1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements classes used to handle lowerings specific to common 11 // object file formats. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #include "llvm/Target/TargetLoweringObjectFile.h" 16 #include "llvm/BinaryFormat/Dwarf.h" 17 #include "llvm/IR/Constants.h" 18 #include "llvm/IR/DataLayout.h" 19 #include "llvm/IR/DerivedTypes.h" 20 #include "llvm/IR/Function.h" 21 #include "llvm/IR/GlobalVariable.h" 22 #include "llvm/IR/Mangler.h" 23 #include "llvm/MC/MCContext.h" 24 #include "llvm/MC/MCExpr.h" 25 #include "llvm/MC/MCStreamer.h" 26 #include "llvm/MC/MCSymbol.h" 27 #include "llvm/Support/ErrorHandling.h" 28 #include "llvm/Support/raw_ostream.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 Ctx = &ctx; 43 // `Initialize` can be called more than once. 44 delete Mang; 45 Mang = new Mangler(); 46 InitMCObjectFileInfo(TM.getTargetTriple(), TM.isPositionIndependent(), *Ctx, 47 TM.getCodeModel() == CodeModel::Large); 48 } 49 50 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 51 delete Mang; 52 } 53 54 static bool isNullOrUndef(const Constant *C) { 55 // Check that the constant isn't all zeros or undefs. 56 if (C->isNullValue() || isa<UndefValue>(C)) 57 return true; 58 if (!isa<ConstantAggregate>(C)) 59 return false; 60 for (auto Operand : C->operand_values()) { 61 if (!isNullOrUndef(cast<Constant>(Operand))) 62 return false; 63 } 64 return true; 65 } 66 67 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) { 68 const Constant *C = GV->getInitializer(); 69 70 // Must have zero initializer. 71 if (!isNullOrUndef(C)) 72 return false; 73 74 // Leave constant zeros in readonly constant sections, so they can be shared. 75 if (GV->isConstant()) 76 return false; 77 78 // If the global has an explicit section specified, don't put it in BSS. 79 if (GV->hasSection()) 80 return false; 81 82 // If -nozero-initialized-in-bss is specified, don't ever use BSS. 83 if (NoZerosInBSS) 84 return false; 85 86 // Otherwise, put it in BSS! 87 return true; 88 } 89 90 /// IsNullTerminatedString - Return true if the specified constant (which is 91 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 92 /// nul value and contains no other nuls in it. Note that this is more general 93 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 94 static bool IsNullTerminatedString(const Constant *C) { 95 // First check: is we have constant array terminated with zero 96 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 97 unsigned NumElts = CDS->getNumElements(); 98 assert(NumElts != 0 && "Can't have an empty CDS"); 99 100 if (CDS->getElementAsInteger(NumElts-1) != 0) 101 return false; // Not null terminated. 102 103 // Verify that the null doesn't occur anywhere else in the string. 104 for (unsigned i = 0; i != NumElts-1; ++i) 105 if (CDS->getElementAsInteger(i) == 0) 106 return false; 107 return true; 108 } 109 110 // Another possibility: [1 x i8] zeroinitializer 111 if (isa<ConstantAggregateZero>(C)) 112 return cast<ArrayType>(C->getType())->getNumElements() == 1; 113 114 return false; 115 } 116 117 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase( 118 const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const { 119 assert(!Suffix.empty()); 120 121 SmallString<60> NameStr; 122 NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix(); 123 TM.getNameWithPrefix(NameStr, GV, *Mang); 124 NameStr.append(Suffix.begin(), Suffix.end()); 125 return Ctx->getOrCreateSymbol(NameStr); 126 } 127 128 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol( 129 const GlobalValue *GV, const TargetMachine &TM, 130 MachineModuleInfo *MMI) const { 131 return TM.getSymbol(GV); 132 } 133 134 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 135 const DataLayout &, 136 const MCSymbol *Sym) const { 137 } 138 139 140 /// getKindForGlobal - This is a top-level target-independent classifier for 141 /// a global object. Given a global variable and information from the TM, this 142 /// function classifies the global in a target independent manner. This function 143 /// may be overridden by the target implementation. 144 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO, 145 const TargetMachine &TM){ 146 assert(!GO->isDeclaration() && !GO->hasAvailableExternallyLinkage() && 147 "Can only be used for global definitions"); 148 149 // Functions are classified as text sections. 150 if (isa<Function>(GO)) 151 return SectionKind::getText(); 152 153 // Global variables require more detailed analysis. 154 const auto *GVar = cast<GlobalVariable>(GO); 155 156 // Handle thread-local data first. 157 if (GVar->isThreadLocal()) { 158 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) 159 return SectionKind::getThreadBSS(); 160 return SectionKind::getThreadData(); 161 } 162 163 // Variables with common linkage always get classified as common. 164 if (GVar->hasCommonLinkage()) 165 return SectionKind::getCommon(); 166 167 // Variable can be easily put to BSS section. 168 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) { 169 if (GVar->hasLocalLinkage()) 170 return SectionKind::getBSSLocal(); 171 else if (GVar->hasExternalLinkage()) 172 return SectionKind::getBSSExtern(); 173 return SectionKind::getBSS(); 174 } 175 176 // If the global is marked constant, we can put it into a mergable section, 177 // a mergable string section, or general .data if it contains relocations. 178 if (GVar->isConstant()) { 179 // If the initializer for the global contains something that requires a 180 // relocation, then we may have to drop this into a writable data section 181 // even though it is marked const. 182 const Constant *C = GVar->getInitializer(); 183 if (!C->needsRelocation()) { 184 // If the global is required to have a unique address, it can't be put 185 // into a mergable section: just drop it into the general read-only 186 // section instead. 187 if (!GVar->hasGlobalUnnamedAddr()) 188 return SectionKind::getReadOnly(); 189 190 // If initializer is a null-terminated string, put it in a "cstring" 191 // section of the right width. 192 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 193 if (IntegerType *ITy = 194 dyn_cast<IntegerType>(ATy->getElementType())) { 195 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 196 ITy->getBitWidth() == 32) && 197 IsNullTerminatedString(C)) { 198 if (ITy->getBitWidth() == 8) 199 return SectionKind::getMergeable1ByteCString(); 200 if (ITy->getBitWidth() == 16) 201 return SectionKind::getMergeable2ByteCString(); 202 203 assert(ITy->getBitWidth() == 32 && "Unknown width"); 204 return SectionKind::getMergeable4ByteCString(); 205 } 206 } 207 } 208 209 // Otherwise, just drop it into a mergable constant section. If we have 210 // a section for this size, use it, otherwise use the arbitrary sized 211 // mergable section. 212 switch ( 213 GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) { 214 case 4: return SectionKind::getMergeableConst4(); 215 case 8: return SectionKind::getMergeableConst8(); 216 case 16: return SectionKind::getMergeableConst16(); 217 case 32: return SectionKind::getMergeableConst32(); 218 default: 219 return SectionKind::getReadOnly(); 220 } 221 222 } else { 223 // In static, ROPI and RWPI relocation models, the linker will resolve 224 // all addresses, so the relocation entries will actually be constants by 225 // the time the app starts up. However, we can't put this into a 226 // mergable section, because the linker doesn't take relocations into 227 // consideration when it tries to merge entries in the section. 228 Reloc::Model ReloModel = TM.getRelocationModel(); 229 if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI || 230 ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI) 231 return SectionKind::getReadOnly(); 232 233 // Otherwise, the dynamic linker needs to fix it up, put it in the 234 // writable data.rel section. 235 return SectionKind::getReadOnlyWithRel(); 236 } 237 } 238 239 // Okay, this isn't a constant. 240 return SectionKind::getData(); 241 } 242 243 /// This method computes the appropriate section to emit the specified global 244 /// variable or function definition. This should not be passed external (or 245 /// available externally) globals. 246 MCSection *TargetLoweringObjectFile::SectionForGlobal( 247 const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const { 248 // Select section name. 249 if (GO->hasSection()) 250 return getExplicitSectionGlobal(GO, Kind, TM); 251 252 if (auto *GVar = dyn_cast<GlobalVariable>(GO)) { 253 auto Attrs = GVar->getAttributes(); 254 if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) || 255 (Attrs.hasAttribute("data-section") && Kind.isData()) || 256 (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly())) { 257 return getExplicitSectionGlobal(GO, Kind, TM); 258 } 259 } 260 261 if (auto *F = dyn_cast<Function>(GO)) { 262 if (F->hasFnAttribute("implicit-section-name")) 263 return getExplicitSectionGlobal(GO, Kind, TM); 264 } 265 266 // Use default section depending on the 'type' of global 267 return SelectSectionForGlobal(GO, Kind, TM); 268 } 269 270 MCSection *TargetLoweringObjectFile::getSectionForJumpTable( 271 const Function &F, const TargetMachine &TM) const { 272 unsigned Align = 0; 273 return getSectionForConstant(F.getParent()->getDataLayout(), 274 SectionKind::getReadOnly(), /*C=*/nullptr, 275 Align); 276 } 277 278 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection( 279 bool UsesLabelDifference, const Function &F) const { 280 // In PIC mode, we need to emit the jump table to the same section as the 281 // function body itself, otherwise the label differences won't make sense. 282 // FIXME: Need a better predicate for this: what about custom entries? 283 if (UsesLabelDifference) 284 return true; 285 286 // We should also do if the section name is NULL or function is declared 287 // in discardable section 288 // FIXME: this isn't the right predicate, should be based on the MCSection 289 // for the function. 290 return F.isWeakForLinker(); 291 } 292 293 /// Given a mergable constant with the specified size and relocation 294 /// information, return a section that it should be placed in. 295 MCSection *TargetLoweringObjectFile::getSectionForConstant( 296 const DataLayout &DL, SectionKind Kind, const Constant *C, 297 unsigned &Align) const { 298 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 299 return ReadOnlySection; 300 301 return DataSection; 302 } 303 304 /// getTTypeGlobalReference - Return an MCExpr to use for a 305 /// reference to the specified global variable from exception 306 /// handling information. 307 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference( 308 const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM, 309 MachineModuleInfo *MMI, MCStreamer &Streamer) const { 310 const MCSymbolRefExpr *Ref = 311 MCSymbolRefExpr::create(TM.getSymbol(GV), getContext()); 312 313 return getTTypeReference(Ref, Encoding, Streamer); 314 } 315 316 const MCExpr *TargetLoweringObjectFile:: 317 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 318 MCStreamer &Streamer) const { 319 switch (Encoding & 0x70) { 320 default: 321 report_fatal_error("We do not support this DWARF encoding yet!"); 322 case dwarf::DW_EH_PE_absptr: 323 // Do nothing special 324 return Sym; 325 case dwarf::DW_EH_PE_pcrel: { 326 // Emit a label to the streamer for the current position. This gives us 327 // .-foo addressing. 328 MCSymbol *PCSym = getContext().createTempSymbol(); 329 Streamer.EmitLabel(PCSym); 330 const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext()); 331 return MCBinaryExpr::createSub(Sym, PC, getContext()); 332 } 333 } 334 } 335 336 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 337 // FIXME: It's not clear what, if any, default this should have - perhaps a 338 // null return could mean 'no location' & we should just do that here. 339 return MCSymbolRefExpr::create(Sym, *Ctx); 340 } 341 342 void TargetLoweringObjectFile::getNameWithPrefix( 343 SmallVectorImpl<char> &OutName, const GlobalValue *GV, 344 const TargetMachine &TM) const { 345 Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false); 346 } 347