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 clones the module and 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 compile the original module using the
PerModuleDefaultPipeline used for non-LTO compilation. We generate
standard object files at the end of this pipeline, which contain machine
code and the new `.llvm.lto` section containing bitcode.
Since the two pipelines operate on different copies of the module, we
can be sure that the bitcode in the `.llvm.lto` section and object code
in `.text` are congruent with the existing output produced by the
default and LTO pipelines.
Original RFC: https://discourse.llvm.org/t/rfc-ffat-lto-objects-support/63977
Earlier versions of this patch were missing REQUIRES lines for llc
related tests in Transforms/EmbedBitcode. Those tests are now under
CodeGen/X86, which should avoid running the check on unsupported
platforms.
The EmbedbBitcodePass also returned PreservedAnalyses::all when adding a
metadata section, which failed expensive checks, since it modified the
module. This is now corrected.
Reviewed By: tejohnson, MaskRay, nikic
Differential Revision: https://reviews.llvm.org/D146776
75a1797044