.05% instructions retired).


https://reviews.llvm.org/D144732

Files:
  llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

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