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/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/MC/MCAsmInfo.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/Dwarf.h" 28 #include "llvm/Support/ErrorHandling.h" 29 #include "llvm/Support/raw_ostream.h" 30 #include "llvm/Target/TargetLowering.h" 31 #include "llvm/Target/TargetMachine.h" 32 #include "llvm/Target/TargetOptions.h" 33 using namespace llvm; 34 35 //===----------------------------------------------------------------------===// 36 // Generic Code 37 //===----------------------------------------------------------------------===// 38 39 /// Initialize - this method must be called before any actual lowering is 40 /// done. This specifies the current context for codegen, and gives the 41 /// lowering implementations a chance to set up their default sections. 42 void TargetLoweringObjectFile::Initialize(MCContext &ctx, 43 const TargetMachine &TM) { 44 Ctx = &ctx; 45 DL = TM.getDataLayout(); 46 InitMCObjectFileInfo(TM.getTargetTriple(), 47 TM.getRelocationModel(), TM.getCodeModel(), *Ctx); 48 } 49 50 TargetLoweringObjectFile::~TargetLoweringObjectFile() { 51 } 52 53 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) { 54 const Constant *C = GV->getInitializer(); 55 56 // Must have zero initializer. 57 if (!C->isNullValue()) 58 return false; 59 60 // Leave constant zeros in readonly constant sections, so they can be shared. 61 if (GV->isConstant()) 62 return false; 63 64 // If the global has an explicit section specified, don't put it in BSS. 65 if (GV->hasSection()) 66 return false; 67 68 // If -nozero-initialized-in-bss is specified, don't ever use BSS. 69 if (NoZerosInBSS) 70 return false; 71 72 // Otherwise, put it in BSS! 73 return true; 74 } 75 76 /// IsNullTerminatedString - Return true if the specified constant (which is 77 /// known to have a type that is an array of 1/2/4 byte elements) ends with a 78 /// nul value and contains no other nuls in it. Note that this is more general 79 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings. 80 static bool IsNullTerminatedString(const Constant *C) { 81 // First check: is we have constant array terminated with zero 82 if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) { 83 unsigned NumElts = CDS->getNumElements(); 84 assert(NumElts != 0 && "Can't have an empty CDS"); 85 86 if (CDS->getElementAsInteger(NumElts-1) != 0) 87 return false; // Not null terminated. 88 89 // Verify that the null doesn't occur anywhere else in the string. 90 for (unsigned i = 0; i != NumElts-1; ++i) 91 if (CDS->getElementAsInteger(i) == 0) 92 return false; 93 return true; 94 } 95 96 // Another possibility: [1 x i8] zeroinitializer 97 if (isa<ConstantAggregateZero>(C)) 98 return cast<ArrayType>(C->getType())->getNumElements() == 1; 99 100 return false; 101 } 102 103 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase( 104 const GlobalValue *GV, StringRef Suffix, Mangler &Mang, 105 const TargetMachine &TM) const { 106 assert(!Suffix.empty()); 107 108 SmallString<60> NameStr; 109 NameStr += DL->getPrivateGlobalPrefix(); 110 TM.getNameWithPrefix(NameStr, GV, Mang); 111 NameStr.append(Suffix.begin(), Suffix.end()); 112 return Ctx->GetOrCreateSymbol(NameStr.str()); 113 } 114 115 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol( 116 const GlobalValue *GV, Mangler &Mang, const TargetMachine &TM, 117 MachineModuleInfo *MMI) const { 118 return TM.getSymbol(GV, Mang); 119 } 120 121 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer, 122 const TargetMachine &TM, 123 const MCSymbol *Sym) const { 124 } 125 126 127 /// getKindForGlobal - This is a top-level target-independent classifier for 128 /// a global variable. Given an global variable and information from TM, it 129 /// classifies the global in a variety of ways that make various target 130 /// implementations simpler. The target implementation is free to ignore this 131 /// extra info of course. 132 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV, 133 const TargetMachine &TM){ 134 assert(!GV->isDeclaration() && !GV->hasAvailableExternallyLinkage() && 135 "Can only be used for global definitions"); 136 137 Reloc::Model ReloModel = TM.getRelocationModel(); 138 139 // Early exit - functions should be always in text sections. 140 const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV); 141 if (!GVar) 142 return SectionKind::getText(); 143 144 // Handle thread-local data first. 145 if (GVar->isThreadLocal()) { 146 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) 147 return SectionKind::getThreadBSS(); 148 return SectionKind::getThreadData(); 149 } 150 151 // Variables with common linkage always get classified as common. 152 if (GVar->hasCommonLinkage()) 153 return SectionKind::getCommon(); 154 155 // Variable can be easily put to BSS section. 156 if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) { 157 if (GVar->hasLocalLinkage()) 158 return SectionKind::getBSSLocal(); 159 else if (GVar->hasExternalLinkage()) 160 return SectionKind::getBSSExtern(); 161 return SectionKind::getBSS(); 162 } 163 164 const Constant *C = GVar->getInitializer(); 165 166 // If the global is marked constant, we can put it into a mergable section, 167 // a mergable string section, or general .data if it contains relocations. 168 if (GVar->isConstant()) { 169 // If the initializer for the global contains something that requires a 170 // relocation, then we may have to drop this into a writable data section 171 // even though it is marked const. 172 switch (C->getRelocationInfo()) { 173 case Constant::NoRelocation: 174 // If the global is required to have a unique address, it can't be put 175 // into a mergable section: just drop it into the general read-only 176 // section instead. 177 if (!GVar->hasUnnamedAddr()) 178 return SectionKind::getReadOnly(); 179 180 // If initializer is a null-terminated string, put it in a "cstring" 181 // section of the right width. 182 if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) { 183 if (IntegerType *ITy = 184 dyn_cast<IntegerType>(ATy->getElementType())) { 185 if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 || 186 ITy->getBitWidth() == 32) && 187 IsNullTerminatedString(C)) { 188 if (ITy->getBitWidth() == 8) 189 return SectionKind::getMergeable1ByteCString(); 190 if (ITy->getBitWidth() == 16) 191 return SectionKind::getMergeable2ByteCString(); 192 193 assert(ITy->getBitWidth() == 32 && "Unknown width"); 194 return SectionKind::getMergeable4ByteCString(); 195 } 196 } 197 } 198 199 // Otherwise, just drop it into a mergable constant section. If we have 200 // a section for this size, use it, otherwise use the arbitrary sized 201 // mergable section. 202 switch (TM.getDataLayout()->getTypeAllocSize(C->getType())) { 203 case 4: return SectionKind::getMergeableConst4(); 204 case 8: return SectionKind::getMergeableConst8(); 205 case 16: return SectionKind::getMergeableConst16(); 206 default: return SectionKind::getMergeableConst(); 207 } 208 209 case Constant::LocalRelocation: 210 // In static relocation model, the linker will resolve all addresses, so 211 // the relocation entries will actually be constants by the time the app 212 // starts up. However, we can't put this into a mergable section, because 213 // the linker doesn't take relocations into consideration when it tries to 214 // merge entries in the section. 215 if (ReloModel == Reloc::Static) 216 return SectionKind::getReadOnly(); 217 218 // Otherwise, the dynamic linker needs to fix it up, put it in the 219 // writable data.rel.local section. 220 return SectionKind::getReadOnlyWithRelLocal(); 221 222 case Constant::GlobalRelocations: 223 // In static relocation model, the linker will resolve all addresses, so 224 // the relocation entries will actually be constants by the time the app 225 // starts up. However, we can't put this into a mergable section, because 226 // the linker doesn't take relocations into consideration when it tries to 227 // merge entries in the section. 228 if (ReloModel == Reloc::Static) 229 return SectionKind::getReadOnly(); 230 231 // Otherwise, the dynamic linker needs to fix it up, put it in the 232 // writable data.rel section. 233 return SectionKind::getReadOnlyWithRel(); 234 } 235 } 236 237 // Okay, this isn't a constant. If the initializer for the global is going 238 // to require a runtime relocation by the dynamic linker, put it into a more 239 // specific section to improve startup time of the app. This coalesces these 240 // globals together onto fewer pages, improving the locality of the dynamic 241 // linker. 242 if (ReloModel == Reloc::Static) 243 return SectionKind::getDataNoRel(); 244 245 switch (C->getRelocationInfo()) { 246 case Constant::NoRelocation: 247 return SectionKind::getDataNoRel(); 248 case Constant::LocalRelocation: 249 return SectionKind::getDataRelLocal(); 250 case Constant::GlobalRelocations: 251 return SectionKind::getDataRel(); 252 } 253 llvm_unreachable("Invalid relocation"); 254 } 255 256 /// SectionForGlobal - This method computes the appropriate section to emit 257 /// the specified global variable or function definition. This should not 258 /// be passed external (or available externally) globals. 259 const MCSection *TargetLoweringObjectFile:: 260 SectionForGlobal(const GlobalValue *GV, SectionKind Kind, Mangler &Mang, 261 const TargetMachine &TM) const { 262 // Select section name. 263 if (GV->hasSection()) 264 return getExplicitSectionGlobal(GV, Kind, Mang, TM); 265 266 267 // Use default section depending on the 'type' of global 268 return SelectSectionForGlobal(GV, Kind, Mang, TM); 269 } 270 271 bool TargetLoweringObjectFile::isSectionAtomizableBySymbols( 272 const MCSection &Section) const { 273 return false; 274 } 275 276 // Lame default implementation. Calculate the section name for global. 277 const MCSection * 278 TargetLoweringObjectFile::SelectSectionForGlobal(const GlobalValue *GV, 279 SectionKind Kind, 280 Mangler &Mang, 281 const TargetMachine &TM) const{ 282 assert(!Kind.isThreadLocal() && "Doesn't support TLS"); 283 284 if (Kind.isText()) 285 return getTextSection(); 286 287 if (Kind.isBSS() && BSSSection != nullptr) 288 return BSSSection; 289 290 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 291 return ReadOnlySection; 292 293 return getDataSection(); 294 } 295 296 /// getSectionForConstant - Given a mergable constant with the 297 /// specified size and relocation information, return a section that it 298 /// should be placed in. 299 const MCSection * 300 TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const { 301 if (Kind.isReadOnly() && ReadOnlySection != nullptr) 302 return ReadOnlySection; 303 304 return DataSection; 305 } 306 307 /// getTTypeGlobalReference - Return an MCExpr to use for a 308 /// reference to the specified global variable from exception 309 /// handling information. 310 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference( 311 const GlobalValue *GV, unsigned Encoding, Mangler &Mang, 312 const TargetMachine &TM, MachineModuleInfo *MMI, 313 MCStreamer &Streamer) const { 314 const MCSymbolRefExpr *Ref = 315 MCSymbolRefExpr::Create(TM.getSymbol(GV, Mang), getContext()); 316 317 return getTTypeReference(Ref, Encoding, Streamer); 318 } 319 320 const MCExpr *TargetLoweringObjectFile:: 321 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding, 322 MCStreamer &Streamer) const { 323 switch (Encoding & 0x70) { 324 default: 325 report_fatal_error("We do not support this DWARF encoding yet!"); 326 case dwarf::DW_EH_PE_absptr: 327 // Do nothing special 328 return Sym; 329 case dwarf::DW_EH_PE_pcrel: { 330 // Emit a label to the streamer for the current position. This gives us 331 // .-foo addressing. 332 MCSymbol *PCSym = getContext().CreateTempSymbol(); 333 Streamer.EmitLabel(PCSym); 334 const MCExpr *PC = MCSymbolRefExpr::Create(PCSym, getContext()); 335 return MCBinaryExpr::CreateSub(Sym, PC, getContext()); 336 } 337 } 338 } 339 340 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const { 341 // FIXME: It's not clear what, if any, default this should have - perhaps a 342 // null return could mean 'no location' & we should just do that here. 343 return MCSymbolRefExpr::Create(Sym, *Ctx); 344 } 345