LTO.cpp revision 360784 
1//===- LTO.cpp ------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "LTO.h"
10#include "Config.h"
11#include "InputFiles.h"
12#include "LinkerScript.h"
13#include "SymbolTable.h"
14#include "Symbols.h"
15#include "lld/Common/Args.h"
16#include "lld/Common/ErrorHandler.h"
17#include "lld/Common/TargetOptionsCommandFlags.h"
18#include "llvm/ADT/STLExtras.h"
19#include "llvm/ADT/SmallString.h"
20#include "llvm/ADT/StringRef.h"
21#include "llvm/ADT/Twine.h"
22#include "llvm/BinaryFormat/ELF.h"
23#include "llvm/Bitcode/BitcodeReader.h"
24#include "llvm/Bitcode/BitcodeWriter.h"
25#include "llvm/IR/DiagnosticPrinter.h"
26#include "llvm/LTO/Caching.h"
27#include "llvm/LTO/Config.h"
28#include "llvm/LTO/LTO.h"
29#include "llvm/Object/SymbolicFile.h"
30#include "llvm/Support/CodeGen.h"
31#include "llvm/Support/Error.h"
32#include "llvm/Support/FileSystem.h"
33#include "llvm/Support/MemoryBuffer.h"
34#include <algorithm>
35#include <cstddef>
36#include <memory>
37#include <string>
38#include <system_error>
39#include <vector>
40
41using namespace llvm;
42using namespace llvm::object;
43using namespace llvm::ELF;
44
45namespace lld {
46namespace elf {
47
48// Creates an empty file to store a list of object files for final
49// linking of distributed ThinLTO.
50static std::unique_ptr<raw_fd_ostream> openFile(StringRef file) {
51  std::error_code ec;
52  auto ret =
53      std::make_unique<raw_fd_ostream>(file, ec, sys::fs::OpenFlags::OF_None);
54  if (ec) {
55    error("cannot open " + file + ": " + ec.message());
56    return nullptr;
57  }
58  return ret;
59}
60
61static std::string getThinLTOOutputFile(StringRef modulePath) {
62  return lto::getThinLTOOutputFile(modulePath,
63                                   config->thinLTOPrefixReplace.first,
64                                   config->thinLTOPrefixReplace.second);
65}
66
67static lto::Config createConfig() {
68  lto::Config c;
69
70  // LLD supports the new relocations and address-significance tables.
71  c.Options = initTargetOptionsFromCodeGenFlags();
72  c.Options.RelaxELFRelocations = true;
73  c.Options.EmitAddrsig = true;
74
75  // Always emit a section per function/datum with LTO.
76  c.Options.FunctionSections = true;
77  c.Options.DataSections = true;
78
79  if (auto relocModel = getRelocModelFromCMModel())
80    c.RelocModel = *relocModel;
81  else if (config->relocatable)
82    c.RelocModel = None;
83  else if (config->isPic)
84    c.RelocModel = Reloc::PIC_;
85  else
86    c.RelocModel = Reloc::Static;
87
88  c.CodeModel = getCodeModelFromCMModel();
89  c.DisableVerify = config->disableVerify;
90  c.DiagHandler = diagnosticHandler;
91  c.OptLevel = config->ltoo;
92  c.CPU = getCPUStr();
93  c.MAttrs = getMAttrs();
94  c.CGOptLevel = args::getCGOptLevel(config->ltoo);
95
96  c.PTO.LoopVectorization = c.OptLevel > 1;
97  c.PTO.SLPVectorization = c.OptLevel > 1;
98
99  // Set up a custom pipeline if we've been asked to.
100  c.OptPipeline = config->ltoNewPmPasses;
101  c.AAPipeline = config->ltoAAPipeline;
102
103  // Set up optimization remarks if we've been asked to.
104  c.RemarksFilename = config->optRemarksFilename;
105  c.RemarksPasses = config->optRemarksPasses;
106  c.RemarksWithHotness = config->optRemarksWithHotness;
107  c.RemarksFormat = config->optRemarksFormat;
108
109  c.SampleProfile = config->ltoSampleProfile;
110  c.UseNewPM = config->ltoNewPassManager;
111  c.DebugPassManager = config->ltoDebugPassManager;
112  c.DwoDir = config->dwoDir;
113
114  c.CSIRProfile = config->ltoCSProfileFile;
115  c.RunCSIRInstr = config->ltoCSProfileGenerate;
116
117  if (config->emitLLVM) {
118    c.PostInternalizeModuleHook = [](size_t task, const Module &m) {
119      if (std::unique_ptr<raw_fd_ostream> os = openFile(config->outputFile))
120        WriteBitcodeToFile(m, *os, false);
121      return false;
122    };
123  }
124
125  if (config->saveTemps)
126    checkError(c.addSaveTemps(config->outputFile.str() + ".",
127                              /*UseInputModulePath*/ true));
128  return c;
129}
130
131BitcodeCompiler::BitcodeCompiler() {
132  // Initialize indexFile.
133  if (!config->thinLTOIndexOnlyArg.empty())
134    indexFile = openFile(config->thinLTOIndexOnlyArg);
135
136  // Initialize ltoObj.
137  lto::ThinBackend backend;
138  if (config->thinLTOIndexOnly) {
139    auto onIndexWrite = [&](StringRef s) { thinIndices.erase(s); };
140    backend = lto::createWriteIndexesThinBackend(
141        config->thinLTOPrefixReplace.first, config->thinLTOPrefixReplace.second,
142        config->thinLTOEmitImportsFiles, indexFile.get(), onIndexWrite);
143  } else if (config->thinLTOJobs != -1U) {
144    backend = lto::createInProcessThinBackend(config->thinLTOJobs);
145  }
146
147  ltoObj = std::make_unique<lto::LTO>(createConfig(), backend,
148                                       config->ltoPartitions);
149
150  // Initialize usedStartStop.
151  for (Symbol *sym : symtab->symbols()) {
152    StringRef s = sym->getName();
153    for (StringRef prefix : {"__start_", "__stop_"})
154      if (s.startswith(prefix))
155        usedStartStop.insert(s.substr(prefix.size()));
156  }
157}
158
159BitcodeCompiler::~BitcodeCompiler() = default;
160
161void BitcodeCompiler::add(BitcodeFile &f) {
162  lto::InputFile &obj = *f.obj;
163  bool isExec = !config->shared && !config->relocatable;
164
165  if (config->thinLTOIndexOnly)
166    thinIndices.insert(obj.getName());
167
168  ArrayRef<Symbol *> syms = f.getSymbols();
169  ArrayRef<lto::InputFile::Symbol> objSyms = obj.symbols();
170  std::vector<lto::SymbolResolution> resols(syms.size());
171
172  // Provide a resolution to the LTO API for each symbol.
173  for (size_t i = 0, e = syms.size(); i != e; ++i) {
174    Symbol *sym = syms[i];
175    const lto::InputFile::Symbol &objSym = objSyms[i];
176    lto::SymbolResolution &r = resols[i];
177
178    // Ideally we shouldn't check for SF_Undefined but currently IRObjectFile
179    // reports two symbols for module ASM defined. Without this check, lld
180    // flags an undefined in IR with a definition in ASM as prevailing.
181    // Once IRObjectFile is fixed to report only one symbol this hack can
182    // be removed.
183    r.Prevailing = !objSym.isUndefined() && sym->file == &f;
184
185    // We ask LTO to preserve following global symbols:
186    // 1) All symbols when doing relocatable link, so that them can be used
187    //    for doing final link.
188    // 2) Symbols that are used in regular objects.
189    // 3) C named sections if we have corresponding __start_/__stop_ symbol.
190    // 4) Symbols that are defined in bitcode files and used for dynamic linking.
191    r.VisibleToRegularObj = config->relocatable || sym->isUsedInRegularObj ||
192                            (r.Prevailing && sym->includeInDynsym()) ||
193                            usedStartStop.count(objSym.getSectionName());
194    const auto *dr = dyn_cast<Defined>(sym);
195    r.FinalDefinitionInLinkageUnit =
196        (isExec || sym->visibility != STV_DEFAULT) && dr &&
197        // Skip absolute symbols from ELF objects, otherwise PC-rel relocations
198        // will be generated by for them, triggering linker errors.
199        // Symbol section is always null for bitcode symbols, hence the check
200        // for isElf(). Skip linker script defined symbols as well: they have
201        // no File defined.
202        !(dr->section == nullptr && (!sym->file || sym->file->isElf()));
203
204    if (r.Prevailing)
205      sym->replace(Undefined{nullptr, sym->getName(), STB_GLOBAL, STV_DEFAULT,
206                             sym->type});
207
208    // We tell LTO to not apply interprocedural optimization for wrapped
209    // (with --wrap) symbols because otherwise LTO would inline them while
210    // their values are still not final.
211    r.LinkerRedefined = !sym->canInline;
212  }
213  checkError(ltoObj->add(std::move(f.obj), resols));
214}
215
216// If LazyObjFile has not been added to link, emit empty index files.
217// This is needed because this is what GNU gold plugin does and we have a
218// distributed build system that depends on that behavior.
219static void thinLTOCreateEmptyIndexFiles() {
220  for (LazyObjFile *f : lazyObjFiles) {
221    if (!isBitcode(f->mb))
222      continue;
223    std::string path = replaceThinLTOSuffix(getThinLTOOutputFile(f->getName()));
224    std::unique_ptr<raw_fd_ostream> os = openFile(path + ".thinlto.bc");
225    if (!os)
226      continue;
227
228    ModuleSummaryIndex m(/*HaveGVs*/ false);
229    m.setSkipModuleByDistributedBackend();
230    WriteIndexToFile(m, *os);
231    if (config->thinLTOEmitImportsFiles)
232      openFile(path + ".imports");
233  }
234}
235
236// Merge all the bitcode files we have seen, codegen the result
237// and return the resulting ObjectFile(s).
238std::vector<InputFile *> BitcodeCompiler::compile() {
239  unsigned maxTasks = ltoObj->getMaxTasks();
240  buf.resize(maxTasks);
241  files.resize(maxTasks);
242
243  // The --thinlto-cache-dir option specifies the path to a directory in which
244  // to cache native object files for ThinLTO incremental builds. If a path was
245  // specified, configure LTO to use it as the cache directory.
246  lto::NativeObjectCache cache;
247  if (!config->thinLTOCacheDir.empty())
248    cache = check(
249        lto::localCache(config->thinLTOCacheDir,
250                        [&](size_t task, std::unique_ptr<MemoryBuffer> mb) {
251                          files[task] = std::move(mb);
252                        }));
253
254  if (!bitcodeFiles.empty())
255    checkError(ltoObj->run(
256        [&](size_t task) {
257          return std::make_unique<lto::NativeObjectStream>(
258              std::make_unique<raw_svector_ostream>(buf[task]));
259        },
260        cache));
261
262  // Emit empty index files for non-indexed files
263  for (StringRef s : thinIndices) {
264    std::string path = getThinLTOOutputFile(s);
265    openFile(path + ".thinlto.bc");
266    if (config->thinLTOEmitImportsFiles)
267      openFile(path + ".imports");
268  }
269
270  if (config->thinLTOIndexOnly) {
271    thinLTOCreateEmptyIndexFiles();
272
273    if (!config->ltoObjPath.empty())
274      saveBuffer(buf[0], config->ltoObjPath);
275
276    // ThinLTO with index only option is required to generate only the index
277    // files. After that, we exit from linker and ThinLTO backend runs in a
278    // distributed environment.
279    if (indexFile)
280      indexFile->close();
281    return {};
282  }
283
284  if (!config->thinLTOCacheDir.empty())
285    pruneCache(config->thinLTOCacheDir, config->thinLTOCachePolicy);
286
287  if (!config->ltoObjPath.empty()) {
288    saveBuffer(buf[0], config->ltoObjPath);
289    for (unsigned i = 1; i != maxTasks; ++i)
290      saveBuffer(buf[i], config->ltoObjPath + Twine(i));
291  }
292
293  if (config->saveTemps) {
294    saveBuffer(buf[0], config->outputFile + ".lto.o");
295    for (unsigned i = 1; i != maxTasks; ++i)
296      saveBuffer(buf[i], config->outputFile + Twine(i) + ".lto.o");
297  }
298
299  std::vector<InputFile *> ret;
300  for (unsigned i = 0; i != maxTasks; ++i)
301    if (!buf[i].empty())
302      ret.push_back(createObjectFile(MemoryBufferRef(buf[i], "lto.tmp")));
303
304  for (std::unique_ptr<MemoryBuffer> &file : files)
305    if (file)
306      ret.push_back(createObjectFile(*file));
307  return ret;
308}
309
310} // namespace elf
311} // namespace lld
312