[PATCH] D146776: [llvm] Preliminary fat-lto-objects support

[Paul Kirth via Phabricator via llvm-commits]

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Fat LTO objects contain both LTO compatible IR, as well as generated
object code. This allows users to defer the choice of whether to use LTO
or not to link-time. This is a feature available in GCC for some time,
and makes the existing -ffat-lto-objects flag functional in the same
way as GCC's.

Within LLVM, we add a new EmbedBitcodePass that serializes the module to
the object file, and expose a new pass pipeline for compiling fat
objects. The new pipeline initially runs the selected (Thin)LTOPrelink
pipeline, after which it will serialize the module into a `.llvm.lto`
section of an ELF file. When compiling for (Thin)LTO, this normally the
point at which the compiler would emit a object file containing the
bitcode and metadata.

After that point we use the PerModuleDefaultPipeline to optimize the
module as close as we can to a normal compilation pipleine used at e.g.,
`-O2` or `-O3`. We generate standard object files at the end of this
pipeline, which contain machine code. This finalized output will also
contain the previously mentioned `.llvm.lto` section containing the
bitcode.

Original RFC: https://discourse.llvm.org/t/rfc-ffat-lto-objects-support/63977


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D146776

Files:
  llvm/include/llvm/Bitcode/EmbedBitcodePass.h
  llvm/include/llvm/Passes/PassBuilder.h
  llvm/lib/Bitcode/Writer/CMakeLists.txt
  llvm/lib/Bitcode/Writer/EmbedBitcodePass.cpp
  llvm/lib/Object/ObjectFile.cpp
  llvm/lib/Passes/PassBuilder.cpp
  llvm/lib/Passes/PassBuilderPipelines.cpp
  llvm/lib/Passes/PassRegistry.def
  llvm/test/Bitcode/embed-multiple.ll
  llvm/test/Bitcode/embed-unsupported-object-format.ll
  llvm/test/Bitcode/embed.ll

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