[PATCH] D117244: [Bitcode] [ThinLTO] Add a new bitcode module record for THINLTO_INDEX_FLAG

[Mingming Liu via Phabricator via llvm-commits]

luna created this revision.
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- The record is of format [uint64_t] (one uint64_t integer). Only last bit is used so far; if last bit is true, the bitcode is minimized for the ThinLTO thin link only (i.e., bitcode generated by `opt --thin-link-bitcode-file=<minimized-bitcode>` or `clang -Xclang -fthin-link-bitcode=<minimized-bitcode>`.

- Writer
  - In BitcodeWriter.cpp, only `ThinLinkBitcodeWriter::write` writes 1, the rest of two bitcode writers write 0.

- Reader
  - In BitcodeReader.cpp, method `BitcodeModule::getLTOInfo` reads this record.
    - Implementation-wise, the method records the read state of {lto subblocks, index flag record}, skip records after index flag is read, and bails out early after both is read.
    - Optimize by using the fact that flag is written before subblocks, but correctness doesn't depend on this.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D117244

Files:
  llvm/include/llvm/Bitcode/BitcodeReader.h
  llvm/include/llvm/Bitcode/LLVMBitCodes.h
  llvm/lib/Bitcode/Reader/BitcodeAnalyzer.cpp
  llvm/lib/Bitcode/Reader/BitcodeReader.cpp
  llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
  llvm/test/Bitcode/bcanalyzer-thinlto-index-flag.ll
  llvm/tools/llvm-dis/llvm-dis.cpp

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