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...erializing between the LTO optimization pipeline and the LTO codegen pipeline. On SPEC, serializing to/from bitcode will change the output executable in 33 benchmarks. If you use `-preserve-bc-use-list-order`, all executables match. How? ==== Have a look at `BitcodeReader::ParseUseLists()` and `predictUseListOrder()` in `lib/Bitcode/Writer/ValueEnumerator.cpp`. Does it degrade smoothly if IR changes? ======================================= Yes. If a `Value`'s use-list has changed in size (e.g., because of bitcode upgrades), its use-list order record is ignored. What does it cost? ==================...