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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 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 Reviewed By: tejohnson, MaskRay, nikic Differential Revision: https://reviews.llvm.org/D146776 |
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The LLVM Compiler Infrastructure
Welcome to the LLVM project!
This repository contains the source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.
The LLVM project has multiple components. The core of the project is itself called "LLVM". This contains all of the tools, libraries, and header files needed to process intermediate representations and convert them into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer.
C-like languages use the Clang frontend. This component compiles C, C++, Objective-C, and Objective-C++ code into LLVM bitcode -- and from there into object files, using LLVM.
Other components include: the libc++ C++ standard library, the LLD linker, and more.
Getting the Source Code and Building LLVM
Consult the Getting Started with LLVM page for information on building and running LLVM.
For information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Getting in touch
Join the LLVM Discourse forums, Discord chat, or #llvm IRC channel on OFTC.
The LLVM project has adopted a code of conduct for participants to all modes of communication within the project.