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 "SymbolTable.h"
13 #include "Symbols.h"
14 #include "lld/Common/Args.h"
15 #include "lld/Common/ErrorHandler.h"
16 #include "lld/Common/Strings.h"
17 #include "lld/Common/TargetOptionsCommandFlags.h"
18 #include "llvm/ADT/SmallString.h"
19 #include "llvm/ADT/StringRef.h"
20 #include "llvm/ADT/Twine.h"
21 #include "llvm/BinaryFormat/ELF.h"
22 #include "llvm/Bitcode/BitcodeWriter.h"
23 #include "llvm/LTO/Config.h"
24 #include "llvm/LTO/LTO.h"
25 #include "llvm/Support/Caching.h"
26 #include "llvm/Support/CodeGen.h"
27 #include "llvm/Support/Error.h"
28 #include "llvm/Support/FileSystem.h"
29 #include "llvm/Support/MemoryBuffer.h"
30 #include <algorithm>
31 #include <cstddef>
32 #include <memory>
33 #include <string>
34 #include <system_error>
35 #include <vector>
36
37 using namespace llvm;
38 using namespace llvm::object;
39 using namespace llvm::ELF;
40 using namespace lld;
41 using namespace lld::elf;
42
43 // Creates an empty file to store a list of object files for final
44 // linking of distributed ThinLTO.
openFile(StringRef file)45 static std::unique_ptr<raw_fd_ostream> openFile(StringRef file) {
46 std::error_code ec;
47 auto ret =
48 std::make_unique<raw_fd_ostream>(file, ec, sys::fs::OpenFlags::OF_None);
49 if (ec) {
50 error("cannot open " + file + ": " + ec.message());
51 return nullptr;
52 }
53 return ret;
54 }
55
56 // The merged bitcode after LTO is large. Try opening a file stream that
57 // supports reading, seeking and writing. Such a file allows BitcodeWriter to
58 // flush buffered data to reduce memory consumption. If this fails, open a file
59 // stream that supports only write.
openLTOOutputFile(StringRef file)60 static std::unique_ptr<raw_fd_ostream> openLTOOutputFile(StringRef file) {
61 std::error_code ec;
62 std::unique_ptr<raw_fd_ostream> fs =
63 std::make_unique<raw_fd_stream>(file, ec);
64 if (!ec)
65 return fs;
66 return openFile(file);
67 }
68
getThinLTOOutputFile(StringRef modulePath)69 static std::string getThinLTOOutputFile(StringRef modulePath) {
70 return lto::getThinLTOOutputFile(
71 std::string(modulePath), std::string(config->thinLTOPrefixReplace.first),
72 std::string(config->thinLTOPrefixReplace.second));
73 }
74
createConfig()75 static lto::Config createConfig() {
76 lto::Config c;
77
78 // LLD supports the new relocations and address-significance tables.
79 c.Options = initTargetOptionsFromCodeGenFlags();
80 c.Options.RelaxELFRelocations = true;
81 c.Options.EmitAddrsig = true;
82
83 // Always emit a section per function/datum with LTO.
84 c.Options.FunctionSections = true;
85 c.Options.DataSections = true;
86
87 // Check if basic block sections must be used.
88 // Allowed values for --lto-basic-block-sections are "all", "labels",
89 // "<file name specifying basic block ids>", or none. This is the equivalent
90 // of -fbasic-block-sections= flag in clang.
91 if (!config->ltoBasicBlockSections.empty()) {
92 if (config->ltoBasicBlockSections == "all") {
93 c.Options.BBSections = BasicBlockSection::All;
94 } else if (config->ltoBasicBlockSections == "labels") {
95 c.Options.BBSections = BasicBlockSection::Labels;
96 } else if (config->ltoBasicBlockSections == "none") {
97 c.Options.BBSections = BasicBlockSection::None;
98 } else {
99 ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
100 MemoryBuffer::getFile(config->ltoBasicBlockSections.str());
101 if (!MBOrErr) {
102 error("cannot open " + config->ltoBasicBlockSections + ":" +
103 MBOrErr.getError().message());
104 } else {
105 c.Options.BBSectionsFuncListBuf = std::move(*MBOrErr);
106 }
107 c.Options.BBSections = BasicBlockSection::List;
108 }
109 }
110
111 c.Options.UniqueBasicBlockSectionNames =
112 config->ltoUniqueBasicBlockSectionNames;
113
114 if (auto relocModel = getRelocModelFromCMModel())
115 c.RelocModel = *relocModel;
116 else if (config->relocatable)
117 c.RelocModel = None;
118 else if (config->isPic)
119 c.RelocModel = Reloc::PIC_;
120 else
121 c.RelocModel = Reloc::Static;
122
123 c.CodeModel = getCodeModelFromCMModel();
124 c.DisableVerify = config->disableVerify;
125 c.DiagHandler = diagnosticHandler;
126 c.OptLevel = config->ltoo;
127 c.CPU = getCPUStr();
128 c.MAttrs = getMAttrs();
129 c.CGOptLevel = args::getCGOptLevel(config->ltoo);
130
131 c.PTO.LoopVectorization = c.OptLevel > 1;
132 c.PTO.SLPVectorization = c.OptLevel > 1;
133
134 // Set up a custom pipeline if we've been asked to.
135 c.OptPipeline = std::string(config->ltoNewPmPasses);
136 c.AAPipeline = std::string(config->ltoAAPipeline);
137
138 // Set up optimization remarks if we've been asked to.
139 c.RemarksFilename = std::string(config->optRemarksFilename);
140 c.RemarksPasses = std::string(config->optRemarksPasses);
141 c.RemarksWithHotness = config->optRemarksWithHotness;
142 c.RemarksHotnessThreshold = config->optRemarksHotnessThreshold;
143 c.RemarksFormat = std::string(config->optRemarksFormat);
144
145 // Set up output file to emit statistics.
146 c.StatsFile = std::string(config->optStatsFilename);
147
148 c.SampleProfile = std::string(config->ltoSampleProfile);
149 for (StringRef pluginFn : config->passPlugins)
150 c.PassPlugins.push_back(std::string(pluginFn));
151 c.DebugPassManager = config->ltoDebugPassManager;
152 c.DwoDir = std::string(config->dwoDir);
153
154 c.HasWholeProgramVisibility = config->ltoWholeProgramVisibility;
155 c.AlwaysEmitRegularLTOObj = !config->ltoObjPath.empty();
156
157 for (const llvm::StringRef &name : config->thinLTOModulesToCompile)
158 c.ThinLTOModulesToCompile.emplace_back(name);
159
160 c.TimeTraceEnabled = config->timeTraceEnabled;
161 c.TimeTraceGranularity = config->timeTraceGranularity;
162
163 c.CSIRProfile = std::string(config->ltoCSProfileFile);
164 c.RunCSIRInstr = config->ltoCSProfileGenerate;
165 c.PGOWarnMismatch = config->ltoPGOWarnMismatch;
166
167 c.OpaquePointers = config->opaquePointers;
168
169 if (config->emitLLVM) {
170 c.PostInternalizeModuleHook = [](size_t task, const Module &m) {
171 if (std::unique_ptr<raw_fd_ostream> os =
172 openLTOOutputFile(config->outputFile))
173 WriteBitcodeToFile(m, *os, false);
174 return false;
175 };
176 }
177
178 if (config->ltoEmitAsm)
179 c.CGFileType = CGFT_AssemblyFile;
180
181 if (!config->saveTempsArgs.empty())
182 checkError(c.addSaveTemps(config->outputFile.str() + ".",
183 /*UseInputModulePath*/ true,
184 config->saveTempsArgs));
185 return c;
186 }
187
BitcodeCompiler()188 BitcodeCompiler::BitcodeCompiler() {
189 // Initialize indexFile.
190 if (!config->thinLTOIndexOnlyArg.empty())
191 indexFile = openFile(config->thinLTOIndexOnlyArg);
192
193 // Initialize ltoObj.
194 lto::ThinBackend backend;
195 auto onIndexWrite = [&](StringRef s) { thinIndices.erase(s); };
196 if (config->thinLTOIndexOnly) {
197 backend = lto::createWriteIndexesThinBackend(
198 std::string(config->thinLTOPrefixReplace.first),
199 std::string(config->thinLTOPrefixReplace.second),
200 config->thinLTOEmitImportsFiles, indexFile.get(), onIndexWrite);
201 } else {
202 backend = lto::createInProcessThinBackend(
203 llvm::heavyweight_hardware_concurrency(config->thinLTOJobs),
204 onIndexWrite, config->thinLTOEmitIndexFiles,
205 config->thinLTOEmitImportsFiles);
206 }
207
208 ltoObj = std::make_unique<lto::LTO>(createConfig(), backend,
209 config->ltoPartitions);
210
211 // Initialize usedStartStop.
212 if (ctx->bitcodeFiles.empty())
213 return;
214 for (Symbol *sym : symtab->symbols()) {
215 if (sym->isPlaceholder())
216 continue;
217 StringRef s = sym->getName();
218 for (StringRef prefix : {"__start_", "__stop_"})
219 if (s.startswith(prefix))
220 usedStartStop.insert(s.substr(prefix.size()));
221 }
222 }
223
224 BitcodeCompiler::~BitcodeCompiler() = default;
225
add(BitcodeFile & f)226 void BitcodeCompiler::add(BitcodeFile &f) {
227 lto::InputFile &obj = *f.obj;
228 bool isExec = !config->shared && !config->relocatable;
229
230 if (config->thinLTOEmitIndexFiles)
231 thinIndices.insert(obj.getName());
232
233 ArrayRef<Symbol *> syms = f.getSymbols();
234 ArrayRef<lto::InputFile::Symbol> objSyms = obj.symbols();
235 std::vector<lto::SymbolResolution> resols(syms.size());
236
237 // Provide a resolution to the LTO API for each symbol.
238 for (size_t i = 0, e = syms.size(); i != e; ++i) {
239 Symbol *sym = syms[i];
240 const lto::InputFile::Symbol &objSym = objSyms[i];
241 lto::SymbolResolution &r = resols[i];
242
243 // Ideally we shouldn't check for SF_Undefined but currently IRObjectFile
244 // reports two symbols for module ASM defined. Without this check, lld
245 // flags an undefined in IR with a definition in ASM as prevailing.
246 // Once IRObjectFile is fixed to report only one symbol this hack can
247 // be removed.
248 r.Prevailing = !objSym.isUndefined() && sym->file == &f;
249
250 // We ask LTO to preserve following global symbols:
251 // 1) All symbols when doing relocatable link, so that them can be used
252 // for doing final link.
253 // 2) Symbols that are used in regular objects.
254 // 3) C named sections if we have corresponding __start_/__stop_ symbol.
255 // 4) Symbols that are defined in bitcode files and used for dynamic
256 // linking.
257 // 5) Symbols that will be referenced after linker wrapping is performed.
258 r.VisibleToRegularObj = config->relocatable || sym->isUsedInRegularObj ||
259 sym->referencedAfterWrap ||
260 (r.Prevailing && sym->includeInDynsym()) ||
261 usedStartStop.count(objSym.getSectionName());
262 // Identify symbols exported dynamically, and that therefore could be
263 // referenced by a shared library not visible to the linker.
264 r.ExportDynamic =
265 sym->computeBinding() != STB_LOCAL &&
266 (config->exportDynamic || sym->exportDynamic || sym->inDynamicList);
267 const auto *dr = dyn_cast<Defined>(sym);
268 r.FinalDefinitionInLinkageUnit =
269 (isExec || sym->visibility != STV_DEFAULT) && dr &&
270 // Skip absolute symbols from ELF objects, otherwise PC-rel relocations
271 // will be generated by for them, triggering linker errors.
272 // Symbol section is always null for bitcode symbols, hence the check
273 // for isElf(). Skip linker script defined symbols as well: they have
274 // no File defined.
275 !(dr->section == nullptr && (!sym->file || sym->file->isElf()));
276
277 if (r.Prevailing)
278 sym->replace(
279 Undefined{nullptr, StringRef(), STB_GLOBAL, STV_DEFAULT, sym->type});
280
281 // We tell LTO to not apply interprocedural optimization for wrapped
282 // (with --wrap) symbols because otherwise LTO would inline them while
283 // their values are still not final.
284 r.LinkerRedefined = sym->scriptDefined;
285 }
286 checkError(ltoObj->add(std::move(f.obj), resols));
287 }
288
289 // If LazyObjFile has not been added to link, emit empty index files.
290 // This is needed because this is what GNU gold plugin does and we have a
291 // distributed build system that depends on that behavior.
thinLTOCreateEmptyIndexFiles()292 static void thinLTOCreateEmptyIndexFiles() {
293 for (BitcodeFile *f : ctx->lazyBitcodeFiles) {
294 if (!f->lazy)
295 continue;
296 std::string path = replaceThinLTOSuffix(getThinLTOOutputFile(f->getName()));
297 std::unique_ptr<raw_fd_ostream> os = openFile(path + ".thinlto.bc");
298 if (!os)
299 continue;
300
301 ModuleSummaryIndex m(/*HaveGVs*/ false);
302 m.setSkipModuleByDistributedBackend();
303 writeIndexToFile(m, *os);
304 if (config->thinLTOEmitImportsFiles)
305 openFile(path + ".imports");
306 }
307 }
308
309 // Merge all the bitcode files we have seen, codegen the result
310 // and return the resulting ObjectFile(s).
compile()311 std::vector<InputFile *> BitcodeCompiler::compile() {
312 unsigned maxTasks = ltoObj->getMaxTasks();
313 buf.resize(maxTasks);
314 files.resize(maxTasks);
315
316 // The --thinlto-cache-dir option specifies the path to a directory in which
317 // to cache native object files for ThinLTO incremental builds. If a path was
318 // specified, configure LTO to use it as the cache directory.
319 FileCache cache;
320 if (!config->thinLTOCacheDir.empty())
321 cache =
322 check(localCache("ThinLTO", "Thin", config->thinLTOCacheDir,
323 [&](size_t task, std::unique_ptr<MemoryBuffer> mb) {
324 files[task] = std::move(mb);
325 }));
326
327 if (!ctx->bitcodeFiles.empty())
328 checkError(ltoObj->run(
329 [&](size_t task) {
330 return std::make_unique<CachedFileStream>(
331 std::make_unique<raw_svector_ostream>(buf[task]));
332 },
333 cache));
334
335 // Emit empty index files for non-indexed files but not in single-module mode.
336 if (config->thinLTOModulesToCompile.empty()) {
337 for (StringRef s : thinIndices) {
338 std::string path = getThinLTOOutputFile(s);
339 openFile(path + ".thinlto.bc");
340 if (config->thinLTOEmitImportsFiles)
341 openFile(path + ".imports");
342 }
343 }
344
345 if (config->thinLTOEmitIndexFiles)
346 thinLTOCreateEmptyIndexFiles();
347
348 if (config->thinLTOIndexOnly) {
349 if (!config->ltoObjPath.empty())
350 saveBuffer(buf[0], config->ltoObjPath);
351
352 // ThinLTO with index only option is required to generate only the index
353 // files. After that, we exit from linker and ThinLTO backend runs in a
354 // distributed environment.
355 if (indexFile)
356 indexFile->close();
357 return {};
358 }
359
360 if (!config->thinLTOCacheDir.empty())
361 pruneCache(config->thinLTOCacheDir, config->thinLTOCachePolicy);
362
363 if (!config->ltoObjPath.empty()) {
364 saveBuffer(buf[0], config->ltoObjPath);
365 for (unsigned i = 1; i != maxTasks; ++i)
366 saveBuffer(buf[i], config->ltoObjPath + Twine(i));
367 }
368
369 if (config->saveTempsArgs.contains("prelink")) {
370 if (!buf[0].empty())
371 saveBuffer(buf[0], config->outputFile + ".lto.o");
372 for (unsigned i = 1; i != maxTasks; ++i)
373 saveBuffer(buf[i], config->outputFile + Twine(i) + ".lto.o");
374 }
375
376 if (config->ltoEmitAsm) {
377 saveBuffer(buf[0], config->outputFile);
378 for (unsigned i = 1; i != maxTasks; ++i)
379 saveBuffer(buf[i], config->outputFile + Twine(i));
380 return {};
381 }
382
383 std::vector<InputFile *> ret;
384 for (unsigned i = 0; i != maxTasks; ++i)
385 if (!buf[i].empty())
386 ret.push_back(createObjFile(MemoryBufferRef(buf[i], "lto.tmp")));
387
388 for (std::unique_ptr<MemoryBuffer> &file : files)
389 if (file)
390 ret.push_back(createObjFile(*file));
391 return ret;
392 }
393