a67208e1c6
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
52 lines
1.8 KiB
C++
52 lines
1.8 KiB
C++
//===- EmbedBitcodePass.cpp - Pass that embeds the bitcode into a global---===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/IPO/EmbedBitcodePass.h"
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#include "llvm/Bitcode/BitcodeWriter.h"
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#include "llvm/Bitcode/BitcodeWriterPass.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Passes/PassBuilder.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/MemoryBuffer.h"
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#include "llvm/TargetParser/Triple.h"
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#include "llvm/Transforms/IPO/ThinLTOBitcodeWriter.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/ModuleUtils.h"
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using namespace llvm;
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PreservedAnalyses EmbedBitcodePass::run(Module &M, ModuleAnalysisManager &AM) {
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if (M.getGlobalVariable("llvm.embedded.module", /*AllowInternal=*/true))
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report_fatal_error("Can only embed the module once",
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/*gen_crash_diag=*/false);
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Triple T(M.getTargetTriple());
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if (T.getObjectFormat() != Triple::ELF)
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report_fatal_error(
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"EmbedBitcode pass currently only supports ELF object format",
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/*gen_crash_diag=*/false);
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std::unique_ptr<Module> NewModule = CloneModule(M);
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MPM.run(*NewModule, AM);
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std::string Data;
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raw_string_ostream OS(Data);
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if (IsThinLTO)
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ThinLTOBitcodeWriterPass(OS, /*ThinLinkOS=*/nullptr).run(*NewModule, AM);
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else
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BitcodeWriterPass(OS, /*ShouldPreserveUseListOrder=*/false, EmitLTOSummary)
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.run(*NewModule, AM);
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embedBufferInModule(M, MemoryBufferRef(Data, "ModuleData"), ".llvm.lto");
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return PreservedAnalyses::all();
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}
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