1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===// 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 // This file implements classes used to handle lowerings specific to common 10 // object file formats. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Target/TargetLoweringObjectFile.h" 15 #include "llvm/BinaryFormat/Dwarf.h" 16 #include "llvm/IR/Constants.h" 17 #include "llvm/IR/DataLayout.h" 18 #include "llvm/IR/DerivedTypes.h" 19 #include "llvm/IR/Function.h" 20 #include "llvm/IR/GlobalVariable.h" 21 #include "llvm/IR/Mangler.h" 22 #include "llvm/IR/Module.h" 23 #include "llvm/MC/MCAsmInfo.h" 24 #include "llvm/MC/MCContext.h" 25 #include "llvm/MC/MCExpr.h" 26 #include "llvm/MC/MCStreamer.h" 27 #include "llvm/MC/SectionKind.h" 28 #include "llvm/Support/ErrorHandling.h" 29 #include "llvm/Target/TargetMachine.h" 30 #include "llvm/Target/TargetOptions.h" 31 using namespace llvm; 32 33 //===----------------------------------------------------------------------===// 34 // Generic Code 35 //===----------------------------------------------------------------------===// 36 37 /// Initialize - this method must be called before any actual lowering is 38 /// done. This specifies the current context for codegen, and gives the 39 /// lowering implementations a chance to set up their default sections. 40 void TargetLoweringObjectFile::Initialize(MCContext &ctx, 41 const TargetMachine &TM) { 42 // `Initialize` can be called more than once. 43 delete Mang; 44 Mang = new Mangler(); 45 initMCObjectFileInfo(ctx, TM.isPositionIndependent(), 46 TM.getCodeModel() == CodeModel::Large); 47 48 // Reset various EH DWARF encodings. 49 PersonalityEncoding = LSDAEncoding = TTypeEncoding = dwarf::DW_EH_PE_absptr; 50 CallSiteEncoding = dwarf::DW_EH_PE_uleb128; 51 52 this->TM = &TM; 53 } 54 55 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 56 delete Mang; 57 } 58 59 unsigned TargetLoweringObjectFile::getCallSiteEncoding() const { 60 // If target does not have LEB128 directives, we would need the 61 // call site encoding to be udata4 so that the alternative path 62 // for not having LEB128 directives could work. 63 if (!getContext().getAsmInfo()->hasLEB128Directives()) 64 return dwarf::DW_EH_PE_udata4; 65 return CallSiteEncoding; 66 } 67 68 static bool isNullOrUndef(const Constant *C) { 69 // Check that the constant isn't all zeros or undefs. 70 if (C->isNullValue() || isa<UndefValue>(C)) 71 return true; 72 if (!isa<ConstantAggregate>(C)) 73 return false; 74 for (auto Operand : C->operand_values()) { 75 if (!isNullOrUndef(cast<Constant>(Operand))) 76 return false; 77 } 78 return true; 79 } 80 81 static bool isSuitableForBSS(const GlobalVariable *GV) { 82 const Constant *C = GV->getInitializer(); 83 84 // Must have zero initializer. 85 if (!isNullOrUndef(C)) 86 return false; 87 88 // Leave constant zeros in readonly constant sections, so they can be shared. 89 if (GV->isConstant()) 90 return false; 91 92 // If the global has an explicit section specified, don't put it in BSS. 93 if (GV->hasSection()) 94 return false; 95 96 // Otherwise, put it in BSS! 97 return true; 98 } 99 100 /// IsNullTerminatedString - Return true if the specified constant (which is 101 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 102 /// nul value and contains no other nuls in it. Note that this is more general 103 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 104 static bool IsNullTerminatedString(const Constant *C) { 105 // First check: is we have constant array terminated with zero 106 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 107 unsigned NumElts = CDS->getNumElements(); 108 assert(NumElts != 0 && "Can't have an empty CDS"); 109 110 if (CDS->getElementAsInteger(NumElts-1) != 0) 111 return false; // Not null terminated. 112 113 // Verify that the null doesn't occur anywhere else in the string. 114 for (unsigned i = 0; i != NumElts-1; ++i) 115 if (CDS->getElementAsInteger(i) == 0) 116 return false; 117 return true; 118 } 119 120 // Another possibility: [1 x i8] zeroinitializer 121 if (isa<ConstantAggregateZero>(C)) 122 return cast<ArrayType>(C->getType())->getNumElements() == 1; 123 124 return false; 125 } 126 127 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase( 128 const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const { 129 assert(!Suffix.empty()); 130 131 SmallString<60> NameStr; 132 NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix(); 133 TM.getNameWithPrefix(NameStr, GV, *Mang); 134 NameStr.append(Suffix.begin(), Suffix.end()); 135 return getContext().getOrCreateSymbol(NameStr); 136 } 137 138 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol( 139 const GlobalValue *GV, const TargetMachine &TM, 140 MachineModuleInfo *MMI) const { 141 return TM.getSymbol(GV); 142 } 143 144 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 145 const DataLayout &, 146 const MCSymbol *Sym) const { 147 } 148 149 void TargetLoweringObjectFile::emitCGProfileMetadata(MCStreamer &Streamer, 150 Module &M) const { 151 MCContext &C = getContext(); 152 SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags; 153 M.getModuleFlagsMetadata(ModuleFlags); 154 155 MDNode *CFGProfile = nullptr; 156 157 for (const auto &MFE : ModuleFlags) { 158 StringRef Key = MFE.Key->getString(); 159 if (Key == "CG Profile") { 160 CFGProfile = cast<MDNode>(MFE.Val); 161 break; 162 } 163 } 164 165 if (!CFGProfile) 166 return; 167 168 auto GetSym = [this](const MDOperand &MDO) -> MCSymbol * { 169 if (!MDO) 170 return nullptr; 171 auto *V = cast<ValueAsMetadata>(MDO); 172 const Function *F = cast<Function>(V->getValue()->stripPointerCasts()); 173 if (F->hasDLLImportStorageClass()) 174 return nullptr; 175 return TM->getSymbol(F); 176 }; 177 178 for (const auto &Edge : CFGProfile->operands()) { 179 MDNode *E = cast<MDNode>(Edge); 180 const MCSymbol *From = GetSym(E->getOperand(0)); 181 const MCSymbol *To = GetSym(E->getOperand(1)); 182 // Skip null functions. This can happen if functions are dead stripped after 183 // the CGProfile pass has been run. 184 if (!From || !To) 185 continue; 186 uint64_t Count = cast<ConstantAsMetadata>(E->getOperand(2)) 187 ->getValue() 188 ->getUniqueInteger() 189 .getZExtValue(); 190 Streamer.emitCGProfileEntry( 191 MCSymbolRefExpr::create(From, MCSymbolRefExpr::VK_None, C), 192 MCSymbolRefExpr::create(To, MCSymbolRefExpr::VK_None, C), Count); 193 } 194 } 195 196 /// getKindForGlobal - This is a top-level target-independent classifier for 197 /// a global object. Given a global variable and information from the TM, this 198 /// function classifies the global in a target independent manner. This function 199 /// may be overridden by the target implementation. 200 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO, 201 const TargetMachine &TM){ 202 assert(!GO->isDeclarationForLinker() && 203 "Can only be used for global definitions"); 204 205 // Functions are classified as text sections. 206 if (isa<Function>(GO)) 207 return SectionKind::getText(); 208 209 // Basic blocks are classified as text sections. 210 if (isa<BasicBlock>(GO)) 211 return SectionKind::getText(); 212 213 // Global variables require more detailed analysis. 214 const auto *GVar = cast<GlobalVariable>(GO); 215 216 // Handle thread-local data first. 217 if (GVar->isThreadLocal()) { 218 if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) { 219 // Zero-initialized TLS variables with local linkage always get classified 220 // as ThreadBSSLocal. 221 if (GVar->hasLocalLinkage()) { 222 return SectionKind::getThreadBSSLocal(); 223 } 224 return SectionKind::getThreadBSS(); 225 } 226 return SectionKind::getThreadData(); 227 } 228 229 // Variables with common linkage always get classified as common. 230 if (GVar->hasCommonLinkage()) 231 return SectionKind::getCommon(); 232 233 // Most non-mergeable zero data can be put in the BSS section unless otherwise 234 // specified. 235 if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) { 236 if (GVar->hasLocalLinkage()) 237 return SectionKind::getBSSLocal(); 238 else if (GVar->hasExternalLinkage()) 239 return SectionKind::getBSSExtern(); 240 return SectionKind::getBSS(); 241 } 242 243 // Global variables with '!exclude' should get the exclude section kind if 244 // they have an explicit section and no other metadata. 245 if (GVar->hasSection()) 246 if (MDNode *MD = GVar->getMetadata(LLVMContext::MD_exclude)) 247 if (!MD->getNumOperands()) 248 return SectionKind::getExclude(); 249 250 // If the global is marked constant, we can put it into a mergable section, 251 // a mergable string section, or general .data if it contains relocations. 252 if (GVar->isConstant()) { 253 // If the initializer for the global contains something that requires a 254 // relocation, then we may have to drop this into a writable data section 255 // even though it is marked const. 256 const Constant *C = GVar->getInitializer(); 257 if (!C->needsRelocation()) { 258 // If the global is required to have a unique address, it can't be put 259 // into a mergable section: just drop it into the general read-only 260 // section instead. 261 if (!GVar->hasGlobalUnnamedAddr()) 262 return SectionKind::getReadOnly(); 263 264 // If initializer is a null-terminated string, put it in a "cstring" 265 // section of the right width. 266 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 267 if (IntegerType *ITy = 268 dyn_cast<IntegerType>(ATy->getElementType())) { 269 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 270 ITy->getBitWidth() == 32) && 271 IsNullTerminatedString(C)) { 272 if (ITy->getBitWidth() == 8) 273 return SectionKind::getMergeable1ByteCString(); 274 if (ITy->getBitWidth() == 16) 275 return SectionKind::getMergeable2ByteCString(); 276 277 assert(ITy->getBitWidth() == 32 && "Unknown width"); 278 return SectionKind::getMergeable4ByteCString(); 279 } 280 } 281 } 282 283 // Otherwise, just drop it into a mergable constant section. If we have 284 // a section for this size, use it, otherwise use the arbitrary sized 285 // mergable section. 286 switch ( 287 GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) { 288 case 4: return SectionKind::getMergeableConst4(); 289 case 8: return SectionKind::getMergeableConst8(); 290 case 16: return SectionKind::getMergeableConst16(); 291 case 32: return SectionKind::getMergeableConst32(); 292 default: 293 return SectionKind::getReadOnly(); 294 } 295 296 } else { 297 // In static, ROPI and RWPI relocation models, the linker will resolve 298 // all addresses, so the relocation entries will actually be constants by 299 // the time the app starts up. However, we can't put this into a 300 // mergable section, because the linker doesn't take relocations into 301 // consideration when it tries to merge entries in the section. 302 Reloc::Model ReloModel = TM.getRelocationModel(); 303 if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI || 304 ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI || 305 !C->needsDynamicRelocation()) 306 return SectionKind::getReadOnly(); 307 308 // Otherwise, the dynamic linker needs to fix it up, put it in the 309 // writable data.rel section. 310 return SectionKind::getReadOnlyWithRel(); 311 } 312 } 313 314 // Okay, this isn't a constant. 315 return SectionKind::getData(); 316 } 317 318 /// This method computes the appropriate section to emit the specified global 319 /// variable or function definition. This should not be passed external (or 320 /// available externally) globals. 321 MCSection *TargetLoweringObjectFile::SectionForGlobal( 322 const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { 323 // Select section name. 324 if (GO->hasSection()) 325 return getExplicitSectionGlobal(GO, Kind, TM); 326 327 if (auto *GVar = dyn_cast<GlobalVariable>(GO)) { 328 auto Attrs = GVar->getAttributes(); 329 if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) || 330 (Attrs.hasAttribute("data-section") && Kind.isData()) || 331 (Attrs.hasAttribute("relro-section") && Kind.isReadOnlyWithRel()) || 332 (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly())) { 333 return getExplicitSectionGlobal(GO, Kind, TM); 334 } 335 } 336 337 if (auto *F = dyn_cast<Function>(GO)) { 338 if (F->hasFnAttribute("implicit-section-name")) 339 return getExplicitSectionGlobal(GO, Kind, TM); 340 } 341 342 // Use default section depending on the 'type' of global 343 return SelectSectionForGlobal(GO, Kind, TM); 344 } 345 346 /// This method computes the appropriate section to emit the specified global 347 /// variable or function definition. This should not be passed external (or 348 /// available externally) globals. 349 MCSection * 350 TargetLoweringObjectFile::SectionForGlobal(const GlobalObject *GO, 351 const TargetMachine &TM) const { 352 return SectionForGlobal(GO, getKindForGlobal(GO, TM), TM); 353 } 354 355 MCSection *TargetLoweringObjectFile::getSectionForJumpTable( 356 const Function &F, const TargetMachine &TM) const { 357 Align Alignment(1); 358 return getSectionForConstant(F.getParent()->getDataLayout(), 359 SectionKind::getReadOnly(), /*C=*/nullptr, 360 Alignment); 361 } 362 363 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection( 364 bool UsesLabelDifference, const Function &F) const { 365 // In PIC mode, we need to emit the jump table to the same section as the 366 // function body itself, otherwise the label differences won't make sense. 367 // FIXME: Need a better predicate for this: what about custom entries? 368 if (UsesLabelDifference) 369 return true; 370 371 // We should also do if the section name is NULL or function is declared 372 // in discardable section 373 // FIXME: this isn't the right predicate, should be based on the MCSection 374 // for the function. 375 return F.isWeakForLinker(); 376 } 377 378 /// Given a mergable constant with the specified size and relocation 379 /// information, return a section that it should be placed in. 380 MCSection *TargetLoweringObjectFile::getSectionForConstant( 381 const DataLayout &DL, SectionKind Kind, const Constant *C, 382 Align &Alignment) const { 383 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 384 return ReadOnlySection; 385 386 return DataSection; 387 } 388 389 MCSection *TargetLoweringObjectFile::getSectionForMachineBasicBlock( 390 const Function &F, const MachineBasicBlock &MBB, 391 const TargetMachine &TM) const { 392 return nullptr; 393 } 394 395 MCSection *TargetLoweringObjectFile::getUniqueSectionForFunction( 396 const Function &F, const TargetMachine &TM) const { 397 return nullptr; 398 } 399 400 /// getTTypeGlobalReference - Return an MCExpr to use for a 401 /// reference to the specified global variable from exception 402 /// handling information. 403 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference( 404 const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM, 405 MachineModuleInfo *MMI, MCStreamer &Streamer) const { 406 const MCSymbolRefExpr *Ref = 407 MCSymbolRefExpr::create(TM.getSymbol(GV), getContext()); 408 409 return getTTypeReference(Ref, Encoding, Streamer); 410 } 411 412 const MCExpr *TargetLoweringObjectFile:: 413 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 414 MCStreamer &Streamer) const { 415 switch (Encoding & 0x70) { 416 default: 417 report_fatal_error("We do not support this DWARF encoding yet!"); 418 case dwarf::DW_EH_PE_absptr: 419 // Do nothing special 420 return Sym; 421 case dwarf::DW_EH_PE_pcrel: { 422 // Emit a label to the streamer for the current position. This gives us 423 // .-foo addressing. 424 MCSymbol *PCSym = getContext().createTempSymbol(); 425 Streamer.emitLabel(PCSym); 426 const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext()); 427 return MCBinaryExpr::createSub(Sym, PC, getContext()); 428 } 429 } 430 } 431 432 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 433 // FIXME: It's not clear what, if any, default this should have - perhaps a 434 // null return could mean 'no location' & we should just do that here. 435 return MCSymbolRefExpr::create(Sym, getContext()); 436 } 437 438 void TargetLoweringObjectFile::getNameWithPrefix( 439 SmallVectorImpl<char> &OutName, const GlobalValue *GV, 440 const TargetMachine &TM) const { 441 Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false); 442 } 443